Question

I apologize upfront for the long question, with multiple subquestions, but the question is really as stated in the title. All that follows is a detailed breakup of different aspects of the question.

In my universal ios game built using cocos2d, I have four categories of images – I want to determine which of them should go into spritesheets and which are better off loaded as individual images. My current guess is that only character animations that run throughout the game provide value being loaded in memory as spritesheets:

1. Character animations that run throughout the game play (except
   when user is in menus): for these, I assume that having the images
   in a spritesheet will reduce runtime memory usage (due to the
   padding of individual files to power of two byte boundaries), hence
   they are candidates for a spritesheet. Is that correct?
2. Small images (about 200 of them) of which 0 to 4 are displayed at any
   time, picked at random, during game play. For these, I am not sure
   if it is worth having all 200 images loaded in memory when only at
   most any 4 are used at a time. So it may be better to directly
   access them as images. Is that correct?
3. A few (about 20) small menu elements like buttons that are used only in static menus: since the menu items are used only during menu display, I assume they are not of much value in improving memory access via spritesheets. Is that correct?
4. A few large images that are used as backgrounds for the menus, the game play scene, etc. Most of these images are as large as the screen resolution. Since the screen resolution is roughly equal to the maximum size of a single image (for example, for ipad retina, 4096 x 4096 image size versus screen size of 2048 x 1536), there isn’t much gain in using spritesheets as at most 1 or 2 large images would fit in one spritesheet. Also, since each of these large files is used only in one scene, loading all these large images as spritesheets in memory upfront seems like an unnecessary overhead. Hence, directly access them as spritesheets. Is that correct?

A couple of common related questions:

a) Packing images used across different scenes into the same spritesheet makes us load them into memory even when only a subset of the images is used in that scene. I assume that is a bad idea. Correct?

b) There is a stutter in the game play only on older devices (iPad 1 and iPhone 3gs). Will spritesheets help reduce such stutter?

c) I assume that spritesheets are only going to benefit runtime memory footprint, and not so much the on disk size of the app archive. I noticed, for instance, that a set of files which are 11.8 MB in size, when put in a spritesheet are 11 Mb – not much of a compression benefit. Is that a valid assumption?

Please let me know your thoughts on my rationale above.

Thanks

Anand

Answer 1

Rule of thumb: everything goes in a spritesheet (texture atlas), unless there’s a good reason not to.

1. Definitely texture atlas.

2. Cocos2d’s caching mechanism will cause all individual images to be cached and remain in memory, so eventually they’ll use more than if they were in a texture atlas. I would only use single images if they’re rarely needed (ie menu icons) and I would also remove them from CCTextureCache just after they’ve been added to the scene, to make sure they will be removed from memory when leaving that menu screen.

3. Assumption may be wrong. A 320×480 image uses 512×512 as a texture in memory. If you have multiple of them, you may be better off having them all in a single texture atlas. Unless you enabled NPOT support in cocos2d. Again, don’t forget that CCTextureCache is caching the textures.

4. Keep in mind that large textures benefit a lot from spritebatching. So even if there may only be 2-3 images in the texture atlas, it can make a difference in performance.

a) It really depends. If your app is running low on memory, you might want to have separate texture atlases. Otherwise it would be wasteful not to pack the images into a single texture atlas (if possible).

b) It depends on what’s causing the stutter. Most common issue: loading resources (ie images) during gameplay, or creating and removing many nodes every frame instead of re-using existing ones.

c) Probably. It may depend on the texture atlas tool (I recommend TexturePacker). It definitely depends on the file format. If you can pack multiple PNG into a single .pvr.ccz texture atlas, you’ll save a lot of memory (and gain loading speed).

For more info refer to my cocos2d memory optimization blog post.