vertex shader interpolants are interpolated at pixel centers by the GPU,
but we were doing our own "pixel-center" adjustment, so we ended-up with
"vertex_uv" at a pixel corner instead of center.
with this change, the vertex shader always compute vertex_uv in
fractional texels and the conversion to texture coordinates is done
in the fxaa code.
The half-pixel adjustment is removed.
This leads to sharper looking images because in addition to shifting
everything by 0.5 pixels, this was essentially applying a box-filter
to the whole picture -- kind of like taking 1 mip level down.
* Fix FXAA computations in mediump
UV coordinates computed in highp should be passed to the FXAA function
in highp as well. This change also fixes a potential division by 0 which
was causing dir1 to have components set to inifinity, thus breaking the
texture sampling calls below. We fix this with an early exit when a
potential division by 0 is detected. The original code contained a bias
to try to avoid this problem but that bias was not always enough. It
was frequent in mediump to cancel out the bias.
* Update shaders/src/fxaa.fs
Co-Authored-By: romainguy <romainguy@curious-creature.com>
