hi i would like to know if we are limite in numbers od decals, or we can add infinite decals, on walls, ground etc
are decals heavy in memory?

thanks

depends on the image size but yes of course they use memory - the larger the image the more memory it needs, same as if your game was 2D.
you just save on the fact it doesn't also need to load an object so there's no polygon's to use up more memory.

yes i know its no polys, but how much decals can i add in a level, 200, 2000, 10000???
it is a limitin GG?

I believe they still have 4 vertices.

Texture wise they use commonly use:

512x512 (standard)
1024x1024 (larger file size - better detail and more sprite animation options)

So it really depends what the size of your texture is. They do use memory but in general much less than a full volumetric object. Also you should consider what shader your using. As that effect also uses memory. Either way its all much less than a complete 3D model. Are you considering making something like Daggerfall? The enemies are just decals

i was thinking to use 512x512 textures for creating tons of garbage, dirt, papers etc on grounds, graph on walls etc

Oh yes that's perfect for detail and optimizing on memory. Games like Fallout 3 made good use of clutter decal.

whats the maximum quantity of decals we can have in a level?

How long is a piece of string?

You can have as many as you want, there's no limit except what your memory can handle, we can't give you a number because every decal is different, it depends on your texture sizes, how much detail is on the texture, and how many poly's ate in your decal model.

Everything uses memory. Maximum amount of decals will be based on how much memory each image takes up, plus a very tiny amount for the plane. 1024 sized textures generally take about 1mb each for example (assuming a normal colour range. Less colours, less memory). So at that size or lower you will fit a lot of images into your game. A 2K texture is normally around 16MB however so 16 times as much memory as the 1024 image. So as you can see, there is no set limit as such, just the 4 Gig of memory allocated to GG. I am unsure if GG still takes up a fair chunk of that 4 gig now or if Lee managed to alter that with some clever coding.

I could be wrong, but another way to save up on memory is by reusing many of the same decals on your map. I believe GG supports instances of objects. So I think using a small amount of variations is better than having like 20 different kinds of debris decals. Even if they total out to being the same amount of entities on your map.

Yeah decals are definitely the least memory intensive ways to fill your map out. To make them really efficient, lower the colour palette. Old School, makes no difference performance-wise these days unfortunately ( remember using well dithered 256 colour textures and less for speed back in the DB days), but will lower the file size considerably. A paint package these days will do a great job and you will hardly notice a difference.