[PATCH 05/10] wined3d: Try to avoid glGetTexImage() for 2D array textures.

[Henri Verbeet]

On 19 April 2016 at 12:17, Józef Kucia <joseph.kucia at gmail.com> wrote:
> The issue is if we do not have ARB_get_texture_sub_image we use
> glGet[Compressed]TexImage() to download the data from texture. This
> type of download is implemented in the patch 6. I want to avoid
> glGet[Compressed]TexImage() for array textures because these functions
> don't allow to download the data for a single wined3d surface. They
> allow to download data for the whole miplevel slice. In the patch 6 a
> block of memory is allocated, the whole miplevel slice is downloaded
> and the data for a single layer is copied to the current wined3d
> surface.
>
> The idea in this patch is to simulate glGetTextureSubImage() using
> reads from the framebuffer. An array array texture is attached to
> framebuffer using glFramebufferTextureLayer() and the data for a
> single wined3d surface is downloaded using glReadPixels(). This works
> in my d3d11 tests which I plan to submit in the following patchset.

Well, yes. But it's not up to surface_download_data() to decide
whether to use GetTexImage() or ReadPixels(). surface_load_sysmem()
should decide if it's more appropriate to use surface_download_data()
or read_from_framebuffer().

That said, I'm not sure read_from_framebuffer() is necessarily an
improvement in terms of (overall) performance. True, GetTexImage()
reads more data than it needs to, but I don't think it's a given that
that's much more expensive than reading a smaller amount of data
unless the texture is particularly large. In cases where you're going
to access the other sub-resources afterwards anyway it may even be an
improvement, provided resource locations etc. are properly updated. On
the other hand, read_from_framebuffer() calls
context_apply_blit_state(), which does a lot of state invalidation. If
multiple of those are batched together that may be ok (but in that
case perhaps surface_download_data() would be too), but if they're
interleaved with draws it may not be an improvement.

I.e., it would be a lot easier to justify this if you could show an
application where it helps.