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View Full Version : DPI vs. PPI - What's the difference?



Gamerprinter
10-13-2007, 06:05 PM
For my own purposes, and this really is for everyone, since I have been getting more and more into Virtual Table Top software of late, I would like to see more maps that are "agnostic" to the game system. What I mean by that is no grid. Grids just get in the way of using a VTT because VTTs have their own grid overlays usually. Also, if text could be removed that would be a great addition as well.

Basically two versions of the map are needed for VTTs. One complete (text and grid) and one clean (no text or grid). The complete one is for GM reference and the clean one is to play on.

Finally, all maps should be created at a scale of 200 pixels equals 5 feet. That will make it usable across almost every VTT with the understanding that some VTTs will need to rescale, but it is easier and cleaner to scale down rather than up.

That said, I would love to eventually play on this map. *hint* *hint*

Can't wait to see what the others produce.

RP, although I've been creating hand-drawn and digital maps for many years, I've never included a grid, until this year, because I wanted my maps to be more universal in use. With your mention of VTT, I see a huge opportunity to my maps to a different and perhaps larger audience.

Its true my business is printing maps, but map creation and distribution is also at the heart of what I do and finding a way to target the VTT audience is one of my goals.

The maps produced here were not saved to any specific pixel resolution to be used specifically for gaming with - I only made them large enough to show detail for anyone who might download a closer view.

That said, I have no problem providing through this thread the final versions of each map at 200 dpi with and without gridlines. (Of course if the outside map is too large, megabyte-wise for this site, I may have to break it up into multiple maps.) 8)

RPMiller
10-13-2007, 08:00 PM
Just to clarify, not 200 dpi (dots per inch), 200 pixels = 5 feet (scale).

Cool. Look forward to seeing them. As for the VTT audience, you should probably immerse yourself into those forums to get an idea of what is expected and what people want to see.

Gamerprinter
10-13-2007, 10:25 PM
If I create my graphics with a scale of 1 inch = 5 feet, and resolution to 200 dpi (dots per inch), isn't that the same as 200 pixels per 5 feet? ;)

RPMiller
10-13-2007, 11:43 PM
You are over complicating it. As I explained in the reply to your PM, we don't care what the dpi is. We aren't printing it. What is a dot on the screen, how does that compare to a dot on a printer? Just keep it simple.

Midgardsormr
10-14-2007, 12:52 AM
Heh. I think the answer to the question is "yes." DPI may not matter to you, but I suspect it might to Gamerprinter!

RPMiller
10-14-2007, 01:12 AM
Not in this instance. He is talking to me specifically about VTTs. There is no need to print when you are playing directly on the screen, but I get the irony of the statement and his name.

Gamerprinter
10-14-2007, 02:45 AM
A digital image is measured in pixels. When a pixel is printed, it is represented by a dot. Therefore dpi and ppi is the same thing - one in its digital format only (pixels), the other in print (dots).

All graphics are measured interchangeably as dpi or ppi - I've got software that refers to both.

A 200 dpi or ppi image is measured as 200 x 200 dots or pixels. Most software I use only refer to dpi, even when an image is never intended to be printed.

So the issue is semantics. Unless you've got some other specific way to describe 200 pixels = 5 feet, there's only one way for me to make it.

So I do believe I am correct. I am not making it more complicated. The software I refer to mentioning ppi was Micrografx Picture Publisher and it nolonger exists. All other graphics software refer to dpi interchangeably with pixels.

Its a simple question. ;)

Robbie
10-14-2007, 11:05 AM
Nice work GP...I'm also quite thoroughly impressed with the exterior map...The interior map shows a LOT of work done, but for my own tastes it may be a wee bit too texture heavy...I'm not saying thats a bad thing, as I'm more of a minimalist in my approach to mapping...I definitely agree that these would go really well with VTT.

As for the dpi/ppi/200 argument...there is no argument really...200 pixels per 5 square feet = 200 dpi scaled print for D&D size miniatures = good for everyone

RPMiller
10-14-2007, 01:40 PM
A digital image is measured in pixels. When a pixel is printed, it is represented by a dot. Therefore dpi and ppi is the same thing - one in its digital format only (pixels), the other in print (dots).

All graphics are measured interchangeably as dpi or ppi - I've got software that refers to both.

A 200 dpi or ppi image is measured as 200 x 200 dots or pixels. Most software I use only refer to dpi, even when an image is never intended to be printed.

