nothing wrote to FBO attachment

kewuwsi · March 26, 2008, 10:02am

Hi, there.
attached is my code for FBO.

[b]
// declare fbo
static GLuint PP_FBO = 0; 
static GLuint PP_PING_TEX = 0;

static int currentShaderProgram = 0;


// init fbo
if( 0 == PP_PING_TEX)
    {
        glGenTextures(1, & PP_PING_TEX);
        glBindTexture(GL_TEXTURE_2D, PP_PING_TEX);
        glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
                 //glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
        //glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB8, 256, 256, 0, GL_LUMINANCE, GL_UNSIGNED_BYTE, NULL);        
    }
    
    if( 0 == PP_FBO)
    {
        glGenFramebuffersEXT(1, & PP_FBO);
        glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, PP_FBO);
        
        // Attach Texture to Framebuffer Color Buffer
        glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT , GL_TEXTURE_2D, PP_PING_TEX, 0);
    
        GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
        assert(status == GL_FRAMEBUFFER_COMPLETE_EXT);
        glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
    }


//use fbo
glUseProgramObjectARB(0);
    
    // render to buffer
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, PP_FBO);
    glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, PP_PING_TEX, 0);
    
    glDrawBuffer( GL_COLOR_ATTACHMENT0_EXT);
    // save current attribute
    glPushAttrib(GL_VIEWPORT_BIT);
     
    // set for fbo rendering
    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();
    gluOrtho2D(0.0, 256.0, 0.0, 256.0);    
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();
    glViewport(0, 0, 256, 256 );
     
    
    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, myTextureName);

    
    // first pass
    //glClearColor(1.0,0.0,0.0,0.0);
    //glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    
    glBegin(GL_QUADS);
    glTexCoord2f(0.0f, 0.0f); glVertex2f( 0.0, 0.0);
    glTexCoord2f(1.0f, 0.0f); glVertex2f( 256.0, 0.0);
    glTexCoord2f(1.0f, 1.0f); glVertex2f( 256.0 ,  256.0);
    glTexCoord2f(0.0f, 1.0f); glVertex2f( 0.0, 256.0);
    glEnd();
    
    unsigned char result[256*256];
    glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);
    glReadPixels(0, 0, 256,256,GL_LUMINANCE,GL_UNSIGNED_BYTE,result);
     for( int i = 0; i &lt; 256; i++)
     {
         for( int j = 0; j &lt; 256; j++)
         {
             int value = (int) result[i*256+j];
             if( value )
             {
                 cout&lt;&lt; (int) result[i*256+j]&lt;&lt;"|";
             }
        }
     }
    glPopMatrix();
    glMatrixMode(GL_PROJECTION);
    glPopMatrix();
    glMatrixMode(GL_MODELVIEW);
     
    glPopAttrib();
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

 [/b]
[/b][/QUOTE]

Please take a look. The problem is, seems nothing is wrote to attachment( nothing printed out). And if I enable the two line after "//first pass", I got all 255s.

really out of my mind. can you please give a clue?


tks

Relic · March 26, 2008, 1:35pm

Check if there had been an error during the rendering. Add a glGetError after the glClear and after the glEnd.
I would expect that this setup won’t work because GL_LUMINANCE is a 1-component format and FBOs have only been specified for rendering to 3- and 4-component formats (RGB and RGBA).
1- and 2-component formats are a special case and if it’s supported then on NVIDIA as described in one of the early FBO presentations and the formats are R and RG, not luminance.

You could render to an RGB texture and read out luminance but that glReadPixels would need to calculate the luminance values on the fly.

kewuwsi · March 26, 2008, 2:29pm

as far as my understanding, FBO is not supporting 1 component internal format. The one component input format is OK.
I did succeed with this type of setting.( have working code ).

I am just expecting glReadPixels(…GL_LUMINANCE…) give me some none zero value.

adding

GLenum err=glGetError();
if (err!=0)
{
printf( “GLERR: %s”, gluErrorString(err));
}

[/b]

after glEnd(); and glClear(). never give me msg.

the thing made me headache is: similar FBO in another class works OK( in same application )… I ma out of mind!

Relic · March 26, 2008, 3:58pm

Oops, my bad, I misread the
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB8, 256, 256, 0, GL_LUMINANCE, GL_UNSIGNED_BYTE, NULL);

So it’s actually an RGB8 texture and the glReadPixels should convert it to luminance and that works somehow with the clear color.

You don’t have a depth buffer attached to the FBO. The clear GL_DEPTH_BUFFER_BIT is useless.

Be aware that the glTexEnv is per texture unit not per texture object so the glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE); affects the glBindTexture(GL_TEXTURE_2D, myTextureName);, but that’s probably what you wanted anyway.

Does it work if you render the quad untextured with a constant color different than the clear color, like (0.5,0,0)?
If yes it’s the texture setup.
How did you setup the myTextureName texture object?

