I'm writing an OpenGL ES (2.0) app and have a custom Quad class which draws a textured quad for my sprites.
Currently, I put the textures of any sprites of the same size into a single file or atlas, then, when drawing, I can simply specify the frame I want. It will the render the correct frame based on how many textures in the atlas and how many total frames etc. Something like so:
mySprite.x = 0; mySprite.y = 0; mySprite frame = 1; drawSprite(mySprite);
The texture Atlas would look something like this: (Simplified for the purpose of this question, in the real project, there are over 200 textures arranged in neat rows/columns, but the important thing is there is order, every texture is the same size as this is necessary for the calculations).
So, in the example above, I can say, there are 2 columns, 1 row and a total of 2 textures.
So, if I specify 1 as my texture (2nd texture), my sprite class can work out (using the columns, rows, total textures etc..) which texture I want and how to get it. As I said this is simplified, the calculations are a little but more involved when there are multiple rows. But it makes doing animations nice and simple as I can simply increment or decrement the current frame.
However, I keep seeing sprite sheets where there is no order. There are different sized textures seemingly thrown into an atlas like so:
I really would like to go down this route as it means I wouldn't have to separate my sprite sheets by object size, and therefore, vastly reduce the number of separate sheets I have to use.
I can't however work out how one would write a calculation to get the correct texture just by specifying the texture number to render.
So how does this work? Do I need to manually work out the texel coordinates of each and every object (and indeed frame), manually (and maybe store this value in an array and associate it to a frame number?) or am I missing a trick here somewhere?