So the issue is semantics. Unless you've got some other specific way to describe 200 pixels = 5 feet, there's only one way for me to make it.

So I do believe I am correct. I am not making it more complicated. The software I refer to mentioning ppi was Micrografx Picture Publisher and it nolonger exists. All other graphics software refer to dpi interchangeably with pixels.

Its a simple question. ;)


Nice work GP...I'm also quite thoroughly impressed with the exterior map...The interior map shows a LOT of work done, but for my own tastes it may be a wee bit too texture heavy...I'm not saying thats a bad thing, as I'm more of a minimalist in my approach to mapping...I definitely agree that these would go really well with VTT.

As for the dpi/ppi/200 argument...there is no argument really...200 pixels per 5 square feet = 200 dpi scaled print for D&D size miniatures = good for everyone

While I understand and appreciate and agree to some extent with your assessment of dpi, it is not entirely accurate. As I've mentioned I have a background in reprographics from 15 odd years ago as well as some formal training in the area of computer graphics. This dpi/ppi argument has been around a long time and unfortunately those that don't fully understand it are in the majority and even the application designers get it wrong sometimes because of the way the older software worked and used the terms.

Here is just one of many links that you can find on the subject, but I picked this one specifically because it has print shop links as well. Please understand that I'm not trying to be a jerk about this, and I agree that we are essentially saying the same thing, and in this instance it isn't important as the software obviously is doing it correctly, but we really should be using the correct terms and it is always good to learn something new. :) I've talked to many reprographics folks over the years and many of them get this wrong because they are usually self taught or don't have the diverse background that would have introduced them to the differences in the terms. Not to mention, as I stated, the software itself is misleading.

http://www.rideau-info.com/photos/mythdpi.html

I would encourage everyone to investigate this further, and if we want to discuss it some more, since it definitely falls into the realm of this site, I think starting a new thread would be a great idea.

ravells
10-14-2007, 02:35 PM
Might be worth moving the relevant posts from here to a new thread too for when people come to look at the threads come judgement day :)

Ravs

RPMiller
10-14-2007, 03:14 PM
Excellent point! Done. :D

Gamerprinter
10-14-2007, 09:18 PM
I just tried to export my exterior map to 1" = 5 feet at 200 dpi, and the file was over 1.8 GB in size! Note the map is currently scaled at 1" = 20 feet and its set dimensions are 54 inches by 43 inches, which means I have to rescale the map to 216 inches x 132 inches at 200 dpi to achieve proper scale.

This makes for a huge file.

The problem of course is that I need to include the idea that I'm going export at that specific scale when I create the map in the first place, so I can create manageable chunks that export easily at proper scale.

In regards to the DPI vs. PPI discussion, points to keep in mind:

1. Pixel or "picture element" is a basic element of color described on a video screen. Your screen has a set resolution that you can rescale if you have the right size monitor and proper video card. This however is measured both in pixels and dots per inch.

A pixel is square and is created using light shot through a monitor into your eyes.

A printed dot is a circle that is created from nozzles of inkjet paper or colored toner from a laser printer and is viewed by white light bouncing off the paper so your eyes can see the color (simplified explanation)

2. Most graphics software, even web graphics is not interdependant of DPI for measurement scale. Digital graphics and dpi can not be separated - not by definition of most graphics applications. They offer no "PPI" option.

I agree that a pixel and a dot are two different things, however software developers don't know the difference.

Regarding software and to help others understand how to create properly scaled map graphics for VTT use, they are treated as the same. You can't treat them as "apples and oranges" that will only confuse others.

Know then that a pixel and a dot are two different things. But if you want to 200 pixels to equal 5 feet in VTT scale, then you have to create your graphics at 1 inch equals 5 feet scale saved at 200 dpi - to get what you're looking for. That's all I'm saying. :?

My only question now is what is the dimensions of a standard map used for VTT use, in other words how many "feet" is shown on the monitor and how many feet are allowed off monitor that you can "scroll/move characters" within the same map?

If I knew that I could optimize my map designs for that scale and dimension in mind and create optimal VTT maps.

RPMiller
10-14-2007, 11:26 PM
Please read through the article I linked to.

kalmarjan
10-14-2007, 11:46 PM
Honestly,

This will only ever come up if you plan to print out your maps. The whole PPI vs DPI point is moot if you are just using your maps for a VTT.

