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OpenGL: glFlush()(1)

无聊到学习 2022-05-06 阅读 90
3dc++

PART ONE

 codes.cpp

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

#define width 150
#define height 150
GLubyte green[height][width][4];
void initGreen()
{
	for (int y = 0; y < height; y++) {
		for (int x = 0; x < width; x++) {
			green[y][x][0] = 0;
			green[y][x][1] = 255;
			green[y][x][2] = 0;
			green[y][x][3] = 255;
		}
	}
}

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);
	glutInitDisplayMode(GLUT_DOUBLE);
	glutCreateWindow("glFlush");

	glClearColor(1, 0, 0, 1);
	glClear(GL_COLOR_BUFFER_BIT);
	
	glDrawBuffer(GL_FRONT);
	initGreen();
	glDrawPixels(width, height, GL_RGBA, GL_UNSIGNED_BYTE, green);
			
	glReadBuffer(GL_BACK);
	grab(600, 600);
	glFlush();
	
	glutMainLoop();
	return 0;
}

PART TWO

代码分析

1、在上一讲的基础上,在mian()函数中调用glFlush()函数,GL_FRONT显示到窗口,GL_BACK被grab到”grab.bmp"。

2、调用glFlush()前,UNPACK-BUFFER被glReadBuffer()设置为GL_BACK,但glFlush()并没有将GL_BACK冲入GL_FRONT用于显示。

3、DOUBLE-BUFFER下, glFlush()实际作用于GL_FRONT。

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