opengl iOS glkit顏色選擇

Question

它是關於使用拾色挑選3d對象的。 我將網格物體（更新：駐留在靜態頭文件中，並在同一項目中的屏幕上很好地顯示）繪制到新的屏幕外幀緩沖區，然后使用glReadPixels標識用戶是否觸摸了它。

遵循Erik M.Buck的《 Learning OpenGLES for iOS》一書中opengles_ch10_1項目中的代碼之后，我編寫了以下代碼，該代碼始終以nslog的方式輸出，以拾取（通過點擊）像素的顏色為0,0,0。

我的代碼是：

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

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

        int tt = [self findMeshByPoint:tapLocation];

        //NSLog( @"tap value: %i", tt );
    }
}

- (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];

    glBindVertexArrayOES(0);

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

    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;

    }
    else
    {
        glBindBuffer(GL_ARRAY_BUFFER,

                     _glVertexAttributeBufferID);
    }

    //glEnableVertexAttribArray(TETerrainPositionAttrib);

    glEnableVertexAttribArray( GLKVertexAttribPosition );

     if(0 == _program)
     {
            [self loadShadersWithName:@"UtilityPickTerrainShader"];

            [self buildFBO];

            NSAssert(0 != _program,

                     @"prepareOpenGL failed to load shaders");
        }

        glUseProgram(_program);

        // Pre-calculate the mvpMatrix

        GLKMatrix4 modelViewProjectionMatrix2 =

        GLKMatrix4Multiply(

                           self.effect.transform.projectionMatrix,

                           self.effect.transform.modelviewMatrix);

        // Standard matrices

        glUniformMatrix4fv(uniforms[UtilityPickTerrainMVPMatrix], 1, 0,

                           modelViewProjectionMatrix2.m);

//        glUniform2fv(uniforms[UtilityPickTerrainDimensionFactors], 1,

//                     &_temp1 ); //self.factors.v);  //I removed this from the shaders

        glUniform1f(uniforms[UtilityPickTerrainModelIndex],

                    1.0f ); // self.modelIndex / 255.0f);

    glBindFramebuffer(GL_FRAMEBUFFER, _pickFBO);

    glViewport(0, 0, 512, 512);

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    //glEnableVertexAttribArray(GLKVertexAttribPosition);

    glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 12 * sizeof(GLfloat), 0);

    NSLog( @"******* 7" );

    [self.effect prepareToDraw];

    glDrawArrays(GL_TRIANGLES, 0, sizeof(parparit51OBJVertices) / sizeof(Vertex));

    NSLog( @"******* 7.5" );

    const GLfloat width = [glView drawableWidth];

    const GLfloat height = [glView drawableHeight];

    NSAssert(0 < width && 0 < height, @"Invalid drawble size");

    // Get info for picked location

    const GLKVector2 scaledProjectionPosition = {

        point.x / width,

        point.y / height

    };

    NSLog( @"******* 8" );

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

    GLint readLocationX = MIN((512 - 1),

                              (512 - 1) * scaledProjectionPosition.x);

    GLint readLocationY = MIN((512 - 1),

                              (512 - 1) * scaledProjectionPosition.y);

    glReadPixels(readLocationX,

                 readLocationY,

                 1,

                 1,

                 GL_RGBA,

                 GL_UNSIGNED_BYTE,

                 pixelColor);


    NSLog(@"pixelColor[0]=%i, pixelColor[1]=%i, pixelColor[2]=%i", pixelColor[0], pixelColor[1], pixelColor[2] );


    // Restore OpenGL state that pickTerrainEffect changed

    glBindFramebuffer(GL_FRAMEBUFFER, 0); // default frame buffer

    glViewport(0, 0, width, height); // full area of glView

    return 0;

}

-(void) buildFBO
{
    if ( 0 == _pickFBO )
    {

        GLuint colorTexture;

        // Create a texture object to apply to model

        glGenTextures(1, &colorTexture);

        glBindTexture(GL_TEXTURE_2D, colorTexture);

        // Set up filter and wrap modes for this texture object

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S,
                        GL_CLAMP_TO_EDGE);

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T,
                        GL_CLAMP_TO_EDGE);

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER,
                        GL_LINEAR);

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,
                        GL_LINEAR_MIPMAP_LINEAR);

        // Allocate a texture image we can render into

        // Pass NULL for the data parameter since we don't need to

        // load image data. We will be generating the image by

        // rendering to this texture.

        glTexImage2D(GL_TEXTURE_2D,

                     0,

                     GL_RGBA,

                     512,

                     512,

                     0,

                     GL_RGBA,

                     GL_UNSIGNED_BYTE,

                     NULL);

        GLuint depthRenderbuffer;

        glGenRenderbuffers(1, &depthRenderbuffer);

        glBindRenderbuffer(GL_RENDERBUFFER, depthRenderbuffer);

        glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16,

                              512, 512);

        glGenFramebuffers(1, &_pickFBO);

        glBindFramebuffer(GL_FRAMEBUFFER, _pickFBO);

        glFramebufferTexture2D(GL_FRAMEBUFFER,

                               GL_COLOR_ATTACHMENT0,

                               GL_TEXTURE_2D, colorTexture, 0);

        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

您的錯誤是您正在嘗試訪問一個Null指針-您幾乎可以肯定將一個從未真正初始化過的數組傳遞給GLKEffect或通過bufferData調用將其提供給OpenGL，因為該錯誤發生在prepareToDraw或glDrawArrays內部。 盡管我無法確認其中的任何一個，但我還是可以看到幾種解釋，因為相關信息是如何分配在perpareToDraw或glDrawArrays中使用的數據。

首先是“ parparit51OBJVertices”可能會在堆上動態分配（您正在調用malloc之類？如何指定數組的大小），在這種情況下，對sizeof的調用將返回錯誤的值（我認為0 ），這可能會導致EXEC_BAD_ACESS。

另一個似乎令人懷疑的部分是，您正在調用glDrawArrays並傳入頂點數，但是在此之前，您將VBO綁定到索引-充其量是浪費的，因為glDrawArrays將忽略索引。 更糟糕的是，如果您試圖繪制一個OBJ（正如變量名所暗示的那樣），那么可能根本就沒有相當多的幾何圖形根本沒有繪制，因此，即使您解決了EXEC_BAD_ACCESS問題，您仍然會得到不好的結果。

Answer 2

我發生了BAD_ACCESS錯誤。 我通過刪除處理索引的代碼解決了這個問題：

if(0 == _indexBufferID)
{
  // Indices haven't been sent to GPU yet
  // Create an element array buffer for mesh indices
    glGenBuffers(1, &_indexBufferID);
    NSAssert(0 != _indexBufferID,
             @"Failed to generate element array buffer");

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexBufferID);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER,
                 sizeof( parparit51OBJIndices ),
                 parparit51OBJIndices,
                 GL_STATIC_DRAW);
}
else
{
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexBufferID);
}

現在，我仍然需要了解為什么上面更新的代碼對於任何拾取的像素總是輸出0,0,0。