[Mesa-dev] [Bug 33934] New: glBlitFramebuffer could have a fast path if there's no scaling

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=33934

           Summary: glBlitFramebuffer could have a fast path if there's no
                    scaling
           Product: Mesa
           Version: git
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: medium
         Component: Mesa core
        AssignedTo: mesa-dev at lists.freedesktop.org
        ReportedBy: neil at linux.intel.com


The Mesa meta implementation of glBlitFramebuffer (which seems to be used by
most of the drivers) performs a render when the source and destination
framebuffers are textures. However I think this function is also commonly used
to copy a region between two textures without scaling. In this case it would be
good if this could boil down to a hardware blit rather than having to submit
geometry.

One way to do this could be to use glCopyTexSubImage2D in the meta code. This
seems to end up being a blit on at least the i965 and Radeon drivers.

This came about because in Clutter we were getting some pressure to use
glBlitFramebuffer instead of glCopyTexSubImage to migrate images between atlas
textures because it is faster on some (non-mesa) drivers. However I noticed
that the opposite is true for Mesa.

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