A better thing to think of is pixel resolution. While you are dealing with a VTT, it is helpful to remember that you are basically rendering your images just like you would for a web page. There are several things to keep in mind:

1) Download/transfer times. Like it not, you are transfering your map to another client/player. While you may appreciate the 200 "DPI" image, the long downloading time on the client end will absolutely impact play. With some VTTs, this can lead to errors, especially if there is functionality built into the VTT for use with maps. (Fog of war etc.)

2) Designing any form of graphics at 200 PPI (or DPI if you prefer) is really a waste while on a monitor. You will have users with high end cards using a maximum of 92 PPI with those images, so you are wasting filesizes.

3) File format. This is a biggie. Some people swear by JPEG, others by GIF, others by PNG or BMP. In the end, it all comes down to what you are doing with the image. I prefer using JPEG with my maps, as it keeps the filesizes down. You will hear people say that this downgrades the image blah blah... which is true if you do not know what you are doing. As long as you are finished with an image, make it into a JPEG with as low as compression you can stand before the image degrades (*Imageready is a good program for this), and you now have a filesize that is 20% of what you started. For tokens, PNG is the way to go **if** there is any element of the token that needs to be transparent. If you can keep your token square, then it is better to use the JPEG format. If your token only has 256 colors, then GIF is a better option.

It is useful to look up the various file formats and see what is right. Better yet, look up tutorials on designing graphics for the web and you will set your mind straight on what needs to be done.

I hope that helps, and if there is any questions, hollar at my PM box, and I will try and help you all out.

Sandeman

kalmarjan
10-15-2007, 12:03 AM
In regards to the DPI vs. PPI discussion, points to keep in mind:

1. Pixel or "picture element" is a basic element of color described on a video screen. Your screen has a set resolution that you can rescale if you have the right size monitor and proper video card. This however is measured both in pixels and dots per inch.

DPI is a measurement for printers ONLY. If you are speaking about screen resolution, it is better to refer to it is PPI.


2. Most graphics software, even web graphics is not interdependant of DPI for measurement scale. Digital graphics and dpi can not be separated - not by definition of most graphics applications. They offer no "PPI" option.

The two major ones, Photoshop, and GIMP measure these by Pixels/Inch, or PPI.


I agree that a pixel and a dot are two different things, however software developers don't know the difference.

Not true. In my experience, it is the user who does not know the difference. You will hardly ever see a setting in creating a new graphic or render that says "X number dots per Inch". The only time you will run into DPI is in print dialog boxes, where it is VERY important. This is usually based off your printer, but sometimes you can set the PPI to closely match the DPI. It is relatively based on the profiles you have set up for your printer within a program like photoshop.


Regarding software and to help others understand how to create properly scaled map graphics for VTT use, they are treated as the same. You can't treat them as "apples and oranges" that will only confuse others.

Know then that a pixel and a dot are two different things. But if you want to 200 pixels to equal 5 feet in VTT scale, then you have to create your graphics at 1 inch equals 5 feet scale saved at 200 dpi - to get what you're looking for. That's all I'm saying. :?

At that scale, each foot would be equivilent to 40 px. So a map size of 50 feet (or 10 5 foot squares) would be 2000 px X 2000 px.


My only question now is what is the dimensions of a standard map used for VTT use, in other words how many "feet" is shown on the monitor and how many feet are allowed off monitor that you can "scroll/move characters" within the same map?

If I knew that I could optimize my map designs for that scale and dimension in mind and create optimal VTT maps.

This would really depend. Again, I submit that it is up to you, and your players. It really helps to think of VTTs as web applications. Having a map that is 100 feet square equaling 4000 x 4000 px would be okay, if your VTT could handle that. Depending on what is on that map though, you could be looking at a significant filesize. If you are running a map that is like one of my photos (at 8.1 MP) then a 3264 x 2448 map size could run at 944 Kb unoptimized. If you were to save that out at 80%, you could be looking at a filesize of 755 Kb, which is not too bad.

I hope that helped out.

