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opengldrawingtextures

Reducing opengl draw calls vs binding smaller textures

I'm making an isometric (2D) game with SFML. I handle the drawing order (depth) by sorting all drawables by their Y position and it works perfectly well.

The game uses an enourmous amount of art assets, such that the npcs, monsters and player graphics alone are contained in their own 4k texture atlas. It is logistically not possible for me to put everything into one atlas. The target devices would not be able to handle textures of that size. Please do not focus on WHY it's impossible, and understand that I simply MUST use seperate files for my textures in this case.

This causes a problem. Let's say I have a level with 2 npcs and 2 pillars. The npcs are in NPCs.png and the pillars are in CastleLevel.png. Depending on where the npcs move, the drawing order (hence the opengly texture binding order) can be different. Let's say the Y positions are sorted like this:

npc1, pillar1, npc2, pillar 2

This would mean that opengl has to switch between the 2 textures twice. My question is, should I:

a) keep the texture atlasses OR

b) divide them all into smaller png files (1 png per npc, 1 png per pillar etc). Since the textures must be changed multiple times anyway, would it improve performance if opengl had to bind smaller textures instead?

Is it worth keeping the texture atlasses because it will SOMETIMES reduce the number of draw calls?

Solution

  • Since the textures must be changed multiple times anyway, would it improve performance if opengl had to bind smaller textures instead?

    Almost certainly not. The cost of a texture bind is fixed; it isn't based on the texture's size.

    It would be better for you to either:

    1. Properly batch your rendering. That is, when you say "draw NPC1", you don't actually draw it yet. You stick some data in an array, and later on, you execute "draw NPCs", which draws all of the NPCs you've buffered in one go.

    2. Use a bigger texture atlas, probably involving array textures. Each layer of the array texture would be one of the atlases you load. This way, you only ever bind one texture to render your scene.

    3. Deal with it. 2D games aren't exactly stressful on the GPU or CPU. The overhead from the additional state changes will not be what knocks you down from 60FPS to 30FPS.