vo_opengl: implement compute shader based EWA kernel

This performs almost 50% faster on my machine (!!), from 4650μs down to
about 3176μs for ewa_lanczossharp.

It's possible we could use a similar approach to speed up the separable
scalers, although with vastly simpler code. For separable scalers we'd
also have the additional huge benefit of only needing padding in one
direction, so we could potentially use a big 256x1 kernel or something
to essentially compute an entire row at once.

1 files changed, 15 insertions, 1 deletions

diff --git a/video/out/opengl/video.c b/video/out/opengl/video.c
index 76b9d829ab..5a4d17e454 100644
--- a/video/out/opengl/video.c
+++ b/video/out/opengl/video.c
@@ -1755,7 +1755,21 @@ static void pass_sample(struct gl_video *p, struct img_tex tex,
     } else if (strcmp(name, "oversample") == 0) {
         pass_sample_oversample(p->sc, scaler, w, h);
     } else if (scaler->kernel && scaler->kernel->polar) {
-        pass_sample_polar(p->sc, scaler, tex.components, p->gl->glsl_version);
+        // Use a compute shader where possible, fallback to the slower texture
+        // fragment sampler otherwise. Also use the fragment shader for
+        // very large kernels to avoid exhausting shmem
+        if (p->gl->glsl_version < 430 || scaler->kernel->f.radius > 16) {
+            pass_sample_polar(p->sc, scaler, tex.components, p->gl->glsl_version);
+        } else {
+            // For performance we want to load at least as many pixels
+            // horizontally as there are threads in a warp (32 for nvidia), as
+            // well as enough to take advantage of shmem parallelism
+            const int warp_size = 32, threads = 256;
+            compute_size_minimum(p, warp_size, threads / warp_size);
+            pass_compute_polar(p->sc, scaler, tex.components,
+                               p->compute_w, p->compute_h,
+                               (float)w / tex.w, (float)h / tex.h);
+        }
     } else if (scaler->kernel) {
         pass_sample_separated(p, tex, scaler, w, h);
     } else {