Sandeman

RPMiller
10-15-2007, 01:20 AM
To add to the above, the point of doing 200 pixels = 5' is so that the user can down sample the graphic to whatever his application uses as its standard scale and most will still keep it at least double the default so that the players can zoom in and the image won't get badly pixelated. The 200 pixels = 5' is the largest scale I've seen used and is actually for Dundjinni which, interestingly enough, is geared specifically for printing out and playing on with miniatures, but the VTT community uses a lot of Dundjinni graphics for its maps and just resizes them as needed. For MapTool, which is what I use, the standard scale is 50 pixels / cell. This is different than mentioned above, but would require a bunch more of explanation so just assume 50 pixels = 5'. So it is pretty easy to just scale everything to 25% of original and call it done.

Regarding graphic formats, yes, there is quite a lot of discussion, but for my taste and most of those that I play with and talk to, PNG is the only way to go. It is a lossless format with a decent compression ratio and supports alpha channels. In fact the only thing it doesn't support is animation so I would definitely go with PNG. JPEG would be your choice for photographs though, but that is yet another discussion.

Anyway, all the information is out there and very well explained and reasons given. What has been posted so far has been a great introduction, and I want to thank Sandeman for his excellent posts as well.

RobA
10-15-2007, 11:24 AM
Wow, this has been an active thread! Figured I'd chime in as well with a couple points of clarification...

I just tried to export my exterior map to 1" = 5 feet at 200 dpi, and the file was over 1.8 GB in size! Note the map is currently scaled at 1" = 20 feet and its set dimensions are 54 inches by 43 inches, which means I have to rescale the map to 216 inches x 132 inches at 200 dpi to achieve proper scale.


What kind of file was it? 1.8 GB is huge - this must be completely uncompressed!



A pixel is square and is created using light shot through a monitor into your eyes.


Not quite :) It all depends on the aspect ratio of your screen and the resolution you are driving it at.



2. Most graphics software, even web graphics is not interdependant of DPI for measurement scale. Digital graphics and dpi can not be separated - not by definition of most graphics applications. They offer no "PPI" option.


I think I must disagree here. Any raster graphic application (pixel based) I have seen used the terms pixels and DPI when defining the image - these two combined define a "print size". Many program, however, default the dpi, and changing it is buried somewhere within the application (if available at all).

The situation is very different for vector based applications, where the image is drawn, usually in real world measurements, and then rasterized out at a particular resolution. For example, if I have a 8"x10" image, and export it at 72dpi I end up with a 576x720px image. If I export the same image at 200dpi I get a 1600x2000 image. Now this gets even more complicated if you are drawing scaled in the application already.

Now this is clouded by Xara, as it is the only program I aware of that is actually a hybrid vector/raster application.



Know then that a pixel and a dot are two different things. But if you want to 200 pixels to equal 5 feet in VTT scale, then you have to create your graphics at 1 inch equals 5 feet scale saved at 200 dpi - to get what you're looking for. That's all I'm saying. :?


Yes - but only if your application is drawing in "real word units", which most "2D paint" program do NOT use.



so that the players can zoom in and the image won't get badly pixelated.

Which is why I'd like to see a pure vector (SVG?) VTT.



PNG is the only way to go. It is a lossless format with a decent compression ratio and supports alpha channels. In fact the only thing it doesn't support is animation so I would definitely go with PNG. JPEG would be your choice for photographs though, but that is yet another discussion.

OR JPEG for any photorealistic rendered map. (just watch the compression artifacts!)

-Rob A>

RPMiller
10-15-2007, 11:33 AM
Excellent post RobA! Thanks for the additional information. Especially regarding Xara, and the vector information.

Yea, the vector VTT idea is routinely brought up. I think the reason there aren't any (that I know of) is because the programming is a lot more complex than just dropping in a raster image. The nice thing is that most folks typically don't zoom in more than 200%, with exceptions of course of what they are looking at at the time. That would be cool to be able to zoom in all the way to read the page of a book lying open on the table. MapTool has sort of taken care of that though in that you can now add an associated image to the object so that if a player hovers their mouse over the object a window pops up with an image.

RobA
10-15-2007, 11:44 AM
I think the reason there aren't any (that I know of) is because the programming is a lot more complex

Hopefully that will change, as the next version of SVG standard will have some adaptive Level-of-Detail definitions. This web site (http://www.svgopen.org/2004/papers/AdaptiveLoD/) describes the current proposal - and it looks pretty nifty.

-Rob A>

RPMiller
10-15-2007, 11:53 AM
Nice! Some of those same algorithms are being used as guidelines in some of the new gigapixel images as well. Yes, I said gigapixel. :shock: If you haven't seen some of the new gigapixel images they are worth googling.

