GLbyte Data in Strange Format -- NPR Technique

Question

I'm working on an edge detection algorithm for a NPR technique. I plan on just using difference of gaussians to find the edges.

I thought that I would take a copy of the current screen, then analyze and recolor the pixels so that I have a map to draw the edges with.

This is my screen copy logic so far:

int width = rd->width();
int height = rd->height();
GLbyte * data = (GLbyte *)malloc( width * height * 3 );
if( data ) {
    glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, data);
}
    float color = 0;
for (int i = 0; i < width; i++)
{
    for (int j = 0; j < height; j++)
    {
        color = data[i*width+j];
    }
}

Seeing as I'm just grabbing everything, I didn't think that the alpha component was necessary to copy. rd is my render device, and data is being output like this:

2Wy2Wy2Wy2Wy2Wy2Wy2Wy2Wy2Wy2Wy2Wy2Wy2Wy2Wy2Wy2Vy2Vy2Vy2Vx2Vx2Vx2Vx2Vx2Vx2Vx2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy2Vy3Vy3Vy3Vy3Vy3Vy2Vy2Vy1Vy1Uy0Uy1Vy1Vy1Vy1Vy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy1Vy1Vy0Vy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vy0Vy0Vy0Vy0Vy0Vz0Vz0Vz0Vz0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Ux0Ux0Ux0Tx0Tx0Tx0Tx0Tx0Ux0Ux0Ux0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx/Tx/Tw/Tw/Tx/Tx0Tx0Tx0Tx/Tx/Tw.Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw-Tw.Tw.Tw.Tw.Tw/Tw/Tw/Tw/Tx/Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Tx0Ux0Ux0Ux0Ux0Ux0Ux0Ux0Ux0Ux0Ux0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vz0Vy0Vy0Vy0Vy0Vy0Vy0Vy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Uy0Vy1Vy1Vy2Vy2Vz2Vz3Wz3Wz3Vz3Vz3Vz3Vz...

And I have no idea how to handle that. I tried reading a value as shown below with the float color but that didn't really help me, as I don't really know what it means. Is each color I'm reading an intensity value of the pixel, or do I need to read three data points in a row to get all the channels?

What is a good way to get the data displayed on the screen, modify it, and redraw it?

score 2 · Accepted Answer · answered Oct 30 '13 at 13:27

2

You are telling glReadPixels that you want to read RGB values in 3 BYTES and you are putting it in a single float value. This cannot work.

Try the following instead:

unsigned char color[3];
for ...
    color[0] = data[3*(i*width+j)];
    color[1] = data[3*(i*width+j)+1];
    color[2] = data[3*(i*width+j)+2];

I haven't tried it so there might be some mistakes. But you get the idea.

You could also tell glReadPixels that you only want GL_RED in GL_FLOAT and put it in a float buffer if you are processing black and white images and only want the intensity. Or GL_LUMINANCE; it's really up to you but you need to be coherent between the parameters you pass to glReadPixels and the way you parse that data.

answered Oct 30 '13 at 13:27

Jean-Simon Brochu

940
9
21

That's what I thought I needed to do, I just wanted to confirm that I wouldn't be grabbing too much data per color I'm trying to load. I didn't want to traverse the array too quickly or slowly. Do you know of a good format for passing that data into so I have a drawable image? – Nathan Tornquist Oct 30 '13 at 14:18
1

The data can be ready exactly as listed above. If the colors are modified, they can be redrawn with glDrawPixels() – Nathan Tornquist Oct 30 '13 at 18:44

GLbyte Data in Strange Format -- NPR Technique

1 Answers1