It was used just for certain corner cases that weren't covered by
{EXT,OES}_draw_elements_base_vertex already, but because of a wrongly
understood extension interaction the EXT/OES multidraw entrypoint (which
isn't implemented on ANGLE) was used from these as well.
And while trying to disable the two extensions for the MeshGLTest I
discovered the test half-expects the ANGLE variant to be implemented and
so I finished it for both the single-draw and multi-draw case.
The extension interaction that makes base-vertex multidraw calls broken
on ANGLE will be fixed in the following commit.
Yes, this is only ever provided by an ANGLE extension, and as you might
have guessed, The Great Google Overlords didn't bother caring to have
this supported outside of their browser.
Since the main point of the low-level API is to make use of the
EXT_multi_draw_arrays / ANGLE_multi_draw / WEBGL_multi_draw extensions
for a reduced driver overhead *and* the builtin gl_DrawID variable, it
just doesn't make sense to provide a fallback to draw() in a loop. When
the extensions are not available, the user has to do extra work in order
to supply proper UBO draw offset for each draw call, and thus the
fallback path has basically no practical use.
On the other hand, draw() taking the MeshView instances is kept as-is --
this one is rather old and so breaking compatibility would be an
unnecessary pain point. Plus, since it has to allocate the contiguous
arrays internally, it's not desirable for any fast-path rendering
anyway.
It doesn't allow anything except full RGBA and 32-bit types to be read
from the framebuffer. Chromium browsers weren't complaining but Firefox
did. I remember a similar restriction from GLES2 but all implementations
I tested with including ANGLE and SwiftShader allow that already, so
it's practically just a WebGL limitation.
This doesn't actually fix the WebGL-specific error (there it
additionally disallows mismatched signedness in a vertex attribute) but
is nevertheless good to have correct.
Unlike most other extensions, this one has to be explicitly enabled in
Emscripten in order to be used. Which thus done as part of other "driver
workarounds" done on startup. To avoid that, the extension can be
explicitly disabled, and thanks to the previous commit the disabling
will be performed before the extension is attempted to be enabled.
Because a MeshView might not be the best thing to have when you are
submitting a batch of thousand draws. It takes a strided array views to
allow for more flexibility, but can also detect if the input is already
contiguous and use it as-is.
UNFORTUNATELY the GL 1.0 legacy still continues to stink and so there
has to be a 64-bit-specific overload which is the *actual* variant that
doesn't allocate because glMultiDrawElements takes a `void**` for INDEX
OFFSETS and it's IN BYTES! Which foolish soul designed such a thing back
in the 1860s, I wonder. There's no reason to not have an index offset
in elements because all indices have to have the same type ANYWAY. And
yes, I wasted about three hours debugging driver crashes because I
THOUGHT this parameter takes offset in elements, not bytes.
Also note: on 32-bit platforms this depends on latest Corrade with the
CORRADE_TARGET_32BIT definition. Spent an embarrassing amount of time
wondering why all local builds but Emscripten work.
Or, the horror when I comment-out half of it and it still kinda passes.
The new code tests it more thoroughly, with actually checking that
different draw produce different results instead of one draw overwriting
the other.
Plus also verifying interaction with gl_VertexID and gl_DrawID to ensure
it really behaves correctly:
- gl_VertexID should get the same value regardless of whether we
perform a multi-draw or a one-by-one fallback, so this catches issues
where the mesh would get reconfigured in some nasty way, losing this
information
- gl_DrawID should get properly populated if and only if we use the
multi-draw code path, so this catches issues where the fallback would
get used instead of the actual multi-draw code path
Vladimír Vondruš