The n_steps variable corresponds to the number of *pairs* of
texture lookups that the blur shader does. For example, when
n_steps = 1, the for-loop reads 1 pixel before and 1 pixel
after the current one.
Our blur shader is heavily inspired in WebRender's blur shader,
the biggest difference being that we calculate the gaussian
samples in the fragment shader itself, and not in the vertex
shader. (This could be an improvement in performance for the
future though!)
WebRender's blur shader calculates n_steps differently than what
we currently do, though. It calculates n_step in such a way that
at least 2 steps are performed for evey non-zero sigma value.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1646>
The shader already operates on floating point sigma, and
there's just no reason for us to force it to be an unsigned
integer. It's still important that sigma must be positive
though.
Make sigma a float, and make sure it's a positive number.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1646>
Yet another way to reduce the instruction count of the fragment
shader. Passing a pair of floats once is virtually free, compared
to computing horizontal and vertical on each fragment run.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1646>
Setting an ortho projection gives us pretty much the same result as
manually calculating the projection matrix. The ortho projection is
actually more "complete" than the custom projection we've been using,
as it also considers z-near and z-far, but in practice the generated
pixels are exactly equal.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1642>
