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Thread: Dpi, scanning and image sizes doubts.

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      Beoner is offline
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    Default Dpi, scanning and image sizes doubts.

    Hey,

    I know there are some other questions about the same topic on the forum, but I have some other questions I coulnd't find on other posts.

    In the future (and I hope it is soon), when I have some better mapping skills, I'll want to start taking some of the requests on the Mapmaking Request forum, specially aiming Paid request - again, considering I'll have enough skill for this mission -, and by what I have read on most requests people usually ask for a size so that they can print it on a A3 paper, or even on a Poster, and thats where I need some help of you.

    When making maps for printing purposes it is needed 300 dpi or more right? What about the pixel size? What would be the better size(px)?
    Also, lets say I draw a map by hand on a A4 paper, when I "photoscan" it my camera takes the picture with 72 dpi, editing the number by PS, from 72 to 300 dpi, brings any issues to the image(except for the size grow)? I mean, does the image loses any quality?
    Also, a scanned A4 paper draw would look pixelated if printed on a A3 paper?

    That is it for now. I'm sorry for the great amount of questions, and if any question got a little confused,

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      Hai-Etlik is offline
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    This is one of those things that is very, very, very simple, but which a lot of people have trouble understanding because they keep trying to overthink it.

    There are three properties at issue.

    Logical Size is the amount of information. This is the number of pixels.
    Physical Size is the size of a physical image, be it a printed image, or one displayed on your screen.
    Resolution is a ratio between these two. In the world of graphics, it's usually measured in dots/pixels per inch, or per centimetre. (DPI, PPI, px/cm, etc) "pixel" and "dot" are pretty much interchangeable here. Resolution is a property of the process of converting between image data, and a physical image.

    Resolution = Logical Size / Physical Size

    If you get that and understand all the implications, that's all you need to know. Most of the rest of this post is going to be trying different ways of explaining that one very simple little equation.

    You can adjust the number of pixels. This is a process called "resampling" and produces artefacts (errors). Increasing the pixel count (Upsampling) also produces problems as a result of having to guess at what was "in between". In most cases the artefacts of take the form of "blurriness". There are many different algorithms for resampling (shifting from one grid of pixels to another) and interpolation (guessing what should go between pixels) and they all have different strength and weaknesses. Downsampling is usually OK if you do it by a large factor: reducing by 10 (10% as many pixels on a side) should look fine, reducing by 0.1 (90% as many pixels on a side) will not look good.

    So, for a file, focus on the logical size. Changing that means resampling, which is generally a bad thing, (except large downsampling).

    Physical size is hopefully fairly simple. Take out a ruler or measuring tape and measure. That's the physical size.

    Resolution is, as I said, a property of the process of converting between physical and logical.

    Many image files have a little tag attached to them that indicates a resolution. This is used to either indicate what resolution the image was scanned at, or to suggest what resolution/size it should be displayed/printed at. This may be wrong, ignored, or just irrelevant. It's really not that important. If you don't have a use for it, it's best to leave this blank.

    Generally, when you display an image on a computer monitor, it is displayed at the resolution of the monitor (A typical modern desktop monitor is about 100 DPI). A lot of people will go on and on about the magical significance of 72 DPI with regard to monitor resolution and particularly Macs. This is nonsense. For images where printing/scanning isn't important, you should generally set the resolution blank to indicate that it's not important. Failing that, use the resolution of the monitor you created it on (probably approximately 100 DPI)

    Some software knows to look at the DPI tag, and will resample the image to appear at the correct size on the screen (assuming the OS reports the screen resolution to the software correctly) As there is resampling going on, this can look a bit blurry, unless the resolution specified in the image is much higher than that of the screen (in which case you get downsampling by a large factor) In GIMP for instance you can turn off "Dot For Dot" mode in the "View" menu to get this behaviour. This is only really all that important if you plan on printing the image and want to see how big it will be.

    Resolution can also be thought of as the physical density of information. Which is one factor impacting quality. Denser information allows more detail, and more precision. But remember that the resolution tag on the image is just a hint as to how big to print the image. Increasing the resolution just means the image will be printed smaller. That certainly means it will be more dense, but it's probably not what you are after. You can compensate by upsampling, but this is couterproductive as you are still just stretching the same information out over the same area. The interpolation will "cancel out" the higher resolution, and you will be at best, no better off, and may well have a worse image.

    If you scanned an A4 sheet, and printed at A3, you would have the same number of pixels, but twice the physical size, and so half the resolution. If you scanned at 600 DPI, you would be able to print at 300 DPI which is probably good enough. If you scanned at 150 DPI, you would print at 75 DPI, which would look fairly obvious unless viewed from a distance.
    ravells, Counlin and Jaxilon like this.

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    As a digital printer (I run Gamer Printshop), printed maps are generally 300 dpi/ppi, however 200 dpi can print well too, the problem only exists for small font labels, the smaller the font the higher the resolution required to minimize pixelization. Most maps for publishers are usually 300 ppi but only to fit within a letter size printed book. However, those looking to print large format maps will require higher resolutions.

    I design using vector software which allows me to create one map, usually designed in sizes from 18" x 24" up to my current map I'm working on which measures 48" x 72". From there I can export at full size 300 dpi, or any resolution down to VT resolution like 50 or 100 ppi. In raster applications like Photoshop or GIMP this is problematic in that the size and resolution have to be determined before you create the map. When dealing with many layers it gets memory intensive, so you might need have more RAM needs and more.
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    Not looking to derail this thread, but I've been looking for a site that has all the sheet sizes for the various formats listed, thought this was following along with this thread nicely. My google fu has only shown me bits and pieces and I'd like a reference I can either print out or place in excel, anyone have a good link? Thanks

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    Honestly, why the need for all the preset sizes - since I use roll-fed stock, up to 42" wide, I can print to any size within my maximum. There is no inherent need to make a map match a specific preset size. Now if you're using a printer that is more limited - print up to it's maximum size. I'm a printer and I don't worry about all the specific sizes...
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    Just as an idea: Wouldn't it be easier if someone just created a thread explaining all this? I mean, I've seen at least dozen people asking about this sort of information, and this way, if a post like that weas on the tutorials area, or even here on general discussion (and locked, so that it would be even easir to find) could be very useful. Any other question that nvolves somehow would be asked on the very same topic.

    It is just a suggestion though.
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    Quote Originally Posted by Counlin View Post
    Just as an idea: Wouldn't it be easier if someone just created a thread explaining all this? I mean, I've seen at least dozen people asking about this sort of information, and this way, if a post like that weas on the tutorials area, or even here on general discussion (and locked, so that it would be even easir to find) could be very useful. Any other question that nvolves somehow would be asked on the very same topic.

    It is just a suggestion though.
    See:

    DPI vs. PPI - What's the difference?

    -RobA >

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    Quote Originally Posted by RobA View Post
    Although I'm not the first to ask this question, you'll see that the quoted thread above is something I started - back in October 2007. It used to be a pinned thread, I haven't looked to see if it still is, though. Read the whole thread, most of these questions are answered there. The map being discussed in that thread was my October 2007 Challenge Map as it might be used for a VTT application.
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      Beoner is offline
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    I'm reading both threads right now, but I may need some more time to finish it, and get everything right.

    I appreciate the help so far, anyone that has anything else to say is welcome to share the knowledge

    Thank you

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