Gamerprinter
10-15-2007, 04:02 PM
The issue with my exterior map is that I scaled it at 1" = 20 feet and it measures 54" x 43", thus the true measure is 1080 feet x 860 feet.

Even if I forget about DPI and stick with pixels only. At 200 pixels every 5 feet, that's 43,200 pixels wide by 34,400 pixels - Xara won't let me export something at that size. In fact Photoshop won't let me create or open an image that is larger than 30,000 x 30,000 pixels.

This means I need to slice my image into multiple pieces, probably in quarters.

I was able to force the dpi down to 50 in Photoshop, but it would still not let me create more than 30,000 pixels in any dimension.

Lastly Kalmarjan describes a maximum normal map size at no large than 100 x 100 feet at 4000 x 4000 pixels. My map is almost 10 times that in dimension. It might be best for me to cut this image into 10 separate maps.

Thoughts?

RPMiller
10-15-2007, 04:13 PM
You must be using an older version of PS. CS3 doesn't have that size restriction.

Breaking it up into smaller pieces would certainly work, but it would be a little clumsy.

Regional maps in VTTs are not that small. In fact, one of the strengths of MapTool is that it practically has no maximum size. Realistically it does, but it depends on what is on it. That also is assuming that you are creating the map directly in MapTool instead of importing an external pic. Some of the tests we've done have been with maps that were a mile long and longer per side. As I suggested before, you should sign up at the various VTT forums and take a look at their capabilities and what others are doing if you are serious about getting into that market.

Gamerprinter
10-15-2007, 04:26 PM
So I should download the newest version of GIMP, since I don't plan to upgrading PS anytime soon. The latest GIMP should be like the newer versions of PS in this respect.

Yeah, I'm using PS 5.5, quite old. I got disgusted with some of the features to CS and never upgraded because of them.

Still Xara won't export the full map at that pixel resolution. Xara still looks at it in DPI and says it is limited by 9 feet in any direction (!)

Gamerprinter
10-15-2007, 06:37 PM
Knowing that I have an older version of PS with a limitation of 30,000 pixels in every dimension, first I cropped the original exterior map to just the compound the octogonal walls and everything within and a few outside trees.

I opened and rescaled the map in PS without resampling and ended up with a map file at 200 pixels = 5 feet. Conversely, the non-essential DPI data is 7.25 dots per inch. Still the file is 25.5 MB in size and in PNG format.

I feel this is still too large, however, I can upload into my FTP account if anyone (RP specifically) wants to download it for their own uses.

Once I get an upgraded version of GIMP I'll probably be able to do the entire map, of course the file size would still get much larger.

Thoughts?

Robbie
10-15-2007, 07:27 PM
Wow...see, for stuff liek this I think targetting VTT with maps of this scale woudl require some forethought...and I'm not slighting your process at all, becuse this all came up after your map was 80% or so done...but forethought would need to be in place to map the subject in sections...

Again though, this takes me back to thinking about the google maps api and how they handle the growing scale of the street level map...I imagine they are definitely usign a vector tool to create the street maps...but they're rendered in various different scales for the tile server...Each tile is 256x256 and the google api does all the work of placing the tiles in order and together...its pretty impressive, and I think a slicing tool would be perfect for that if a VTT took advantage of a tile server for maps...something to think about at least...

In other news...I'm so very glad to see that this conversation is staying professional and mature. There's obviously going to be differing opinions, and there's nothign wrong with being headstrong about being corrected, but the fact that this stayed mature makes me very happy...kudos and rep to all.

Gamerprinter
10-15-2007, 08:08 PM
Xara Xtreme has a very effective slicing tool within, so creating multiple pieces of a specific size is no real problem.

One descrepancy I've found, however, is that "shadows" are attached alogrythymically different than other objects. When I slice an area that includes a shadow, but not its "casting object", it doesn't slice off. I am forced to redraw a polyline copy filled black and given an equal amount of transparency to look like the shadow. This could be quite disconcerting around many or complex shadowing in a sliced area.

Of course I can always export it as a graphic image, then import it back in to slice it up. But the graphic version is not editable as it would be in Xara (.xar) format.

