OpenGL ES. Scrolling 3 layer starfield textures gets me from 60 -> 40 FPS

Question

I need to draw the background for a 2D space scrolling shooter. I need to implement 3 layers of stars: one distant nebula (moving really slow) in the background, one layer of far away stars (moving slow) and one layer of close stars (moving normal) on top of the other two.

The way i first tried this was using 3 textures of 320 x 480 that were transparent pngs of the stars. I used GL_BLEND and SRC_ALPHA, ONE_MINUS_SRC_ALPHA.

The results were not great even on the 3GS. On the first generation devices the FPS dropped to 40..50 so i think i'm doing this the wrong way.

When i disable the GL_BLEND everything works great even on the 1st gen devices and the FPS is back to 60 again... so it's must be the fact that i'm trying to belnd large transparent textures.

The problem is i don't know how to do it some other way...

Should i draw only the first nebula like an opaque texture and then try to emulate the middle and top star layer with small points moving around the screen?

Is there any other approach on the blending issue? How can i speed up the rendering process? Is one big texture (tileset) the answer?

Please help me cuz i'm stuck here and i can't get out.

Answer 1

You might want to use multi-texturing to see if that speeds it up. Each multi-texture stage can be assigned a unique transformation matrix, so you should be able to translate each layer at different speeds.

I believe all iPhone models support two texture stages, so you should be able to combine two of your layers into a single draw call. You might still need to resort to blending for the third layer.

Also note that alpha testing could be faster than alpha blending.

Good luck!

Answer 2

I don't know how you want your stars to look like, but you might want to try to move them from a texture to geometry by using GL_POINTS in the DrawElements or DrawArrays maybe just replace the top two layers with layers of geometry. You can manipulate the points using PointSize, PointSizePointerOES and PointParameter to modify the rendering of the points.

Answer 3

The back nebula should definitely be opaque; everything else is getting drawn on top of it, and I assume the only thing behind it is black. Also, prideout has a point: assuming your star layers can have effectively 1-bit alpha, that's definitely something you can try. Failing that, the GL_POINTS technique Harald mentions would work as well.