vo_opengl: implement more HDR tonemapping algorithms

This is now a configurable option, with tunable parameters.

I got inspiration for these algorithms off wikipedia. "simple" seems to
work pretty well, but not well enough to make it a reasonable default.

Some other notable candidates:

- Local functions (e.g. based on local contrast or gradient)
- Clamp with soft knee (linear up to a point)
- Mapping in CIE L*Ch. Map L smoothly, clamp C and h.
- Color appearance models

These will have to be implemented some other time.

Note that the parameter "peak_src" to pass_tone_map should, in
principle, be auto-detected from the SEI information of the source file
where available. This will also have to be implemented in a later
commit.

1 files changed, 40 insertions, 0 deletions

diff --git a/video/out/opengl/video_shaders.c b/video/out/opengl/video_shaders.c
index c63f32ca7d..237e1f39d2 100644
--- a/video/out/opengl/video_shaders.c
+++ b/video/out/opengl/video_shaders.c
@@ -313,6 +313,46 @@ void pass_delinearize(struct gl_shader_cache *sc, enum mp_csp_trc trc)
     }
 }
 
+// Tone map from one brightness to another
+void pass_tone_map(struct gl_shader_cache *sc, float peak_src, float peak_dst,
+                   enum tone_mapping algo, float param)
+{
+    // First we renormalize to the output range
+    float scale = peak_src / peak_dst;
+    GLSLF("color.rgb *= vec3(%f);\n", scale);
+
+    // Then we use some algorithm to map back to [0,1]
+    switch (algo) {
+    case TONE_MAPPING_CLIP:
+        GLSL(color.rgb = clamp(color.rgb, 0.0, 1.0);)
+        break;
+
+    case TONE_MAPPING_SIMPLE: {
+        float contrast = isnan(param) ? 0.5 : param,
+              offset = (1.0 - contrast) / contrast;
+        GLSLF("color.rgb = color.rgb / (color.rgb + vec3(%f));\n", offset);
+        GLSLF("color.rgb *= vec3(%f);\n", (scale + offset) / scale);
+        break;
+    }
+
+    case TONE_MAPPING_GAMMA: {
+        float gamma = isnan(param) ? 1.8 : param;
+        GLSLF("color.rgb = pow(color.rgb / vec3(%f), vec3(%f));\n",
+              scale, 1.0/gamma);
+        break;
+    }
+
+    case TONE_MAPPING_LINEAR: {
+        float coeff = isnan(param) ? 1.0 : param;
+        GLSLF("color.rgb = vec3(%f) * color.rgb;\n", coeff / scale);
+        break;
+    }
+
+    default:
+        abort();
+    }
+}
+
 // Wide usage friendly PRNG, shamelessly stolen from a GLSL tricks forum post.
 // Obtain random numbers by calling rand(h), followed by h = permute(h) to
 // update the state. Assumes the texture was hooked.