glReadPixels on iOS with multisampling - ios

Question

I'm trying to get pixels from framebuffer with multisampling. It returns only zeros. I do call glResolveMultisampleFramebufferAPPLE as suggested here and here, but I can not figure out whats the problem in my case.
first of all I create non-multisampled framebuffer with color attachment:
GLuint framebuffer, colorRenderbuffer;
glGenFramebuffersOES(1, &framebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
glGenRenderbuffersOES(1, &colorRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_RGBA8_OES, w, h);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, colorRenderbuffer);
then create multisample framebuffer with color and depth attachment:
GLuint sampleFramebuffer, sampleColorRenderbuffer, sampleDepthRenderbuffer;
glGenFramebuffersOES(1, &sampleFramebuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, sampleFramebuffer);
glGenRenderbuffersOES(1, &sampleColorRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_RGBA8_OES, w, h);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
glGenRenderbuffersOES(1, &sampleDepthRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, 4, GL_DEPTH_COMPONENT16_OES, w, h);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);
then clear framebuffers:
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
glClear(GL_COLOR_BUFFER_BIT);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, sampleFramebuffer);
glViewport(0, 0, w, h);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
do my drawing (this is Cocos3D drawing code):
[cc3Layer visit];
then resolve buffers:
glBindFramebufferOES(GL_DRAW_FRAMEBUFFER_APPLE, framebuffer);
glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, sampleFramebuffer);
glResolveMultisampleFramebufferAPPLE();
glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, framebuffer);
and then get all zeros:
glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
I skipped two gl checks for success with creating framebuffer, since they are creating successfully. Where is the error in my code?

Answer 1

You should bind the non-multisample color-renderbuffer BEFORE read pixels.
like that:
glResolveMultisampleFramebufferAPPLE()
glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, framebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

Answer 2

the problem was not in OpenGL, but in Cocos3d rendering (-visit did not set up some properties for drawing, but -drawScene did). Here is the working code:
+(UIImage*) takeScreenshotFromScreenRect:(CGRect)rect withResultSize:(CGSize)outSize
{
CCDirector *director = [CCDirector sharedDirector];
director.nextDeltaTimeZero = YES;
rect.origin.x *= CC_CONTENT_SCALE_FACTOR();
rect.origin.y *= CC_CONTENT_SCALE_FACTOR();
rect.size.width *= CC_CONTENT_SCALE_FACTOR();
rect.size.height *= CC_CONTENT_SCALE_FACTOR();
int w = rect.size.width;
int h = rect.size.height;
int winW = director.winSizeInPixels.width;
int winH = director.winSizeInPixels.height;
GLuint bufferLength = w * h * 4;
GLubyte* buffer = (GLubyte*)malloc(bufferLength);
[director pause];
static GLuint framebuffer = 0, colorRenderbuffer;
static GLuint sampleFramebuffer, sampleColorRenderbuffer, sampleDepthRenderbuffer;
if (framebuffer == 0)
{
glGenFramebuffersOES(1, &framebuffer);
glGenRenderbuffersOES(1, &colorRenderbuffer);
glGenFramebuffersOES(1, &sampleFramebuffer);
glGenRenderbuffersOES(1, &sampleColorRenderbuffer);
glGenRenderbuffersOES(1, &sampleDepthRenderbuffer);
}
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, colorRenderbuffer);
glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_RGBA8_OES, winW, winH);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, colorRenderbuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, sampleFramebuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, director.openGLView.pixelSamples, GL_RGBA8_OES, winW, winH);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, sampleColorRenderbuffer);
glBindRenderbufferOES(GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);
glRenderbufferStorageMultisampleAPPLE(GL_RENDERBUFFER_OES, director.openGLView.pixelSamples, GL_DEPTH_COMPONENT16_OES, winW, winH);
glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_DEPTH_ATTACHMENT_OES, GL_RENDERBUFFER_OES, sampleDepthRenderbuffer);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, framebuffer);
glClear(GL_COLOR_BUFFER_BIT);
glClearColor(150.0/255, 190.0/255, 255.0/255, 1);
glBindFramebufferOES(GL_FRAMEBUFFER_OES, sampleFramebuffer);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glClearColor(150.0/255, 190.0/255, 255.0/255, 1);
[director drawScene];
glBindFramebufferOES(GL_DRAW_FRAMEBUFFER_APPLE, framebuffer);
glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, sampleFramebuffer);
glResolveMultisampleFramebufferAPPLE();
glBindFramebufferOES(GL_READ_FRAMEBUFFER_APPLE, framebuffer);
glReadPixels(rect.origin.x, rect.origin.y, w, h, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
GLenum attachments[] = {GL_COLOR_ATTACHMENT0_OES, GL_DEPTH_ATTACHMENT_OES};
glDiscardFramebufferEXT(GL_READ_FRAMEBUFFER_APPLE, 2, attachments);
// restoring render buffers from cocos
ES1Renderer *renderer = [[CCDirector sharedDirector].openGLView valueForKey:#"renderer_"];
glBindFramebuffer(GL_FRAMEBUFFER_OES, renderer.msaaFrameBuffer);
glBindFramebuffer(GL_RENDERBUFFER_OES, renderer.msaaColorBuffer);
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, buffer, bufferLength, NULL);
[director resume];
int bitsPerComponent = 8;
int bitsPerPixel = 32;
int bytesPerRow = 4 * w;
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
CGImageRef iref = CGImageCreate(w, h, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
uint32_t* pixels = (uint32_t*)malloc(bufferLength);
CGContextRef context = CGBitmapContextCreate(pixels, outSize.width, outSize.height, 8, outSize.width * 4, CGImageGetColorSpace(iref), kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGContextTranslateCTM(context, 0, outSize.height);
CGContextScaleCTM(context, 1.0f, -1.0f);
switch (director.deviceOrientation)
{
case CCDeviceOrientationPortrait:
break;
case CCDeviceOrientationPortraitUpsideDown:
CGContextRotateCTM(context, CC_DEGREES_TO_RADIANS(180));
CGContextTranslateCTM(context, -outSize.width, -outSize.height);
break;
case CCDeviceOrientationLandscapeLeft:
CGContextRotateCTM(context, CC_DEGREES_TO_RADIANS(-90));
CGContextTranslateCTM(context, -outSize.height, 0);
break;
case CCDeviceOrientationLandscapeRight:
CGContextRotateCTM(context, CC_DEGREES_TO_RADIANS(90));
CGContextTranslateCTM(context, outSize.width * 0.5f, -outSize.height);
break;
}
CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, outSize.width, outSize.height), iref);
CGImageRef imageFromContext = CGBitmapContextCreateImage(context);
UIImage *outputImage = [UIImage imageWithCGImage:imageFromContext];
CGDataProviderRelease(provider);
CGImageRelease(iref);
CGContextRelease(context);
free(buffer);
free(pixels);
return outputImage;
}