寫入然後從iPhone上的屏幕外FBO讀取;在模擬器上工作，但不在設備上？

Question

我想在iPhone上做一些圖像處理，基於來自Apple的GLImageProcessing示例。

最終，我想要做的是將圖像加載到紋理中，在示例代碼中執行一個或多個操作（色調，飽和度，亮度等），然後再讀取結果圖像用於以後的處理/保存。大多數情況下，這絕不需要觸摸屏幕，所以我認爲FBO可能是一條走向。首先，我已經拼湊了一個創建屏幕外FBO的小例子，繪製它，然後將數據作爲圖像讀出。在模擬器中完美工作時，我很興奮，然後因爲意識到自己剛剛在實際設備上出現黑屏而感到沮喪。

聲明：我的OpenGL已經夠老了，我已經學習了很多OpenGL ES的學習曲線，而且我從來沒有做過很多紋理嚮導。我知道該設備在幀緩衝區訪問方面具有與模擬器不同的特性（必須在屏幕外FBO和設備交換，在模擬器上直接訪問），但我一直無法找到我一直在做錯的事情，即使經過相當廣泛的搜索。

有什麼建議嗎？

// set up the offscreen FBO sizes 

int renderBufferWidth = 1280; 
int renderBufferHeight = 720; 

// now the FBO 
GLuint fbo = 0; 
glGenFramebuffersOES(1, &fbo); 
glBindFramebufferOES(GL_FRAMEBUFFER_OES, fbo); 

GLuint renderBuffer = 0; 
glGenRenderbuffersOES(1, &renderBuffer); 
glBindRenderbufferOES(GL_RENDERBUFFER_OES, renderBuffer); 
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, 
         GL_RGBA8_OES, 
         renderBufferWidth, 
         renderBufferHeight); 

glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, 
          GL_COLOR_ATTACHMENT0_OES, 
          GL_RENDERBUFFER_OES, 
          renderBuffer); 

GLenum status = glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES); 
if (status != GL_FRAMEBUFFER_COMPLETE_OES) { 
    NSLog(@"Problem with OpenGL framebuffer after specifying color render buffer: %x", status); 
} 

// throw in a test drawing 
glClearColor(0.5f, 0.5f, 0.5f, 1.0f); 
glClear(GL_COLOR_BUFFER_BIT); 


static const GLfloat triangleVertices[] = { 
    -0.5f, -0.33f, 
    0.5f, -0.33f, 
    -0.5f, 0.33f 
}; 

static const GLfloat triangleColors[] = { 
    1.0, 0.0, 0.0, 0.5, 
    0.0, 1.0, 0.0, 0.5, 
    0.0, 0.0, 1.0, 0.5 
}; 

GLint backingWidth = 320; 
GLint backingHeight = 480; 

NSLog(@"setting up view/model matrices"); 
glMatrixMode(GL_PROJECTION); 
glLoadIdentity(); 

glMatrixMode(GL_MODELVIEW); 
glLoadIdentity(); 


glVertexPointer(2, GL_FLOAT, 0, triangleVertices); 
glEnableClientState(GL_VERTEX_ARRAY); 
glColorPointer(4, GL_FLOAT, 0, triangleColors); 
glEnableClientState(GL_COLOR_ARRAY); 



// draw the triangle 
glDrawArrays(GL_TRIANGLE_STRIP, 0, 3); 


// Extract the resulting rendering as an image 
int samplesPerPixel = 4; // R, G, B and A 
int rowBytes = samplesPerPixel * renderBufferWidth; 
char* bufferData = (char*)malloc(rowBytes * renderBufferHeight); 
if (bufferData == NULL) { 
    NSLog(@"Unable to allocate buffer for image extraction."); 
} 

// works on simulator with GL_BGRA, but not on device 
glReadPixels(0, 0, renderBufferWidth, 
      renderBufferHeight, 
      GL_BGRA, 
      GL_UNSIGNED_BYTE, bufferData); 
NSLog(@"reading pixels from framebuffer"); 

// Flip it vertically - images read from OpenGL buffers are upside-down 
char* flippedBuffer = (char*)malloc(rowBytes * renderBufferHeight); 
if (flippedBuffer == NULL) { 
    NSLog(@"Unable to allocate flipped buffer for corrected image."); 
} 

for (int i = 0 ; i < renderBufferHeight ; i++) { 
    bcopy(bufferData + i * rowBytes, 
      flippedBuffer + (renderBufferHeight - i - 1) * rowBytes, 
      rowBytes); 
} 

// unbind my FBO 
glBindFramebufferOES(GL_FRAMEBUFFER_OES, 0); 

// Output the image to a file 


CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); 
int bitsPerComponent = 8; 

CGBitmapInfo bitmapInfo = kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder32Host; 
CGContextRef contextRef = CGBitmapContextCreate(flippedBuffer, 
               renderBufferWidth, 
               renderBufferHeight, 
               bitsPerComponent, 
               rowBytes, colorSpace, bitmapInfo); 
if (contextRef == nil) { 
    NSLog(@"Unable to create CGContextRef."); 
} 

CGImageRef imageRef = CGBitmapContextCreateImage(contextRef); 

if (imageRef == nil) { 
    NSLog(@"Unable to create CGImageRef."); 
} else { 
    if (savedImage == NO) { 
     UIImage *myImage = [UIImage imageWithCGImage:imageRef]; 
     UIImageWriteToSavedPhotosAlbum(myImage, nil, nil, nil); 
     savedImage = YES; 
    } 
} 

編輯：

答案，當然是位圖格式應該是GL_RGBA，不GL_BGRA：

// works on simulator with GL_BGRA, but not on device 
glReadPixels(0, 0, renderBufferWidth, 
     renderBufferHeight, 
     **GL_RGBA**, 
     GL_UNSIGNED_BYTE, bufferData); 

Answer 1

安德魯回答自己：

答案，是位圖格式應該是GL_RGBA，而不是GL_BGRA

// works on simulator with GL_BGRA, but not on device 
glReadPixels(0, 0, renderBufferWidth, 
          renderBufferHeight, 
          GL_RGBA, // <-- 
          GL_UNSIGNED_BYTE, bufferData);