[Bug 101592] Very slow performance when rendering scenes with transparency, probably caused by excessive copying (intel_miptree_map())

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=101592

--- Comment #4 from Tapani Pälli <lemody at gmail.com> ---
(In reply to Steve Holland from comment #3)
> Comparison system running exactly the same application, but running Fedora 25
> mesa-dri-drivers-13.0.4-3.fc25 and older Intel graphics "Haswell-ULT
> Integrated Graphics Controller (rev 0b)" is a bit sluggish (say ~3 fps) at a
> particular rendering operation (transparent text over transparent image).
> Newer system (Mesa 17.1.3, Kaby Lake HD Graphics 620, Fedora 26 beta) on the
> same operation is 0.3 fps. 
> 
> Same operations with software rendering forced (by LIBGL_ALWAYS_SOFTWARE=1)
> are 10+fps on both platforms
> 
> Just rendering the image is fast in all cases, transparent or not.
> Overlaying the text character-by-character with glutBitmapCharacter() seems
> to be what is slowing things down.

My recommendation is to implement text rendering using other methods. It is
quite unlikely that there is interest to optimize legacy glBitmap path. From
the trace it also seems like the implementation of glutBitmapCharacter() is
very unoptimal and might not even cache the font and you need to call it for
each character.

You could speed this phase up significantly for example by using traditional
method of rendering textured quads (that use texture atlas which contains the
font glyphs). This pushes the abstraction down a bit but will definitely be
worth it.

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