opengl glkit color-picking glreadpixels always returns zero colored pixels (0,0,0)

Question

The following code can be a complete template for color-picking with glkit. I just need to understand why all the pixels in the offscreen buffer are colored (0,0,0) as printed below to the nslog. Note: the vertices array is defined in an header file as a const array it is displayed well on the screen in another section of my project (my problem is with the offscreen).

Header file:

typedef struct {
    float Position[3];
    float Color[4];
    float TexCoord[2]; 
    float Normal[3]; 
} Vertex;

//Vertices array format: {{vertex.x, vertex.y, vertex.z}, {color.R, color.G, color.B, color.alpha}, {texture.U, texture.V}, {normal.x, normal.y, normal.z}},
const Vertex parparit51OBJVertices[] = {
   {{0.057, -0.088, -0.155},{1,1,1,1},{0.848, 0.810}, {0.329, -0.157, -0.931}},
   {{0.056, -0.035, -0.165},{1,1,1,1},{0.848, 0.811}, {0.338, -0.139, -0.931}}, ......

In the viewController code:

GLuint _pickFBO = 0;
int32_t glVertexAttributeBufferID = 0;

- (IBAction) tapGesture:(id)sender
{
    if ([(UITapGestureRecognizer *)sender state] == UIGestureRecognizerStateEnded) {

        NSLog( @"In tap ended" );

        CGPoint tapLocation = [(UITapGestureRecognizer *)sender locationInView:self.view];

       int tt = [self findMeshByPoint:tapLocation];
    }
}

- (NSUInteger)findMeshByPoint:(CGPoint)point
{
    //In openGL the y axis starts from the bottom of the screen
    point.y = self.view.bounds.size.height - point.y;

    GLKView *glView = (GLKView *)self.view;
    NSAssert([glView isKindOfClass:[GLKView class]],
             @"View controller's view is not a GLKView");

    // Make the view's context current
    [EAGLContext setCurrentContext:glView.context];

    _height = ((GLKView *)self.view).drawableHeight;
    _width = ((GLKView *)self.view).drawableWidth;

    self.effect.useConstantColor = GL_TRUE;
    self.effect.colorMaterialEnabled = GL_TRUE;
    self.effect.light0.diffuseColor = GLKVector4Make(1.0f,1.0f,1.0f,1.0f);

    glBindVertexArrayOES(0);

        glDisable(GL_DITHER);
        glEnable(GL_DEPTH_TEST);
        glLineWidth(2.0F);

       // Important to turn light off !!!
        self.effect.light0.enabled = GL_TRUE;

        glDisableVertexAttribArray(GLKVertexAttribTexCoord0);

    //this crashes the code
    //glEnableVertexAttribArray(GLKVertexAttribColor);

        self.effect.constantColor = GLKVector4Make( 0.0f,  //This should be meshId/255.0f
                                                   0.8f, 0.8f, 1.0f);

    if(0 == _glVertexAttributeBufferID)
    {
        GLuint  glName;

        glGenBuffers(1,                // STEP 1
                     &glName);
        glBindBuffer(GL_ARRAY_BUFFER,  // STEP 2
                     glName);
        glBufferData(                  // STEP 3
                     GL_ARRAY_BUFFER,  // Initialize buffer contents
                     sizeof(parparit51OBJVertices), parparit51OBJVertices,
                     GL_STATIC_DRAW);            // Hint: cache in GPU memory

        _glVertexAttributeBufferID = glName;

        GLenum err = glGetError();
        if (err != GL_NO_ERROR) {
            NSLog(@"Error creating buffer %i. glError: 0x%04X", glName, err);
        }
    }
    else
    {
        glBindBuffer(GL_ARRAY_BUFFER,
                     _glVertexAttributeBufferID);
    }

     [self buildFBO];

    glBindFramebuffer(GL_FRAMEBUFFER, _pickFBO);
    //glViewport(0, 0, _width, _height);

