Question

I am learning to use shaders in OpenGL ES.

As an example: Here’s my playground fragment shader which takes the current video frame and makes it grayscale:

varying highp vec2 textureCoordinate;

uniform sampler2D videoFrame;

void main() {
    highp vec4 theColor = texture2D(videoFrame, textureCoordinate);
    highp float avrg = (theColor[0] + theColor[1] + theColor[2]) / 3.0;
    theColor[0] = avrg; // r
    theColor[1] = avrg; // g
    theColor[2] = avrg; // b
    gl_FragColor = theColor;
}

theColor represents the current pixel. It would be cool to also get access to the previous pixel at this same coordinate.

For sake of curiousity, I would like to add or multiply the color of the current pixel to the color of the pixel in the previous render frame.

How could I keep the previous pixels around and pass them in to my fragment shader in order to do something with them?

Note: It’s OpenGL ES 2.0 on the iPhone.

Answer 1

You need to render the previous frame to a texture, using a Framebuffer Object (FBO), then you can read this texture in your fragment shader.