chroma min/max amb mostreig bilinear subpíxel

source/core/rendering/sdl3gpu/msl/postfx_frag.msl.h

@@ -22,7 +22,7 @@ struct PostVOut {
 
 struct PostFXUniforms {
     float vignette_strength;
-    float chroma_strength;
+    float chroma_min;
     float scanline_strength;
     float screen_height;
     float mask_strength;
@@ -32,7 +32,7 @@ struct PostFXUniforms {
     float pixel_scale;
     float time;
     float flicker;
-    float pad2;
+    float chroma_max;
     // vec4 #3 — paràmetres de scanlines (exposats per preset YAML)
     float scan_dark_ratio;
     float scan_dark_floor;
@@ -40,6 +40,18 @@ struct PostFXUniforms {
     float pad3;
 };
 
+// Mostreig bilinear horitzontal d'un canal RGB. Evita el "tic-tac" del sampler
+// NEAREST quan l'offset de chroma és subpíxel.
+static float sampleBilinearX(float2 uv_target, int channel, texture2d<float> scene, sampler samp) {
+    float2 tex_size = float2(scene.get_width(), scene.get_height());
+    float px = uv_target.x * tex_size.x - 0.5f;
+    float p_floor = floor(px);
+    float f = px - p_floor;
+    float4 c0 = scene.sample(samp, float2((p_floor + 0.5f) / tex_size.x, uv_target.y));
+    float4 c1 = scene.sample(samp, float2((p_floor + 1.5f) / tex_size.x, uv_target.y));
+    return mix(c0[channel], c1[channel], f);
+}
+
 static float3 rgb_to_ycc(float3 rgb) {
     return float3(
          0.299f*rgb.r + 0.587f*rgb.g + 0.114f*rgb.b,
@@ -94,9 +106,14 @@ fragment float4 postfx_fs(PostVOut                 in    [[stage_in]],
         colour = base;
     }
 
-    float ca = u.chroma_strength * 0.005f * (1.0f + 0.15f * sin(u.time * 7.3f));
-    colour.r = scene.sample(samp, uv + float2(ca, 0.0f)).r;
-    colour.b = scene.sample(samp, uv - float2(ca, 0.0f)).b;
+    // Chroma — varia entre chroma_min i chroma_max via sinusoidal; si min == max
+    // queda estàtic. Mostreig bilinear horitzontal per evitar el "tic-tac" del
+    // NEAREST sampler amb offsets subpíxel.
+    if (u.chroma_min > 0.0f || u.chroma_max > 0.0f) {
+        float ca = mix(u.chroma_min, u.chroma_max, 0.5f + 0.5f * sin(u.time * 7.3f)) * 0.005f;
+        colour.r = sampleBilinearX(uv + float2(ca, 0.0f), 0, scene, samp);
+        colour.b = sampleBilinearX(uv - float2(ca, 0.0f), 2, scene, samp);
+    }
 
     if (u.gamma_strength > 0.0f) {
         float3 lin = pow(colour, float3(2.4f));

source/core/rendering/sdl3gpu/sdl3gpu_shader.cpp

@@ -234,7 +234,8 @@ namespace Rendering {
     void SDL3GPUShader::setPostFXParams(const PostFXParams& params) {
         uniforms_.vignette_strength = params.vignette;
         uniforms_.scanline_strength = params.scanlines;
-        uniforms_.chroma_strength = params.chroma;
+        uniforms_.chroma_min = params.chroma_min;
+        uniforms_.chroma_max = params.chroma_max;
         uniforms_.mask_strength = params.mask;
         uniforms_.gamma_strength = params.gamma;
         uniforms_.curvature = params.curvature;

source/core/rendering/sdl3gpu/sdl3gpu_shader.hpp

@@ -15,7 +15,7 @@
 struct PostFXUniforms {
     // vec4 #0
     float vignette_strength;
-    float chroma_strength;
+    float chroma_min;
     float scanline_strength;
     float screen_height;
     // vec4 #1
@@ -27,7 +27,7 @@ struct PostFXUniforms {
     float pixel_scale;
     float time;
     float flicker;
-    float pad2;
+    float chroma_max;
     // vec4 #3 — paràmetres de scanlines exposats al preset YAML
     float scan_dark_ratio;
     float scan_dark_floor;
@@ -92,7 +92,7 @@ namespace Rendering {
 
         PostFXUniforms uniforms_{
             .vignette_strength = 0.0F,
-            .chroma_strength = 0.0F,
+            .chroma_min = 0.0F,
             .scanline_strength = 0.0F,
             .screen_height = 0.0F,
             .mask_strength = 0.0F,
@@ -102,7 +102,7 @@ namespace Rendering {
             .pixel_scale = 1.0F,
             .time = 0.0F,
             .flicker = 0.0F,
-            .pad2 = 0.0F,
+            .chroma_max = 0.0F,
             // Defaults canònics: 2/3 brillant + 1/3 fosca, brillantor 42%, suavitzat complet (1 px físic).
             .scan_dark_ratio = 0.333F,
             .scan_dark_floor = 0.42F,

source/core/rendering/shader_backend.hpp

@@ -19,7 +19,10 @@ namespace Rendering {
     struct PostFXParams {
         float vignette{0.0F};
         float scanlines{0.0F};
-        float chroma{0.0F};
+        // Aberració cromàtica — varia entre min i max via sinusoidal; si coincideixen
+        // queda estàtica. Permet un "floor" perquè la imatge mai sigui lliure de chroma.
+        float chroma_min{0.0F};
+        float chroma_max{0.0F};
         float mask{0.0F};
         float gamma{0.0F};
         float curvature{0.0F};