Check other state, blending, current color etc.

If none of the above helps and things are working in other places of your code, one way would be to do an OpenGL command trace and see what exactly is different there.

kewuwsi · March 27, 2008, 6:28am

I used the following code:

glColor3f(1.0, 1.0, 0.0);
glBegin(GL_TRIANGLES);
glVertex2f(0.0, 0.0);
glVertex2f( 256.0, 0.0);
glVertex2f( 256.0 , 256.0);
glEnd();

[/b]

instead of quad drawing part, still nothing is printed out.
really weird…

NiCo1 · March 27, 2008, 6:32am

You should also add

glDisable(GL_TEXTURE_2D);

before the code you posted otherwise the test is rather pointless.

kewuwsi · March 27, 2008, 7:21am

OK, now the code: still nothing out.

[b]
//use fbo 
    glUseProgramObjectARB(0);
    // render to buffer 
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, PP_FBO);
    glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, PP_PING_TEX, 0);
    glDrawBuffer( GL_COLOR_ATTACHMENT0_EXT); 
    // save current attribute 
    glPushAttrib(GL_VIEWPORT_BIT); 
    // set for fbo rendering 
    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();
    gluOrtho2D(0.0, 256.0, 0.0, 256.0);
    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();
    glViewport(0, 0, 256, 256 );
    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, myTextureName);
    // first pass 
    //glClearColor(1.0,0.0,0.0,0.0); 
    //glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    
    glDisable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, 0);
    
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0, 1.0, 0.0);
    glLineWidth(5.0);
    glBegin(GL_TRIANGLES);
    glVertex2f(0.0, 0.0);
    glVertex2f( 256.0, 0.0);
    glVertex2f( 256.0, 256.0);
    glEnd();
    GLenum err = glGetError();
    if (err!=0) {
        printf("GLERR:  %s", gluErrorString(err));
    }
    //drawQuad();
    unsigned char result[256*256];
    glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);
    glReadPixels(0, 0, 256,256,GL_LUMINANCE,GL_UNSIGNED_BYTE,result);
    for( int i = 0; i &lt; 256; i++) 
    { 
        for( int j = 0; j &lt; 256; j++)
        { 
            int value = (int) result[i*256+j];
            if( value )
            { 
                cout&lt;&lt; (int) result[i*256+j]&lt;&lt;"|";
            } 
        } 
    } 
    glPopMatrix();
    glMatrixMode(GL_PROJECTION);
    glPopMatrix();
    glMatrixMode(GL_MODELVIEW);
    glPopAttrib();
 [/b]
[/b][/QUOTE]

NiCo1 · March 27, 2008, 8:02am

Well, my guess is that you forgot to reset some state during another part of your program. Here’s the code I tried:


    int sizex = 8;
    int sizey = 8;

    GLuint PP_FBO = 0;
    GLuint PP_PING_TEX = 0;

    if ( 0 == PP_PING_TEX)
    {
        glGenTextures(1, & PP_PING_TEX);
        glBindTexture(GL_TEXTURE_2D, PP_PING_TEX);
        glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB8, sizex, sizey, 0, GL_LUMINANCE, GL_UNSIGNED_BYTE, NULL);
    }
    if ( 0 == PP_FBO)
    {
        glGenFramebuffersEXT(1, & PP_FBO);
        glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, PP_FBO);
        glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT , GL_TEXTURE_2D, PP_PING_TEX, 0);
        glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
    }
    glUseProgramObjectARB(0);
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, PP_FBO);
    glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, PP_PING_TEX, 0);
    glDrawBuffer( GL_COLOR_ATTACHMENT0_EXT);

    glPushAttrib(GL_VIEWPORT_BIT);

    glMatrixMode(GL_PROJECTION);
    glPushMatrix();
    glLoadIdentity();
    gluOrtho2D(0.0, sizex, 0.0, sizey);

    glMatrixMode(GL_MODELVIEW);
    glPushMatrix();
    glLoadIdentity();

    glViewport(0, 0, sizex, sizey );

    glClearColor(0.0,0.0,0.0,0.0);
    glClear(GL_COLOR_BUFFER_BIT);

    glColor3f(0.5, 0.0, 0.0);

    glDisable(GL_TEXTURE_2D);

    glBegin(GL_TRIANGLES);
    glVertex2f(0.0, 0.0);
    glVertex2f( sizex, 0.0);
    glVertex2f( sizex, sizey);
    glEnd();

    unsigned char result[sizex*sizey];
    glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);
    glReadPixels(0, 0, sizex,sizey,GL_LUMINANCE,GL_UNSIGNED_BYTE,result);

    fprintf(stderr,"
");

    for ( int i = 0; i < sizey; i++)
    {
        for ( int j = 0; j < sizex; j++)
        {
			fprintf(stderr,"%03d|",(int) result[i*sizex+j]);
        }
        fprintf(stderr,"
");
    }

    glPopMatrix();
    glMatrixMode(GL_PROJECTION);
    glPopMatrix();
    glMatrixMode(GL_MODELVIEW);
    glPopAttrib();
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