    //???
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glEnableVertexAttribArray(GLKVertexAttribPosition);
    glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 12 * sizeof(GLfloat), 0);
    glEnableVertexAttribArray(GLKVertexAttribNormal);
    glVertexAttribPointer(GLKVertexAttribNormal, 3, GL_FLOAT, GL_FALSE,  12 * sizeof(GLfloat), (GLvoid*) (sizeof(float) * 9));

    GLKMatrix4 modelViewMatrixForParparit = GLKMatrix4MakeTranslation(0.0f, 0.3f, -3.5f );
    modelViewMatrixForParparit = GLKMatrix4Scale(modelViewMatrixForParparit, 1.0, 1.0, 1.0);
    self.effect.transform.modelviewMatrix = modelViewMatrixForParparit;

    self.effect.constantColor = GLKVector4Make( 0.8f, 0.3f, 0.3f, 1.0f );

    [self.effect prepareToDraw];
    glDrawArrays(GL_TRIANGLES, 0, sizeof(parparit51OBJVertices) / sizeof(Vertex));

    const GLfloat width = [glView drawableWidth];
    const GLfloat height = [glView drawableHeight];
    NSAssert(0 < width && 0 < height, @"Invalid drawble size");


    int blackPixelsCounter = 0;
    int coloredPixelsCounter = 0;
    GLubyte savePixelColor[4] = {0, };
    bool bFoundDifferentColors = NO;

    GLubyte pixelColor[4];  // Red, Green, Blue, Alpha color

    glReadPixels(50,
                 50,
                 1,
                 1,
                 GL_RGBA,
                 GL_UNSIGNED_BYTE,
                 pixelColor);

    //#ifdef DEBUG
    {  // Report any errors
        GLenum error = glGetError();
        if(GL_NO_ERROR != error)
        {
            NSLog(@"GL Error: 0x%x", error);
        }
    }
    //#endif

    savePixelColor[0] = pixelColor[0];
    savePixelColor[1] = pixelColor[1];
    savePixelColor[2] = pixelColor[2];

    for (GLint xx=0; xx<_width; xx++) {
        for (GLint yy=0; yy<_height; yy++) {

            glReadPixels(xx,
                         yy,
                         1,
                         1,
                         GL_RGBA,
                         GL_UNSIGNED_BYTE,
                         pixelColor);

            //#ifdef DEBUG
            {  // Report any errors
                GLenum error = glGetError();
                if(GL_NO_ERROR != error)
                {
                    NSLog(@"GL Error: 0x%x", error);
                }
            }
            //#endif

            if ( (savePixelColor[0] != pixelColor[0]) || (savePixelColor[1] != pixelColor[1]) || (savePixelColor[2] != pixelColor[2]) )
            {
               bFoundDifferentColors = YES;
            }

            if ( (pixelColor[0] !=0) || (pixelColor[1] !=0) || (pixelColor[2] !=0) )  {
                //NSLog(@"pixelColor[0]=%i, pixelColor[1]=%i, pixelColor[2]=%i", pixelColor[0], pixelColor[1], pixelColor[2] );


                coloredPixelsCounter++;
            }
            else
            {
                blackPixelsCounter++;
            }
        }
     }

     NSLog( @"colored pixels=%i  black pixels=%i", coloredPixelsCounter, blackPixelsCounter );
     if ( bFoundDifferentColors )
     {
         NSLog( @"Found at least 2 different pixels colors in picking buffer !" );
     }
    else
    {
        NSLog( @"All pixels have the same color: %i, %i, %i",  savePixelColor[0], savePixelColor[1], savePixelColor[2]);
    }

    NSLog( @"******* 9" );


    //--- at the end !!! -------

    // Restore OpenGL state that pickTerrainEffect changed
    glBindFramebuffer(GL_FRAMEBUFFER, 0); // default frame buffer
    //glViewport(0, 0, _width, _height); // full area of glView

