[Mesa-dev] Spec interpretation question: glBlitFramebuffer behavior when src rectangle out of range

[Ian Romanick]

On 05/14/2012 07:30 AM, Paul Berry wrote:
> I'm trying to figure out how glBlitFramebuffer() is supposed to behave
> when the source rectangle exceeds the bounds of the read framebuffer.
> For example, if the read framebuffer has width 4 and height 1, and the
> draw framebuffer has width 100 and height 100, what should be the result
> of these two calls?
>
> glBlitFramebuffer(-1, 0, 3, 1, 0, 0, 9, 1, GL_COLOR_BUFFER_BIT, GL_NEAREST);
> glBlitFramebuffer(-1, 0, 3, 1, 0, 0, 9, 1, GL_COLOR_BUFFER_BIT, GL_LINEAR);
>
> (In other words, the source rect is 4 pixels wide with 1 pixel falling
> off the left hand edge of the read framebuffer, and the destination rect
> is 9 pixels wide with all pixels falling within the draw framebuffer).
>
>
> Here is the relevant text from the spec (e.g. GL 4.2 core spec, p316):
>
> "The actual region taken from the read framebuffer is limited to the
> intersection of the source buffers being transferred, which may include
> the color buffer selected by the read buffer, the depth buffer, and/or
> the stencil buffer depending on mask. The actual region written to the
> draw framebuffer is limited to the intersection of the destination
> buffers being written, which may include multiple draw buffers, the
> depth buffer, and/or the stencil buffer depending on mask. Whether or
> not the source or destination regions are altered due to these limits,
> the scaling and offset applied to pixels being transferred is performed
> as though no such limits were present."

This is trying to describe the case where buffers attached to the source 
FBO have different sizes.  If the color buffer is 64x64, the depth 
buffer is 32x32, and the selected buffers is GL_COLOR_BUFFER_BIT | 
GL_DEPTH_BUFFER_BIT, then the source region is treated as though it's 
32x32.  If the color buffer is 64x64, the depth buffer is 32x32, and the 
selected buffers is only GL_COLOR_BUFFER_BIT, then the source region is 
treated as though it's 64x64.

> And then later:
>
> "If a linear filter is selected and the rules of LINEAR sampling would
> require sampling outside the bounds of a source buffer, it is as though
> CLAMP_TO_EDGE texture sampling were being performed. If a linear filter
> is selected and sampling would be required outside the bounds of the
> specified source region, but within the bounds of a source buffer, the
> implementation may choose to clamp while sampling or not."

The last sentence here is also about the mismatched buffer size 
scenario.  If there are two color buffers attached to the source FBO, 
one being 32x32 and the other being 64x64, a blit of

	glBlitFramebuffer(0, 0, 64, 64, 0, 0, 128, 128,
			GL_COLOR_BUFFER_BIT,
			GL_LINEAR);

would sample outside the 32x32 intersection region of the source 
buffers.  However, one of the color buffers has pixel data outside that 
region.  The implementation may or may not sample those pixels.

> The behaviour I observe on my nVidia system is: in GL_NEAREST mode,
> destination pixels that map to a source pixel outside the read
> framebuffer are clipped out of the blit, and are left unchanged.  So, in

So, the source framebuffer has a single 32x32 color buffer, the blit

	glBlitFramebuffer(0, 0, 64, 64, 0, 0, 128, 128,
			GL_COLOR_BUFFER_BIT,
			GL_LINEAR);

only modifies the destination pixels (0, 0) - (64, 64)?

While I can see that as being a valid design choice, it's not the one 
the ARB made.  I can't find anything in the spec, even going back to 
GL_EXT_framebuffer_blit, to support this behavior.

> the GL_NEAREST call above, the first two destination pixels are left
> unchanged.  In GL_LINEAR mode, the same set of pixels is clipped off as
> in GL_LINEAR mode, and the remaining pixels are interpolated as though
> no clipping had occurred, with CLAMP_TO_EDGE behaviour occurring for
> situations where linear interpolation would have required reading a
> non-existent pixel from the read framebuffer.  Notably, this means that
> the nVidia driver is not simply reducing the size of the source and
> destination rectangles to eliminate the clipped off pixels, because
>
> glBlitFramebuffer(-1, 0, 3, 1, 0, 0, 9, 1, GL_COLOR_BUFFER_BIT, GL_LINEAR);
>
> does *not* produce equivalent interpolation to
>
> glBlitFramebuffer(0, 0, 3, 1, 2, 0, 9, 1, GL_COLOR_BUFFER_BIT, GL_LINEAR);
>
>
> Mesa, on the other hand, never clips.  The behaviour of destination
> pixels that map to a source pixel outside the read framebuffer depends
> on whether the read framebuffer is backed by a texture or a
> renderbuffer.  If it's backed by a texture, then those pixels are
> rendered with CLAMP_TO_EDGE behaviour, regardless of whether the blit is
> GL_LINEAR or GL_NEAREST.  If it's backed by a renderbuffer, then garbage
> is written to those pixels.

I can't find anything in the spec to support writing garbage either. 
Any idea what the garbage is?  I'm a little surprised that the behavior 
is different for textures and renderbuffers.

> Any opinions on what the correct behaviour is?