C++ glReadPixels() “数据”参数用法？
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glReadPixels() "data" argument usage?
提问by CustardBun
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.
我正在尝试使用 glReadPixels 从图像中获取颜色数据。我应该使用 glReadPixels 但我似乎无法弄清楚。这是一个更大项目的一部分，但现在我只想知道如何正确使用它。
I looked it up and got something like this:
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);
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)
但我不确定我应该把什么作为最后一个论点，当我这样做时，我什至会如何使用它。帮助将不胜感激！（即：如何使用它或如何获取颜色的简单示例）
回答by datenwolf
datatakes a pointer to some buffer you prepared for glReadPixels to put the data into. Like this:
data获取一个指向您为 glReadPixels 准备的缓冲区的指针，以将数据放入其中。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);
}
回答by genpfault
Usage example:
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;
}
Use the mouse to get the RGBA value for a pixel.
使用鼠标获取像素的 RGBA 值。
回答by Tim
datais the pointer to the pixel data you're trying to read. Take a look at some example code, and look a few lines abovethat 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.
data是指向您尝试读取的像素数据的指针。查看一些示例代码，并查看该调用上方的几行以了解它们如何初始化它。通常它只是大小的分配，例如x * y * depth. 你会把它作为&data. 尝试读取已知颜色的 1x1 像素块，看看它返回什么样的信息。
