Sprites in game programming, multiple files vs one "texture" ?

Question

Pardon me if my lingo is not correct as I'm new to game programming. I've been looking at some open source projects and noticed that some sprites are split up into several files, all of which are grouped together to make a 2d object look like it's animating. That's straight forward. Then I'll see a different approach, with the 2d object all in one png file or something similar, all next to each other.

Is there an advantage of using one approach to another? Should sprites be in separate files? Why are they sometimes all on one sheet?

Answer 1

The former approach is typically more straightforward and easy to program, so you see a lot of it in open source projects.

The second approach is more efficient on modern graphics hardware, because it allows you to draw multiple different sprites from one large texture by specifying different u,v coordinates to select each individual sprite from the composite sheet. Because u,v coordinates can be streamed along with vertex data to a shader, this allows you to draw a large group of sprites much more efficiently than you could if you had to switch textures (which means changing shader state) for each poly. That means you can draw more sprites per millisecond, and thus get more on screen.

Answer 2

Every time you switch your currently bound texture you incur a penalty (sometimes a very big one if the system runs out of memory and starts paging textures in and out). So the more things you can draw with one texture the better. Going to extremes, if you never switched texture bindings, you'd incur 0 penalty.

On the other hand, video cards limit the maximum size of a texture, so you can only group smaller textures into a big one so much. The older the card the smaller the texture size you can use. So if you want to make your game work on a large variety of cards, you have to limit your textures to a more normal size (or have different sets of textures for different cards).

Another problem is that sometimes the stuff in your virtual world just doesn't pertain itself to being grouped like this. While you can have a big texture with every little decoration for your UI (window frames, buttons, etc), you're gonna have a harder time to use a single texture for different enemies because they might not even appear on the screen at the same time, or you might be unable to draw them one after the other because of the back-to-front drawing scheme necessary for transparency.

Answer 3

Not so long ago one reason to use packed sprites instead of seperate ones was that graphics hardware was limited to power-of-two textures (256, 512, 1024, ...). So you would waste a good amount of memory by not packing the sprites as you would have to enlarge everything to power-of-two dimensions before you could upload it. Packing multiple sprites into a single texture worked around that.

Another reason is that its much quicker to load one big image file from the HD then it is to load hundreds of small ones. This is still the case as file access comes with quite a large overhead per file, so the less files you have the faster things become. And especially with small sprites you can easily turn hundred files into a single one, so the saving can be quite noticable.

There are however also reason against having everything in one texture. For one OpenGL is no longer limited to power-of-two textures, so any size will work. But more importantly, packing everything in one texture has negative side effects. When you for example have lots of scaling in a game you have to be careful about the borders of your sprites, as colors will blend into neighboring sprites giving you ugly artifacts. You can avoid that to a certain degree by adding extra space around your sprites, but its not a perfect solution. Having everything in one texture also limits what you can do with the image. For certain effects, such as a waterfall for example, you might want to do the animation by simply offsetting the UV coordinates of the texture, you can't do that so easily when everything is packed into a single texture.