OpenGL, alpha blending with packed buffer and multiple textures

Question

I'm drawing a fairly simple 2D scene containing only rectangles. I have one FloatBuffer into which I put X, Y, Z, R, G, B, A, U, and V data for each vertex. I draw using glDrawArrays and GL_TRIANGLE_STRIP, keeping the rectangles separate with degenerate vertices.

To facilitate the use of multiple textures, I keep separate float arrays for each texture's draw calls. The texture is binded, the float array is put into the FloatBuffer, and I draw. Then the next texture is then binded and this continues until I have drawn all of my textures for this render.

I use an Orthographic projection so I can use the Z coordinates and GL_DEPTH_TEST for setting depth independently of the draw order.

To use alpha blending, every piece of advice on the internet seems to say:

GLES20.glEnable(GLES20.GL_BLEND);
GLES20.glBlendFunc(GLES20.GL_SRC_ALPHA, GLES20.GL_ONE_MINUS_SRC_ALPHA);

This works fine per each texture's "draw", because I have the draw calls sorted in the buffer from back to front before drawing. I have no way to correctly draw texture2 under a partially transparent texture1 because of the depth test and texture1 being drawn before texture2. texture1 chops off the overlapping part of texture2 because the depth test says that texture1 is in front of texture2.

The only ways I see around this are 1) only using 1 texture in the whole program, and 2) not using transparent textures. Neither of these are acceptable options.

Basically, I need a way to have alpha blending without needing to sort back-to-front. Is this possible?

Answer 1

It sounds like you might need to do Depth Peeling. Here's a PDF that shows how to do it.