Overlapping Shader Effects in Opengl ES 2.0

Question

Is it possible to overlap shader effects in OpenGL ES 2.0? (not using FBOs)

How to use the result of a shader with another shader without having to do a glReadPixels and push again the processed pixels?

The next pseudo-code is what I'm trying to achieve:

// Push RGBA pixels into the GPU
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, Pixels_To_Render);

// Apply first shader effect
glUseProgram( FIRST_SHADER_HANDLE);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

// Apply the second shader effect sampling from the result of the first shader effect
glUseProgram( SECOND_SHADER_HANDLE );
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

// Get the overall result
glReadPixels(......)

Answer 1

I presume you're talking about pixel processing with fragment shaders?

With the OpenGL ES 2.0 core API, you can't get pixels from the destination framebuffer into the fragment shader without reading them back from the GPU.

But if you're on a device/platform that supports a shader framebuffer fetch extension (EXT_shader_framebuffer_fetch on at least iOS, NV_shader_framebuffer_fetch in some other places), you're in luck. With that extension, a fragment shader can read the fragment data from the destination framebuffer for the fragment it's rendering to (and only that fragment). This is great for programmable blending or pixel post-processing effects because you don't have to incur the performance penalty of a glReadPixels operation.

Declare that you're using the extension with #extension GL_EXT_shader_framebuffer_fetch : require, then read fragment data from the gl_LastFragData[0] builtin. (The subscript is for the rendering target index, but you don't have multiple render targets unless you're using OpenGL ES 3.0, so it's always zero.) Process it however you like and write to gl_FragColor or gl_FragData as usual.