So the other descrepancy on export size limitations is almost mute, in that I can (in most cases) slice the map to optimal sizes for VTT easily. ;)

Kepli
10-16-2007, 08:50 AM
Many thanks for the explanation and especially the link RPMiller. I knew almost all of this already, but it is tough to explain to others. Now I can just point them to the link and let them read the complete story :D

RPMiller
10-16-2007, 12:05 PM
My pleasure Kepli! It was a real stickler for me for some time as well so I can't really fault those that are going by age old assumptions. Now that the information is out there is no excuse for ignorance any longer. ;) Hopefully folks will look into it more and it will finally start to click for them like it did for me.

P.S. You have a typo at the end of your sig. Fortunately no one reads them, otherwise they would have pointed mine out a long time ago. ;)

Midgardsormr
12-23-2007, 12:12 PM
Gamerprinter, I'm working on a map that is ultimately going to be printed at poster size, around 20" X 30". I'm not sure what equipment the print shop will be using, but I was concerned about what dpi I should aim at. My understanding is that 200 - 300 is the recommended target, but I wanted to be sure, so I looked up the guidelines on your web site, where you recommend 72 dpi to users of Photoshop, et al.

So will 72 dpi provide sufficient quality for a good-looking poster sized print? I started at 300, so I can rescale easily enough.

Gamerprinter
12-23-2007, 02:00 PM
72 dpi works as long as the map file created was made at your correct dimension and scaled at 72 dpi when you export the file.

If you convert a higher or lower resolution file to 72 dpi or rescale your map to 20 x 30 when it wasn't that in the first place - then 72 dpi is not what you want - it will look crappy.

Using high resolution like 300 dpi is required if your map file has to be rescaled to fit the dimensions and resolution you ask.

In other words, if you don't have to rescale 72 dpi is fine, if you do have to rescale start with a higher resolution...

I hope that wasn't too complicated? 8)

Midgardsormr
12-23-2007, 06:03 PM
Not at all. I just wanted to be sure I wouldn't lose too much crispness in the labels at the lower resolution, and I have little experience with designing for print.

Thanks!

Redrobes
12-23-2007, 07:47 PM
Gosh, what a thread ! So much information and so much misunderstanding.

The other day on some board I saw that some chap ask for a high res board game and the map was produced here by somebody. He then said that this map was 7200 x 4800 pixels and was for a board 24x18 inches and at 600dpi. Now my math is not stellar but even I can see something is amiss there.

(Edit -- Ahh found it... http://ritz21.deviantart.com/art/Mythology-Board-Map-70277509 got the numbers a little wrong but comment still applies ! Well I guess you can print it at 600dpi but it wont be 600dpi)

One thing that has been overlooked is that most image formats embed the DPI into the file spec. A png file has its DPI written into the format as does BMP. I am fairly sure TIFF does but I dont know about GIF or JPG. Most paint packages allow you to scale by DPI as well as image size. You can scale in PSP by a different DPI and keep the image the same res with its Actual Size box.

A post earlier was suggesting that it does not make any difference with a VTT what the DPI is. I believe that *in general* this is incorrect as some of them scale the map based on its pixel res. I.e if you want something 5feet across then it had better be exactly 200 pixels if you want all the icons on top to be in scale which makes large maps very difficult in those VTTs.

Since I write ViewingDale I can add a little more info too about VTTs. For some of them - namely mine - the res does not fundamentally matter as it is a zoom browser where you can enter any image for any scale tho whether its of high enough res to make it usable is another matter. So for me 200 pixels per 5ft scale may not be enough in certain circumstances - for example if you wanted to print that map for miniatures at 1:60 real scale on a commercial print job. So I would welcome the large gigapixel images for poster sized maps - bring it on !

ViewingDale maps can be export at any DPI up to about 7200. The scale of that image is independent of the DPI but the real size, DPI and map scale are interrelated so that you can ask for an image which is 200 pixels for 5ft if you so wished.

Finally, I would suggest that if you do want to use a map in a VTT then its better not to have a grid on it as all VTTs that I know of can put one on at the selected scale. DM's also like for it not to have fixed items like monsters and stuff like that where icons will be used on top and moved dynamically. Now how dynamic something has to be before people like it is very variable.

As far as I can see, an image is just a bunch of pixels. If that image is supposed to exist in the real world at a certain size then it has a PPI / DPI value for that real world size. That value might be very different on screen or when scaled to print via a printer driver.

RobA
12-26-2007, 10:12 AM
Most paint packages allow you to scale by DPI as well as image size. You can scale in PSP by a different DPI and keep the image the same res with its Actual Size box.