//#ifdef DEBUG
    {  // Report any errors
        GLenum error = glGetError();
        if(GL_NO_ERROR != error)
        {
            NSLog(@"GL Error: 0x%x", error);
        }
    }
//#endif

    NSLog( @"******* 10" );


    return 0;

}


//tap-11
-(void) buildFBO
{
    NSLog(@"before: buildFBO._pickFBO=%i", _pickFBO );

    if ( 0 == _pickFBO )
    {

        NSLog(@"buildFBO._pickFBO=%i", _pickFBO );

        GLuint colorRenderbuffer;
        //GLuint framebuffer;

        glGenFramebuffers(1, &_pickFBO);
        glBindFramebuffer(GL_FRAMEBUFFER, _pickFBO);

        glGenRenderbuffers(1, &colorRenderbuffer);
        glBindRenderbuffer(GL_RENDERBUFFER, colorRenderbuffer);
        glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8_OES, _width, _height);
        glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER,  colorRenderbuffer);

        GLuint depthRenderbuffer;
        glGenRenderbuffers(1, &depthRenderbuffer);
        glBindRenderbuffer(GL_RENDERBUFFER, depthRenderbuffer);
        glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, _width, _height);
        glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthRenderbuffer);

        if(glCheckFramebufferStatus(GL_FRAMEBUFFER) !=
           GL_FRAMEBUFFER_COMPLETE)
        {
            NSLog(@"failed to make complete framebuffer object %x", glCheckFramebufferStatus(GL_FRAMEBUFFER));
            //+++tbd+++UtilityPickTerrainEffectDestroyFBO(fboName);
            return;
        }

        //#ifdef DEBUG
        //    {  // Report any errors
        GLenum error = glGetError();
        if(GL_NO_ERROR != error)
        {
            NSLog(@"GL Error: 0x%x", error);
        }
        //    }
        //#endif
    }
}

Answer 1

I think you need to close the texture out before reading from it. My color picker implementation is working fine. But I remember that for about 2 days I was also getting only 0,0,0,0.

Here's a clue for you. Is the pixel you're getting (0,0,0,0) or (0,0,0,1)? Cause you only mention that it's (0,0,0). Since you have a clearColor to (0,0,0,1), if you're getting an alpha of zero it's not reading the buffer at all.

I recommend doing it in 2 steps. One method to draw the color picker. Another to read from it. here's my exact code for reading my color buffer:

-(void)process3DTouch
{    
  if (colorPickerNeedsRedrawn) {
    [self drawColorPicker];
  }
  glBindFramebuffer(GL_FRAMEBUFFER, pickerFramebuffer);  // speed note - will bind twice if we just had to draw it
  glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, textureInfo[TEX_PICKER].texture, 0);

  Byte pixelColor[4] = {0,0,0,0};
  glReadPixels(colorPickerTouchPosition.x * backingScale, (backingHeight - (colorPickerTouchPosition.y * backingScale)), 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixelColor);
  ColorSpec pickerColor = ColorSpecMake((float)pixelColor[0]/255, (float)pixelColor[1]/255, (float)pixelColor[2]/255, (float)pixelColor[3]/255);

  //NSLog(@"what is pixelColor at %f,%f = %f %f %f %f", colorPickerTouchPosition.x, colorPickerTouchPosition.y, pickerColor.r,pickerColor.g,pickerColor.b,pickerColor.a);

  if (pickerColor.a == 0.0) {
    whatTouched=TOUCHED_NOTHING;
    whatSecondaryTouched=TOUCHED_NOTHING;
    NSLog(@"touched nothing");
    processColorPicker=false;
    return;
  }
  // now look up the item index from the color
  int itemIndex = [self itemIndexFromColor:pickerColor];

  [self handleTouchedItem: itemIndex];

  processColorPicker=false;
}

Another good debugging method for color pickers is to draw the color picker and then draw the color picker texture to the screen so you can see if you have something else going on in the drawing of the picker.