OpenGL: glReadBuffer

PART ONE

 codes.cpp

#include <iostream>
#include <GL/freeglut.h>
using namespace std;

void grab(GLint w, GLint h)
{
	GLubyte* image;
	GLint dataLength = w * 4 * h; //四通道
	image = (GLubyte*)malloc(dataLength);// 分配CPU内存
	if (image == 0) {
		exit(0);
	}
	else {
		//将GPU缓存数据四通道解包存到CPU内存
		glPixelStorei(GL_UNPACK_ALIGNMENT, 4);
		glReadPixels(0, 0, w, h, GL_BGR_EXT, GL_UNSIGNED_BYTE, image);
	}

	//打开"rb"文件 获取文件头
	GLubyte BMP_Head[54];
	FILE* rbFile;
	rbFile = fopen("test.bmp", "rb");
	if (rbFile == 0) {
		exit(0);
	}
	else {
		fread(BMP_Head, sizeof(BMP_Head), 1, rbFile);
		fclose(rbFile);
	}

	//打开"wb"文件
	FILE* wbFile;
	wbFile = fopen("grab.bmp", "wb");
	if (wbFile == 0) {
		exit(0);
	}
	else {
		// 写入文件头
		fwrite(BMP_Head, sizeof(BMP_Head), 1, rbFile);
		//寻址到0x0012 写入文件大小
		fseek(wbFile, 0x0012, SEEK_SET);
		fwrite(&w, sizeof(w), 1, wbFile);
		fwrite(&h, sizeof(h), 1, wbFile);
		//寻址到0x0012 写入文件数据
		fseek(wbFile, 0, SEEK_END);
		fwrite(image, dataLength, 1, wbFile);
		//文件生成结束
		fclose(wbFile);
	}

	free(image);
}

int main(int argc, char** argv)
{
	glutInit(&argc, argv);
	glutCreateWindow("glReadBuffer");

	glClearColor(0, 1, 0, 1);
	glClear(GL_COLOR_BUFFER_BIT);	

	glReadBuffer(GL_FRONT);
	grab(600,600);

	glutMainLoop();
	return 0;
}

PART TWO

代码分析

1、在grab()函数前调用glReadBuffer()，将UNPACK-BUFFER更改为GL_FRONT。

2、窗口显示白色，grab到"grab.bmp"的是红色。UNPACK-BUFFER并不是glReadBuffer()设置的GL_FRONT，而是与glClear()对应的GL_BACK。

3、结论: GL_FRONT并不能作为UNPACK-BUFFER.