Wow! I am amazed at how often this topic keeps coming up :)

Just a clarification on your point. Changing the DPI of a file SHOULD not change any of the pixels of the image. So changing the DPI should not be called "Image Scaling", and IMHO any program that puts the "change DPI" option under image scaling is doing users a huge disfavor by continuing to spread confusion. The proper place for "change DPI" should either be in the print settings, as this is the only place it has meaning....when printing.



A post earlier was suggesting that it does not make any difference with a VTT what the DPI is. I believe that *in general* this is incorrect as some of them scale the map based on its pixel res. I.e if you want something 5feet across then it had better be exactly 200 pixels if you want all the icons on top to be in scale which makes large maps very difficult in those VTTs.


The 200px/5ft scale was stated (AFAIK) by Dundjinni and has become standard for photo-realistic/artistic battlemaps (as they are intended to be printed and 200DPI gives a nice print result). But again, DPI has nothing to do with the px/ft representation of an image. Some VTT programs may use this as a default setting when using consistently sized art, but hopefully they aren't tied to it. Personally, if I was doing VTT work I'd stick with a DPI (or more correctly PPI, when referring to screens) of 75-100 which is closer to actual screen resolutions.



As far as I can see, an image is just a bunch of pixels. If that image is supposed to exist in the real world at a certain size then it has a PPI / DPI value for that real world size. That value might be very different on screen or when scaled to print via a printer driver.

DING-DING-DING We have a winner in the "most succinct explanation of why DPI/PPI is irrelevant when discussing an image (file, that is) category"!

-Rob A>

Midgardsormr
12-26-2007, 11:34 AM
Wow! I am amazed at how often this topic keeps coming up :)


My fault. I bumped it with my question to GP.

I thought it might be helpful to see the Resample dialogue from CorelPaint and look at how changes in DPI settings affect both the print size and the resolution of an image when working in that package.

The left column of the first image is how the dialogue looks when examining the image in inches. The right column is how it looks when looking at the pixels. I started with my current WIP, which is ~26 X 20 inches at 300 dpi. As you can see, that makes the image 7629 X 5894 px.

When I changed the dpi to 75 but kept the size in inches did not change. When printed, this map will still be 26 X 20. However, the total number of pixels in the image has changed to 1907 X 1474, which will make it far smaller on the computer screen--only 25% of its original size in each dimension.

If you check "Maintain original size" in CorelPaint then change the DPI, the pixels remain constant, but the print size changes. I attached a second image to show the effects of checking that box. Note the new print size is 100 X 78 inches, but the size in pixels is still 7600 X 5800 (and is greyed out--it cannot be changed in this mode).

In Corel, the DPI is set as part of defining the image--You can change any two: DPI, size in pixels, and print size. It's a bit like an electrical calculation with voltage, current, and resistance. You can change two, and those two combined determine the third. If you're designing for the screen, only pixels matter, though, so you can ignore the other two. If you think there's a chance that someone might one day print the map, though, it's probably worth at least glancing at them.

RobA
12-26-2007, 12:48 PM
And I still think that CorelPaint (any many other programs) do a great disservice to their users by doing it that way. All it does is lead to confusion...

-Rob A>

Carnifex
02-26-2008, 12:25 PM
The proper place for "change DPI" should either be in the print settings, as this is the only place it has meaning....when printing.



Not entirely correct. Every professional graphic designer sets the ppi and printing size in Photoshop so that the image shows up correctly in Indesign/Quark.

Carnifex
02-26-2008, 03:05 PM
72 dpi works as long as the map file created was made at your correct dimension and scaled at 72 dpi when you export the file.

If you convert a higher or lower resolution file to 72 dpi or rescale your map to 20 x 30 when it wasn't that in the first place - then 72 dpi is not what you want - it will look crappy.

Using high resolution like 300 dpi is required if your map file has to be rescaled to fit the dimensions and resolution you ask.

In other words, if you don't have to rescale 72 dpi is fine, if you do have to rescale start with a higher resolution...

I hope that wasn't too complicated? 8)
I really recommend at least 150 dpi for printing and 300 if you want the best quality. It all depends on the quality of the print. In offset printing you should know the resolution of the raster (lpi (lines per inch)) - and you should have lpi x 2 in dpi.

Also scaling down is usually not a problem (say from 600 to 150 dpi).



