vo_gpu: split --linear-scaling into two separate options

Since linear downscaling makes sense to handle independently from
linear/sigmoid upscaling, we split this option up. Now,
linear-downscaling is its own option that only controls linearization
when downscaling and nothing more. Likewise, linear-upscaling /
sigmoid-upscaling are two mutually exclusive options (the latter
overriding the former) that apply only to upscaling and no longer
implicitly enable linear light downscaling as well.

The old behavior was very confusing, as evidenced by issues such
as #6213. The current behavior should make much more sense, and only
minimally breaks backwards compatibility (since using linear-scaling
directly was very uncommon - most users got this for free as part of
gpu-hq and relied only on that).

Closes #6213.

video/out/gpu/video.c

@@ -373,8 +373,9 @@ const struct m_sub_options gl_video_conf = {
         SCALER_OPTS("tscale", SCALER_TSCALE),
         OPT_INTRANGE("scaler-lut-size", scaler_lut_size, 0, 4, 10),
         OPT_FLAG("scaler-resizes-only", scaler_resizes_only, 0),
-        OPT_FLAG("linear-scaling", linear_scaling, 0),
         OPT_FLAG("correct-downscaling", correct_downscaling, 0),
+        OPT_FLAG("linear-downscaling", linear_downscaling, 0),
+        OPT_FLAG("linear-upscaling", linear_upscaling, 0),
         OPT_FLAG("sigmoid-upscaling", sigmoid_upscaling, 0),
         OPT_FLOATRANGE("sigmoid-center", sigmoid_center, 0, 0.0, 1.0),
         OPT_FLOATRANGE("sigmoid-slope", sigmoid_slope, 0, 1.0, 20.0),
@@ -423,6 +424,8 @@ const struct m_sub_options gl_video_conf = {
         OPT_REPLACED("opengl-fbo-format", "fbo-format"),
         OPT_REPLACED("opengl-dumb-mode", "gpu-dumb-mode"),
         OPT_REPLACED("opengl-gamma", "gamma-factor"),
+        OPT_REMOVED("linear-scaling", "Split into --linear-upscaling and "
+                    "--linear-downscaling"),
         {0}
     },
     .size = sizeof(struct gl_video_opts),
@@ -2332,13 +2335,18 @@ static void pass_scale_main(struct gl_video *p)
 
     // Pre-conversion, like linear light/sigmoidization
     GLSLF("// scaler pre-conversion\n");
-    bool use_linear = p->opts.linear_scaling || p->opts.sigmoid_upscaling;
+    bool use_linear = false;
+    if (downscaling) {
+        use_linear = p->opts.linear_downscaling;
 
-    // Linear light downscaling results in nasty artifacts for HDR curves due
-    // to the potentially extreme brightness differences severely compounding
-    // any ringing. So just scale in gamma light instead.
-    if (mp_trc_is_hdr(p->image_params.color.gamma) && downscaling)
-        use_linear = false;
+        // Linear light downscaling results in nasty artifacts for HDR curves
+        // due to the potentially extreme brightness differences severely
+        // compounding any ringing. So just scale in gamma light instead.
+        if (mp_trc_is_hdr(p->image_params.color.gamma))
+            use_linear = false;
+    } else if (upscaling) {
+        use_linear = p->opts.linear_upscaling || p->opts.sigmoid_upscaling;
+    }
 
     if (use_linear) {
         p->use_linear = true;
@@ -3494,9 +3502,9 @@ static bool check_dumb_mode(struct gl_video *p)
         return false;
 
     // otherwise, use auto-detection
-    if (o->target_prim || o->target_trc || o->linear_scaling ||
-        o->correct_downscaling || o->sigmoid_upscaling || o->interpolation ||
-        o->blend_subs || o->deband || o->unsharp)
+    if (o->target_prim || o->target_trc || o->correct_downscaling ||
+        o->linear_downscaling || o->linear_upscaling || o->sigmoid_upscaling ||
+        o->interpolation || o->blend_subs || o->deband || o->unsharp)
         return false;
     // check remaining scalers (tscale is already implicitly excluded above)
     for (int i = 0; i < SCALER_COUNT; i++) {
@@ -3652,8 +3660,11 @@ static void check_gl_features(struct gl_video *p)
                   p->opts.target_trc != MP_CSP_TRC_AUTO || p->use_lut_3d;
 
     // mix() is needed for some gamma functions
-    if (!have_mglsl && (p->opts.linear_scaling || p->opts.sigmoid_upscaling)) {
-        p->opts.linear_scaling = false;
+    if (!have_mglsl && (p->opts.linear_downscaling ||
+                        p->opts.linear_upscaling || p->opts.sigmoid_upscaling))
+    {
+        p->opts.linear_downscaling = false;
+        p->opts.linear_upscaling = false;
         p->opts.sigmoid_upscaling = false;
         MP_WARN(p, "Disabling linear/sigmoid scaling (GLSL version too old).\n");
     }

video/out/gpu/video.h

@@ -112,8 +112,9 @@ struct gl_video_opts {
     float tone_mapping_param;
     float tone_mapping_desat;
     int gamut_warning;
-    int linear_scaling;
     int correct_downscaling;
+    int linear_downscaling;
+    int linear_upscaling;
     int sigmoid_upscaling;
     float sigmoid_center;
     float sigmoid_slope;