Photoshop need help with map area measuring.

[feanaaro]

I would not think so, it's actually all white shapes, though maybe with antialiasing... However, since I am subtracting a magic-wand selection with 0 tolerance, any pixel which is even 0.1% opaque should remain in the resulting selection (it's the buildings layer from my city map, btw). That's why I don't get it.


Edit: actually it must be antialiasing, since the discrepancy disappear by de-selecting the option in the magic wand tool. Sorry for not having noticed that myself.

[Azélor]

I would not think so, it's actually all white shapes, though maybe with antialiasing... However, since I am subtracting a magic-wand selection with 0 tolerance, any pixel which is even 0.1% opaque should remain in the resulting selection (it's the buildings layer from my city map, btw). That's why I don't get it.


Edit: actually it must be antialiasing, since the discrepancy disappear by de-selecting the option in the magic wand tool. Sorry for not having noticed that myself.

that's not really how it works. Based on observation, opacity and colors are two very different things and magic wands only recolonize colors not transparency. So if you have a shape with a very blurry and faint surrounding, there is a good chance it get caught with the magic wand too.

If you want to select a specific area that have transparency, I think you might have to use the alpha layers...

[Azélor]

For example, using a shape with 2 different opacities.

Add a layer mask for the shape
Select all the pixels of the shape, copy
go in the alpha layer channel and create a new alpha layer, paste

to get the amount of pixels, just use the magic wand as before (on the alpha layer with all other layers invisible) . The trick is that transparent pixels become opaque so only then you can select them.