Here's a little mini-guide to scaling things in Photoshop:
If you want to change only the dpi (to 200 dpi) but keep the pixel size:
Image > Image Size (or alt+ctrl+I)
Make sure "Scale styles" and "constrain proportions" are checked. "Resample image" should NOT be checked.
Type "200" in the resolution box and click OK.

If you want to scale the image and increase both pixel size and resolution (to 200 dpi):
Image > Image Size (or alt+ctrl+I)
Make sure "Scale styles", "constrain proportions" AND "Resample image" are checked.
Type "200" in the resolution box and click OK.

Does it make sense?

Be careful when you increase the number of pixels of a picture - you'll always loose quality.

alucard339
02-26-2008, 05:36 PM
Thanks for the mini guide, I always find it confusing to work it out.

Like my original wold map: its in 150 DPI, but at a size so huge (12000 by 8000 for 70mb) that its not fun to work with for small change.

ciao,
Alu.

RobA
02-27-2008, 12:19 AM
Not entirely correct. Every professional graphic designer sets the ppi and printing size in Photoshop so that the image shows up correctly in Indesign/Quark.

We can agree to disagree then. You are accepting that what is done by a particular software package or combination of packages is "correct". While this may be what is necessary to get it to work properly, it does not make it "right".

(And the fact that you even need to provide a mini-tutorial just reinforces my point that the applications have confused the issue to the point of being incomprehensible to most users. :P )

-Rob A>

Carnifex
02-27-2008, 09:24 AM
We can agree to disagree then. You are accepting that what is done by a particular software package or combination of packages is "correct". While this may be what is necessary to get it to work properly, it does not make it "right".

(And the fact that you even need to provide a mini-tutorial just reinforces my point that the applications have confused the issue to the point of being incomprehensible to most users. :P )

-Rob A>
Not quite :)

Well, the way I see it, the way PS does it is the only way to do it when you work professionally with layout. This is the way it have worked since the dawn of DTP. If you have any other suggestion to make it work you're welcome.

To import the image to InDesign/Quark/Pagemaker in sceen resolution would be a pain (this is possible nowadays if you want) and then you'd have to check the DPI of every image in the layout program every time you want to resize it. That would be impossible. Some programs automatically resize the images to a certain dpi - but that is not optimal imo when you want maximum control and that would complicate certain other things as well (b/w images for example where you want 1200 ppi for printing).

(When you make a PDF for offset print you can also set the DPI to limit the size in case you used too large images.)

(PS is not just any application - it's industry standard. But I agree that they could make the image size much more clear.)

I also agree with you that dpi/ppi is completely useless until you want to print something - the only thing that matters until then is the number of pixels.

Carnifex
02-27-2008, 10:26 AM
Here is just one of many links that you can find on the subject, but I picked this one specifically because it has print shop links as well. [U]Please understand... (CUT) ...misleading.

http://www.rideau-info.com/photos/mythdpi.html



I really must say that while the most information on the linked page is correct he doesn't get it all correct.

Here's some links to Wikipedia that's relevant:
http://en.wikipedia.org/wiki/Dots_per_inch
http://en.wikipedia.org/wiki/Lines_per_inch
http://en.wikipedia.org/wiki/Halftone

And here's an alternate way to describe dpi, ppi, and lpi. IMHO a more accurate than the photo guide above:
http://desktoppub.about.com/cs/intermediate/a/meas_resolution.htm

The thing is that higher resolution does not always increase the quality of the printed image and in order to keep the image size down (in mb) it's better with as low dpi as possible but still high enough (about two times the lpi).

RPMiller
05-12-2008, 03:56 PM
Bumping because it seems to be a hot topic due to the May challenge. I've also made it sticky so that it will be easier to find in the future.

RPMiller
05-12-2008, 04:06 PM
And here's an alternate way to describe dpi, ppi, and lpi. IMHO a more accurate than the photo guide above:
http://desktoppub.about.com/cs/intermediate/a/meas_resolution.htm
You must have posted this after I had to take my sabbatical. Sorry I missed it on my return.

That last link you posted is an excellent one! That should hopefully solidify everything else that was said in this thread. Repped for the link!

RobA
08-16-2010, 12:19 PM
A bit of thread necromancy.

http://www.webdesignerdepot.com/2010/02/the-myth-of-dpi/

Is another discussion on this which reiterates my own opinion that DPI only matters when printing and should be abolished otherwise ;)

-Rob A>