Question

cannot render texture to framebuffer (blank screen)

Asked by: collages

I have read numerous articles/tutorials online regarding drawing a texture to a framebuffer in opengl, but I am having no luck getting it to work.  I am creating a framebuffer, drawing a texture to the fbo, and then drawing the fbo texture to the screen but nothing displays.  I am at a loss.  If i comment out the fbo code and draw the embedded texture directly to the screen, that works.  I am using OpenGL ES 1.1  

// INITIALIZATION
	glEnable(GL_TEXTURE_2D);
	glEnable(GL_BLEND);
	glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
	
	// Load Test Texture
	glGenTextures(1, &textureTest);
	glBindTexture(GL_TEXTURE_2D, textureTest);			// Bind Our Texture
	glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);	// Linear Filtered
	glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);	// Linear Filtered
	[self loadTexture: @"texture"]; // calls glTexImage2D with image data from png
	
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	glViewport(0,0, 320.0f,480.0f);
	glOrthof(0,320.0f, 480.0f,0, -1.0f, 1.0f); // left, right, bottom, top
	glMatrixMode(GL_MODELVIEW);
	
	//FBO Setup
	glGenTextures(1, &textureFBO); // texture handle
	glBindTexture(GL_TEXTURE_2D, textureFBO); //bind it.
	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, 512, 512, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL); 
	glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);	// Linear Filtered
	glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);	// Linear Filtered
	
	glGenFramebuffersOES(1, &FBO); // frame buffer 
	glBindFramebufferOES(GL_FRAMEBUFFER_OES, FBO); //bind the frame buffer
	
	// attach texture to framebuffer color buffer
	glFramebufferTexture2DOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_TEXTURE_2D, textureFBO, 0);
	if(glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES) != GL_FRAMEBUFFER_COMPLETE_OES)  printf("CANNOT CREATE FRAMEBUFFER\n");
 
 
// MAIN DRAWING LOOP
	static const Vertex3D vertices[] = {
        {0.0,  100.0, 0.0},
        { 100.0,  100.0, 0.0},
        { 0.0, 200.0, 0.0},
        { 100.0, 200.0, 0.0}
    };
	static const GLfloat texCoords[] = {
        0.0, 1.0,
        1.0, 1.0,
        0.0, 0.0,
        1.0, 0.0
    };
	
	glClear(GL_COLOR_BUFFER_BIT);
	glLoadIdentity();
	
	// Render to texture
	glBindFramebufferOES(GL_FRAMEBUFFER_OES, FBO);
	
	// Draw test texture
	glBindTexture(GL_TEXTURE_2D, textureTest);
	glEnableClientState(GL_VERTEX_ARRAY);
	glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glVertexPointer(3, GL_FLOAT, 0, vertices);
	glTexCoordPointer(2, GL_FLOAT, 0, texCoords);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
	glDisableClientState(GL_VERTEX_ARRAY);
	glDisableClientState(GL_TEXTURE_COORD_ARRAY);
	glBindTexture(GL_TEXTURE_2D, 0); //disable any bound textures.
	// End draw test texture
	
	// Render to screen
	glBindFramebufferOES(GL_FRAMEBUFFER_OES, 0); 
	
	// Draw FBO texture
	glBindTexture(GL_TEXTURE_2D, textureFBO);
	glEnableClientState(GL_VERTEX_ARRAY);
	glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glVertexPointer(3, GL_FLOAT, 0, vertices);
	glTexCoordPointer(2, GL_FLOAT, 0, texCoords);
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
	glDisableClientState(GL_VERTEX_ARRAY);
	glDisableClientState(GL_TEXTURE_COORD_ARRAY);
	glBindTexture(GL_TEXTURE_2D, 0); //disable any bound textures.
	// End draw FBO texture

                                  
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Asked On
2009-10-29 at 21:11:15ID24857048
Tags

fbo

,

framebuffer

,

opengl

,

texture

Topics

OpenGL Graphics & Game Programming

,

3D Game Programming

Participating Experts
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Comments
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Answers

 

by: ikeworkPosted on 2009-10-30 at 10:23:09ID: 25705079

Hi collages,

If you just want to draw a bitmap(texture) on the screen, you can do that with glDrawPixels without using any extensions:

Man Page:

  http://www.opengl.org/documentation/specs/man_pages/hardcopy/GL/html/gl/drawpixels.html

Example:

  http://www.cs.mun.ca/~blangdon/opengl/glDrawPixels.html


ike

 

by: collagesPosted on 2009-10-30 at 11:05:01ID: 25705419

I would like to get framebuffer objects working though so I can create textures on initialization.  My first usage would be to create a sprite atlas (one large texture made up of lots of separate textures), so when rendering lots of 2D objects on the screen, I can just bind that one atlas texture and change the UV to select the appropriate images (as an optimization technique).

 

by: ikeworkPosted on 2009-10-30 at 11:10:27ID: 25705467

You can do that too without the extension

 

by: collagesPosted on 2009-10-30 at 11:18:54ID: 25705545

I thought the only way to draw to a texture was to bind it to a frame buffer?

 

by: ikeworkPosted on 2009-10-30 at 11:23:08ID: 25705584

>> I thought the only way to draw to a texture was to bind it to a frame buffer?

Why do you want to draw into the texture in the extra frame buffer and then copy it to the back-buffer?
Its actually more work than less. Why not drawing in the normal back-buffer in the first place, the result is the same, but less work and and easier program.

 

by: collagesPosted on 2009-10-30 at 11:28:50ID: 25705630

I read that the performance of glDrawPixels is very slow, and I don't want to have to call this on every frame.  Also, transformations, alpha blending and applying special effects are easier and faster to run on a quad (2 triangles) with a mapped texture.

 

by: ikeworkPosted on 2009-10-30 at 11:29:27ID: 25705633

If you relly want to render to a texture, you can capture the normal back buffer, still easier code without the need of extensions. Here is an example:

http://nehe.gamedev.net/data/lessons/lesson.asp?lesson=36

 

by: ikeworkPosted on 2009-10-30 at 11:32:16ID: 25705652

>> I read that the performance of glDrawPixels is very slow

I didnt mean using glDrawPixels. Just render your textures normally mapped on polygons, like you would do it into the extra render target.
How did you want to render into the extra target, mapped textures on polygons, right?
Just do this, but not into the extra render-target, but into the normal back-buffer

 

by: collagesPosted on 2009-11-02 at 07:35:09ID: 25720287

I was able to get my code working.  I forgot to re-bind the main frame buffer that renders directly to the screen:

// Render to screen
glBindFramebufferOES(GL_FRAMEBUFFER_OES, 0);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, 1);

20120131-EE-VQP-002

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