This is the output:


127|127|127|127|127|127|127|127|
000|127|127|127|127|127|127|127|
000|000|127|127|127|127|127|127|
000|000|000|127|127|127|127|127|
000|000|000|000|127|127|127|127|
000|000|000|000|000|127|127|127|
000|000|000|000|000|000|127|127|
000|000|000|000|000|000|000|127|

which is correct.

kewuwsi · March 27, 2008, 8:06am

yeah, I think so.
it works in one of my another class( in the same application).

but what is the “state” will effect this part?

NiCo1 · March 27, 2008, 8:14am

Well, there’s a bunch of reasons why this doesn’t work. That’s the hard part about debugging. Some reasons are obvious and some are not.

For example, if you bound an object to the pixel pack buffer and forgot to unbind it when performing glReadPixels(…,result) , then it will treat result as an offset to the pixel pack buffer and it will not read to your system memory at address ‘result’.

or you enabled cullface somewhere and the triangle is being clipped because of the wrong winding rule. Or you have blending still enabled. Or you have lighting enabled and the object is not facing the light. There’s too much to sum up.

I usually bypass all other parts of my code and than iteratively enable parts of it and see if I still get the expected result. The moment this changes, it means that the problem lies in the last part you enabled. It’s trial and error, takes some time but always works.

kewuwsi · March 27, 2008, 8:45am

well, somehow I made it give me some reasonable output.
before using FBO, my apps calls another render(), the code of this render is like this.

[b]
glActiveTexture(GL_TEXTURE1);

    glBindTexture(GL_TEXTURE_2D, myTexture1);

    glEnable(GL_TEXTURE_2D);

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glActiveTexture(GL_TEXTURE2);
    
    glBindTexture(GL_TEXTURE_2D, myTexture2);
    

    glEnable(GL_TEXTURE_2D);
    

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glActiveTexture(GL_TEXTURE3);

    glBindTexture(GL_TEXTURE_2D, myTexture3);
    

    glEnable(GL_TEXTURE_2D);

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

    glActiveTexture(GL_TEXTURE0);
 [/b]
[/b][/QUOTE]
 if I remove this render() call, I can got "000|128|" as you did...
I think the problem is: for this three glEnable(TEXTURE_2D); I have to disable them one by one......... remember had this problem before, but did not figure out the underlay theory....


tks

kewuwsi · March 27, 2008, 9:13am

yeah!
It is this part effect my FBOs.
But I do need these three textures too in shaders. So just disable them is not the solution.

can anybody point me a right direction?

tks

Relic · March 27, 2008, 9:57am

Hmm, read more docs.
Shaders don’t care at all about the fixed function glEnable texture state. They use texture image units and the shader uniforms you setup imply which texture image unit is used to fetch texture data.

NiCo1 · March 27, 2008, 10:29am

Indeed, if you’re using shaders you don’t need to enable the texture stages. One of the things I like about shaders actually
But since the OP is using glUseProgramObjectARB(0) he’s back to fixed functionality for that part.

kewuwsi · March 27, 2008, 11:26am

tks!
finally I will use my shaders instead of glUseProgramObjectARB(0).
I went back to the fixed functionality just want to see whether the FBO is working properly.

The part puzzles me is," why the glEnable(GL_TEXTURE_2D) effects my FBOs?" I mean, no matter I use fixed functionality or use my shaders, if I keep the “glEnable(GL_TEXTURE_2D)” there. I can’t get anything. If I remove it, it just works!

NiCo1 · March 27, 2008, 1:21pm

The glEnable(GL_TEXTURE_2D) does not affect your FBO, it affects your drawing routine. If you enabled multiple texture stages with glActiveTexture, they remain active. You wanted to draw the texture bound to glActiveTexture(GL_TEXTURE0) but GL_TEXTURE1, GL_TEXTURE2,etc. were still active which means that they will also be applied (modulated/replaced/…) to the object you are drawing.

kewuwsi · March 27, 2008, 2:02pm

thanks for explanation.
Then why the try just draw a triangle without any texture( glDisable(GL_TEXTURE_2D) ) rendered nothing to FBO attachment?

kewuwsi · March 27, 2008, 2:08pm

you are saying that the (modulated/replaced/…) is applied to the triangle drawing?

NiCo1 · March 27, 2008, 2:27pm

Yes. You said yourself that it works if you disable the texture units that you activated in an earlier pass. Because you did not deactivate them they will be applied to the triangle.

NiCo1 · March 27, 2008, 2:28pm

Because the glEnable(GL_TEXTURE_2D) and glDisable(GL_TEXTURE_2D) only apply to the texture unit you activated last using glActiveTexture()