Combine multiple opengl fragment shaders

Question

I'm wanting to port some image processing work to OpenGL for performance using OpenGL ES. I've got a very simple thresholding algorithm in place but I'd like to combine additional filters to the image (such as contrast).

My first thought would be to complete this by using multiple fragment shaders. However, I'd like to do this pretty quickly so would this cause a lot of state change? The only method I've read about is to do this by working on a texture and then calling 'use program' multiple times.

Is there a more efficient way to do this? Ideally, I'd like to perform a contrast stretch and a histogram balance as part of the steps.

Unless I can combine this into a single shader would an FBO work for me here?

I'm a bit new to OpenGL (in case you couldn't tell).

Thanks!

Simon

Answer 1

You can't "merge" fragment shader unless you do it manually, so the only sane choice is to do "ping-pong" rendering using FBOs. You have 2 FBOs, draw to one and read from another, then switch FBOs and repeat, switching fragment shaders between rendering.

Answer 2

Ping pong rendering is quite nice for this purpose, actually! Here's a code sample that works within LibGDX, where there is a 'batch' object that actually draws to the screen, the input to which we can capture with FrameBuffers:

FrameBuffer ping = fbo; // the framebuffer containing your rendered texture
for (ShaderProgram shader : shaders) {
    pong.clear();
    pong.begin();
    batch.begin();
    batch.setShader(shader);
    batch.draw(ping.getColorBufferTexture(), 0, 0, width, height);
    batch.end();
    pong.end();
    ping = pong;
}
batch.begin();
batch.draw(pong.getColorBufferTexture(), 0, 0, width, height);
batch.end();

The clear call to pong (in my case, making a new framebuffer) is expensive, so preallocating might offer some speedups. This is more of a followup to Matias' answer than an answer in itself, but wouldn't fit in a comment.