How to copy backbuffer contents into a texture?

[glSun]

Hi,

I am trying to copy the contents of the backbuffer into a texture to reuse it later. However, all my attempts failed, because I have not enough know-how of how to capture textures from the backbuffer.

Could some please fill-in/correct the code below? I need help with steps 4 and 6.

#define CX 144
#define CY 144
 
GLuint img;
 
//	Step 1: create window
glutInitWindowSize (CX, CY);
glutInitDisplayMode ( GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
glutCreateWindow ("glut");
glClearColor(0.0f, 0.0f, 0.0f, 0.5f);
 
//	Step 2: clear the scene
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
//	Step 3: draw a red square
glColor3f(1, 0, 0);
glMatrixMode (GL_MODELVIEW);
glRectf (-0.2f, -0.2f, 0.2f, 0.2f);
 
//	Step 4: save backbuffer contents into texture (save the whole visible area)
glGenTextures(1,&img);
glBindTexture(GL_TEXTURE_2D,img);
// And now? Use glCopyTexImage2D? Use glCopyTexSubImage2D? How does it work?
 
//	Step 5: clear the scene again
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
//	Step 6: draw a quad with the size of the whole screen using the captured texture
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D,img);
glBegin(GL_QUADS);
	glTexCoord2f(?,?); glVertex2i(-1,1);
	glTexCoord2f(?,?); glVertex2i(1,1);
	glTexCoord2f(?,?); glVertex2i(1,-1);				
	glTexCoord2f(?,?); glVertex2i(-1,-1);
glEnd();
glDisable(GL_TEXTURE_2D);

[ZbuffeR]

Step 4: it works like that :
http://www.opengl.org/sdk/docs/man/x...TexImage2D.xml

Step 6: depends on your projection, but with default projection x and y will be [-1;1] accross the screen

[glSun]

Step 4: it works like that :
http://www.opengl.org/sdk/docs/man/x...TexImage2D.xml

Sorry, I should have informed you that I have passed the RTFM phase already ;-) Seriously, I have read the manual, but I don't understand it. I am a complete beginner.
I have googled for examples and I have tried some code I found on the internet, but I just can't seem to make it work. I miss some fundamental understanding of OpenGL here.

Therefore I'd really appreciate if someone could post some example code for step 4 and 6 to help better understand how it works.

[mhagain]

Tell us about the texture you're copying it into. What's it's format, it's width, it's height? Also, can you show us the glCopyTexSubimage2D line that didn't work?

[BionicBytes]

Step 4 - read back buffer into a texture is easy:

Bind (texture);
glCopyTexSubImage2D(GL_TEXTURE_2D,0,0,0, 0,0,texture.image.sizex, texture.image.sizey);


Step 6 - display the quad/texture
You can't do what you have done:
glBegin(GL_QUADS);
glTexCoord2f(?,?); glVertex2i(-1,1);
glTexCoord2f(?,?); glVertex2i(1,1);
glTexCoord2f(?,?); glVertex2i(1,-1);
glTexCoord2f(?,?); glVertex2i(-1,-1);
glEnd();

...because the vertex positions will get multiplied by the ModelView matrix and projection matrices by the fixed function pipeline and or shaders. Are you using GL CORE profile or COMPATABILITY?

Instead, you need to pass in the width and height of the quad you are drawing:

glBegin (GL_Quads);
glTexCoord2f (0,1); glVertex2f (x,y); //top left
glTexCoord2f (1,1); glVertex2f (x+w,y); //top right
glTexCoord2f (1,0); glVertex2f (x+w,y+h); //bottom right
glTexCoord2f (0,0); glVertex2f (x,y+h); //bottom left
glEnd;

[glSun]

Thanks for your comments. I tried to apply what you have said. Now I see something on the screen, but it is now what I had expected. My code looks like this now:

//	Step 4: save backbuffer contents into texture (save the whole visible area)
glGenTextures(1,&img);
glBindTexture(GL_TEXTURE_2D,img);
glCopyTexSubImage2D(GL_TEXTURE_2D,0,0,0, 0,0,CX,CY);
 
//	Step 5: clear the scene again
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
//	Step 6: draw a quad with the size of the whole screen using the captured texture
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D,img);
glBegin (GL_QUADS);
  glTexCoord2f (0,1); glVertex2f (-1,1); //top left
  glTexCoord2f (1,1); glVertex2f (1,1); //top right
  glTexCoord2f (1,0); glVertex2f (1,-1); //bottom right
  glTexCoord2f (0,0); glVertex2f (-1,-1); //bottom left
glEnd();
glDisable(GL_TEXTURE_2D);
 
glutSwapBuffers();

Since I want to capture the whole visible area of my window, I set the coordinates of glVertex2f accordingly (applying the width of the texture as advised).

However, the result is that the red sqare is extended over the whole visible area. Actually I wanted to capture also the black part, that's why I have set the width and height in glCopyTexSubImage2D to CX and CY (width and height of the window)

See the attached images (I expected that "before" and "after" look equally)

Before the capture:


After rendering the captured texture:


Thanks in advance for any help!


EDIT - @BionicBytes: How can I tell whether I am using GL CORE profile or COMPATABILITY?

[BionicBytes]

Compatability or Core is an option set in the context creation code. You had to have specified this in your initialisation. How did you do that?
I guess you have compatibility because you don't appear to have shaders running and are using immediate mode (with quads). These were removed from the 'core' profile!

[mhagain]

What's your viewport x, y, width and height? And the projection matrix you're using when drawing? Also, the values of CX and CY? And also the img texture should be just created once during startup (or if the window size changes), not every frame.

[glSun]

Compatability or Core is an option set in the context creation code. You had to have specified this in your initialisation. How did you do that?

All initialization I do is described in step 1. Do I have do do more than that?

//	Step 1: create window
glutInitWindowSize (CX, CY);
glutInitDisplayMode ( GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
glutCreateWindow ("glut");
glClearColor(0.0f, 0.0f, 0.0f, 0.5f);

What's your viewport x, y, width and height? And the projection matrix you're using when drawing? Also, the values of CX and CY? And also the img texture should be just created once during startup (or if the window size changes), not every frame.

Viewport: Not specified. I really use only the code posted above. Should I set the viewport to the visible area of the window?

Projection matrix: Not specified. What would be a useful value here?

CX, CY: Both are 144. That is also the width and height of the window. I want to capture the complete client area of the window.

Regarding the texture: I will change it so that the texture is created only once. What is the problem if it is created every frame? Memory leaks?

[BionicBytes]

Your using glut - a helper package. I guess that does some defaults.
I suggest you google HeNe which has some excellent begginer tutorials which will take you from the absolute basics to intermediate level. You are missing the fundaments of the whole GL thing and it would take a series of tutorials to explain what those things are!
I suggest you read up on projection matrix, modelview matrix and view port. Then you'll be able to anser your own questions!