glDrawPixels and FBO

OpenGL OpenGL: Basic Coding
1

Hi,

I’m trying to copy to GL_COLOR_ATTACHMENT0_EXT an array stored in CPU. To this end I use glDrawPixels after attaching glFramebufferTexture2DEXT.

The code bellow is not what I want to do but I’m testing the correctness of the code:


GLuint colorTex;
glGenTextures(1, &colorTex);
glBindTexture(GL_TEXTURE_2D, colorTex);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, SCREEN_RES_X, SCREEN_RES_Y, 0, GL_RGB, GL_UNSIGNED_BYTE, NULL);
glBindTexture(GL_TEXTURE_2D, 0);

GLuint fbo=0;
glGenFramebuffersEXT(1, &fbo);

//render to texture to avoid scissors test by qglwidget
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);

//depth cannot be rendered without color
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT,GL_COLOR_ATTACHMENT0_EXT,GL_TEXTURE_2D,colorTex,0);

GLubyte *pixels=new GLubyte[SCREEN_RES*3];
//create a red image
for(int i=0;i<SCREEN_RES;i++)
{
  pixels[i*3]=255; 
  pixels[i*3+1]=pixels[i*3+2]=0;
}

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glDrawPixels(SCREEN_RES_X,SCREEN_RES_Y,GL_RGB,GL_UNSIGNED_BYTE,pixels);

GLubyte * color=new GLubyte[SCREEN_RES*3];
glReadPixels(0,0,SCREEN_RES_X,SCREEN_RES_Y,GL_RGB,GL_UNSIGNED_BYTE,color);

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT,0);
	
glDeleteFramebuffersEXT(1, &fbo);
glDeleteTextures(1,&colorTex);

The problem is that color is all white, like DrawPixels has no effect. Am I doing something wrong?

Thank you.

2

I think you are missing calls to glDrawBuffer/glReadBuffer to specify the FBO attachment glDrawPixels/glReadPixels work with.

3

This:


GLint db;
glGetIntegerv(GL_DRAW_BUFFER,&db);

called before glDrawPixels returns GL_COLOR_ATTACHMENT0_EXT, which is reasonable since:


glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT,GL_COLOR_ATTACHMENT0_EXT,GL_TEXTURE_2D,colorTex,0);

I’m attaching GL_COLOR_ATTACHMENT0_EXT as the color buffer.

In addition:


GLuint colorTex;
glGenTextures(1, &colorTex);
glBindTexture(GL_TEXTURE_2D, colorTex);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, SCREEN_RES_X, SCREEN_RES_Y, 0, GL_RGB, GL_UNSIGNED_BYTE, NULL);
glBindTexture(GL_TEXTURE_2D, 0);

GLuint fbo=0;
glGenFramebuffersEXT(1, &fbo);

//render to texture to avoid scissors test by qglwidget
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fbo);

//depth cannot be rendered without color
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT,GL_COLOR_ATTACHMENT0_EXT,GL_TEXTURE_2D,colorTex,0);

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glColor3f(1,0,0);
glPolygonMode(GL_FRONT, GL_FILL);
glBegin(GL_QUADS);
glVertex2f(0.2,0.2);
glVertex2f(0.4,0.5);
glVertex2f(0.4,0.4);
glVertex2f(0.2,0.4);
glEnd();

GLubyte * color=new GLubyte[SCREEN_RES*3];
glReadPixels(0,0,SCREEN_RES_X,SCREEN_RES_Y,GL_RGB,GL_UNSIGNED_BYTE,color);

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT,0);
	
glDeleteFramebuffersEXT(1, &fbo);
glDeleteTextures(1,&colorTex);

works fine. Any other ideas?