glReadPixels()"数据"参数用法？

Question

我正在尝试使用glReadPixels从图像中获取颜色数据.我应该使用glReadPixels,但我似乎无法搞清楚.它是一个更大的项目的一部分,但现在我想要的只是知道如何正确使用它.

我查了一下,得到了这样的东西:

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

但我不确定我应该在最后一个论点中加入什么,当我这样做时,我甚至会使用它.真的很感激帮助!(即:如何使用它的简单示例,或如何获得颜色)

Answer 1

data获取指向您为glReadPixels准备的缓冲区的指针.像这样:

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);
}

Answer 2

用法示例:

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

int mx = 0, my = 0;
void display()
{
    glClearColor( 0, 0, 0, 1 );
    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_TRIANGLES );
    glColor3ub( 255, 0, 0 );
    glVertex2f( -1, -1 );
    glColor3ub( 0, 255, 0 );
    glVertex2f( 1, -1 );
    glColor3ub( 0, 0, 255 );
    glVertex2f( 0, 1 );
    glEnd();

    // 4 bytes per pixel (RGBA), 1x1 bitmap
    unsigned char pixels[ 1 * 1 * 4 ] = { 0 };
    glReadPixels( mx, my, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixels );
    std::cout << "r: " << static_cast< int >( pixels[ 0 ] ) << '\n';
    std::cout << "g: " << static_cast< int >( pixels[ 1 ] ) << '\n';
    std::cout << "b: " << static_cast< int >( pixels[ 2 ] ) << '\n';
    std::cout << "a: " << static_cast< int >( pixels[ 3 ] ) << '\n' << std::endl;

    glutSwapBuffers();
}

void mouse( int x, int y )
{
    mx = x;
    my = glutGet( GLUT_WINDOW_HEIGHT ) - y;
    glutPostRedisplay();
}

int main( int argc, char **argv )
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutInitWindowSize( 800, 600 );
    glutCreateWindow( "glReadPixels() example" );
    glutDisplayFunc( display );
    glutPassiveMotionFunc( mouse );
    glutMainLoop();
    return 0;
}

使用鼠标获取像素的RGBA值.