Fix texture atlas generation when source sprite is larger than generated atlas

[Giwayume]

Fixes #41414.

The problem: atlas images getting cut off.

https://github.com/godotengine/godot/blob/3.4/editor/import/resource_importer_texture_atlas.cpp#L134

In this line, "xf" starts at the left margin in the original sprite frame (source) image. Which in my case is a 512x512 image with the eyeball in the middle and a lot of empty space around it. This 512x512 dimension is set to the src_width and src_height variables respectively.

The variables width and height in this function refer to the final atlas texture's width and height, which is 256x252 in this case. So the entire generated texture atlas (256x252) is smaller than a single source animation frame (512x512). That exposes this bug.

It looks like in this loop it's limiting by the generated atlas's width when it's actually looping through the source image's pixels.

for (int xi = (xf > 0 ? int(xf) : 0); xi < (xt < width ? xt : width - 1); xi++) {

So instead it should limit by the source image's width.

for (int xi = (xf > 0 ? int(xf) : 0); xi < (xt < src_width ? xt : src_width - 1); xi++) {

This indeed produces the correct results. And it makes sense why the images were getting cut off before.

There's already code that guarantees you're not drawing outside of the destination atlas texture's dimensions.

https://github.com/godotengine/godot/blob/3.4/editor/import/resource_importer_texture_atlas.cpp#L147

This width/src_width variable swap seems to have been a type-o. I don't think the same coding problem is present on the y axis, because my sprites would have been clipped off vertically as well.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[boruok]

I met some graphical issue: region is fine, but it cutout right size of texture.
here example, right red border doesn't render

[Giwayume]

@boruok Can you send the source images?

[boruok]

@Giwayume sure
New Game Project.zip

[Giwayume]

Uh, I'm not sure I understand. is fence_grass.png supposed to be a generated texture atlas? That's the only image I see. What about the images you generated it from?

[boruok]

@Giwayume if you generate atlas from given texture you can see that right red border doesn't render.

[Giwayume]

Interesting edge case.

[Giwayume]

The atlas generation algorithm even in 3.4.stable is adding unnecessary extra space in the atlas in this case. There's more than 1 pre-existing problem here.

EDIT:

Eh... now that I think about it, it's rounding up to the nearest power of 2. Probably designed to do that for memory management.

[boruok]

@Giwayume im pretty sure that is src_width - 1

for (int xi = (xf > 0 ? int(xf) : 0); xi < (xt < src_width ? xt : src_width - 1); xi++) {
...
for (int xi = (xf < src_width ? int(xf) : src_width - 1); xi >= (xt > 0 ? xt : 0); xi--) {
...

[Giwayume]

The original is (atlas_)width - 1, curious if this problem occurs on a 256px wide image in 3.4.stable.

Actually, it adds 4px padding to everything so probably can't reproduce the problem in stable due to that. Bug saved by padding.

It's gonna take me an hour to rebuild Godot after switching branches to find out either way. =(