Downsampling Displaybuffer to FBO is Failing to Downsample

[troycorbinz]

Hi all, thanks for looking. 

I am trying to take a screenshot of a Multi-sampled display. From what I've read, I need to blit it to a standard FBO, which will perform the downsample, and then I can render my FBO to a file. Unfortunately, the screenshot is mostly black with several thumbnail like images at the bottom which I assume are the samples. Here is the relevant code:

Creating the FBO:

Buffer = ByteBuffer.allocateDirect(1*4).order(ByteOrder.nativeOrder()).asIntBuffer(); // allocate a 1 int byte buffer
		glGenFramebuffersEXT( Buffer );
		FBOID = Buffer.get();
		glBindFramebufferEXT( GL_FRAMEBUFFER_EXT, FBOID );
		ColorTexture = glGenTextures();
		
		glBindTexture( GL_TEXTURE_2D, ColorTexture );
		glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
		glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, SizeX, SizeY, 0, GL_RGBA, GL_INT, (java.nio.ByteBuffer) null);
		glFramebufferTexture2DEXT(
				GL_FRAMEBUFFER_EXT,
				GL_COLOR_ATTACHMENT0_EXT,
				GL_TEXTURE_2D,
				ColorTexture,
				0);
		
		int error = glCheckFramebufferStatusEXT( GL_FRAMEBUFFER_EXT );

		DepthID = glGenRenderbuffersEXT();
		glBindRenderbufferEXT( GL_RENDERBUFFER_EXT, DepthID );
		glRenderbufferStorageEXT(
				GL_RENDERBUFFER_EXT,
				GL_DEPTH_COMPONENT24,
				SizeX,
				SizeY);
		glFramebufferRenderbufferEXT(
				GL_FRAMEBUFFER_EXT,
				GL_DEPTH_ATTACHMENT_EXT,
				GL_RENDERBUFFER_EXT,
				DepthID);
		
		int error = glCheckFramebufferStatusEXT( GL_FRAMEBUFFER_EXT );

Blitting from the Display Buffer to my FBO:

int width = Display.getDisplayMode().getWidth();
		int height= Display.getDisplayMode().getHeight();
		int bpp = Display.getDisplayMode().getBitsPerPixel();
		FBO buffer = new FBO(width,height); // Create the FBO we'll use for our screenshot
		glBindFramebufferEXT(EXTFramebufferBlit.GL_READ_FRAMEBUFFER_EXT, 0); // Read from the Display Buffer
		glBindFramebufferEXT(EXTFramebufferBlit.GL_DRAW_FRAMEBUFFER_EXT, buffer.getFBO());
		EXTFramebufferBlit.glBlitFramebufferEXT(0, 0, width, height, 0, 0, width, height, GL_COLOR_BUFFER_BIT, GL_NEAREST);
		ByteBuffer bbuffer = BufferUtils.createByteBuffer(width * height * bpp);
		glReadBuffer(buffer.getFBO()); // Assigns the FBOID we got earlier
		glReadPixels(0, 0, width, height, GL11.GL_RGBA, GL11.GL_UNSIGNED_BYTE, bbuffer );

I've tried a number of variations, including rendering the screen directly onto the FBO, but the result is the same. And as you can see, I am not creating the Color Buffer of the FBO with glRenderbufferStorageMultisampleEXT, so it should not be multisampled, correct?

How can I go about downsampling the FBO so that I get a good glReadPixels for a screenshot? Thanks for looking and thanks for any help you can provide. BTW, this is using LWJGL 2.8.5.

[spasi]

You cannot use BlitFramebufferEXT to resolve a window-system provided MSAA framebuffer. The source framebuffer has to be an FBO with multisampled renderbuffer(s) attached. In your case, it'd be best to render to a multisampled FBO, blit to the window framebuffer (which should be single-sampled), then do the ReadPixels from that.

[troycorbinz]

Thanks for the advice. What blit do I need to use to go from the MSAA FBO to the window framebuffer? Or is BlitFrameBufferEXT fine with window framebuffers that are not multisampled?

[spasi]

BlitFrameBufferEXT. It works on window framebuffers, except on multisampled ones. So, bind READ_FRAMEBUFFER_EXT to the multisampled FBO, DRAW_FRAMEBUFFER_EXT to 0 and do the blit.

[troycorbinz]

Got it working. Thanks for all your help!