glReadPixels() “data” argument usage?

问题:

I'm trying to use glReadPixels to get color data from an image. I'm supposed to be using glReadPixels but I can't seem to figure it out. It's part of a much larger project, but right now all I want is to know how to properly use this.

I looked it up and got something like this:

    void glReadPixels(GLint x, 
       GLint y, 
       GLsizei width, 
       GLsizei height, 
       GLenum format, 
       GLenum type, 
       GLvoid* data);

But I'm not sure what I should be putting in as that last argument, and when I do, how I would even use it. Help would really be appreciated! (ie: a simple example of how to use it, or how to get the color)

回答1:

data takes a pointer to some buffer you prepared for glReadPixels to put the data into. Like this:

switch(format) {
case GL_BGR:
case GL_RGB:
    components = 3; break;

case GL_BGRA:
case GL_RGBA:
    components = 4; break;

case GL_ALPHA:
case GL_LUMINANCE:
    components = 1; break;
}

GLubyte *data = malloc(components * width * height);
if( data ) {
    glReadPixels(0, 0, width, height, format, GL_UNSIGNED_BYTE, data);
}

回答2:

Usage example:

#include <GL/glut.h>
#include <vector>
#include <iostream>

using namespace std;

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-10, 10, -10, 10, -1, 1);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glScalef(5,5,5);
    glBegin(GL_QUADS);
    glColor3ub(255,0,0);
    glVertex2f(-1,-1);
    glColor3ub(0,255,0);
    glVertex2f(1,-1);
    glColor3ub(0,0,255);
    glVertex2f(1,1);
    glColor3ub(255,255,255);
    glVertex2f(-1,1);
    glEnd();

    glutSwapBuffers();
}

void mouse( int x, int y )
{
    // 4 bytes per pixel (RGBA), 1x1 bitmap
    vector< unsigned char > pixels( 1 * 1 * 4 );
    glReadPixels
        ( 
        x, glutGet( GLUT_WINDOW_HEIGHT ) - y, 
        1, 1, 
        GL_RGBA, GL_UNSIGNED_BYTE, &pixels[0] 
        );

    cout << "r: " << (int)pixels[0] << endl;
    cout << "g: " << (int)pixels[1] << endl;
    cout << "b: " << (int)pixels[2] << endl;
    cout << "a: " << (int)pixels[3] << endl;
    cout << endl;
}

int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE);

    glutInitWindowSize(800,600);
    glutCreateWindow("glReadPixels()");

    glutDisplayFunc(display);
    glutPassiveMotionFunc(mouse);
    glutMainLoop();
    return 0;
}

Use the mouse to get the RGBA value for a pixel.

回答3:

data is the pointer to the pixel data you're trying to read. Take a look at some example code, and look a few lines above that call to find out how they're initializing it. Usually it will just be an allocation of size something like x * y * depth. You'd pass it in as &data. Try reading a 1x1 pixel block of known color and see what kind of information it gives back.