feat(postfx): afegir push constants i efectes chromatic aberration + scanlines

- PostFXUniforms struct (vignette_strength, chroma_strength, scanline_strength, time)
- Shader MSL actualitzat: aberració cromàtica RGB + scanlines sin-wave + vinyeta paramètrica
- Pipeline postfx declara num_uniform_buffers=1 (buffer(0) en MSL)
- Engine acumula temps i fa SDL_PushGPUFragmentUniformData cada frame
- Valors per defecte: vignette=1.5, chroma=0, scanlines=0 (comportament idèntic a l'anterior)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

source/engine.cpp

@@ -417,6 +417,9 @@ void Engine::calculateDeltaTime() {
 }
 
 void Engine::update() {
+    // Accumulate time for PostFX uniforms
+    postfx_uniforms_.time += delta_time_;
+
     // Actualizar visibilidad del cursor (auto-ocultar tras inactividad)
     Mouse::updateCursorVisibility();
 
@@ -824,6 +827,7 @@ void Engine::render() {
         SDL_BindGPUGraphicsPipeline(pass2, gpu_pipeline_->postfxPipeline());
         SDL_GPUTextureSamplerBinding scene_tsb = {offscreen_tex_->texture(), offscreen_tex_->sampler()};
         SDL_BindGPUFragmentSamplers(pass2, 0, &scene_tsb, 1);
+        SDL_PushGPUFragmentUniformData(cmd, 0, &postfx_uniforms_, sizeof(PostFXUniforms));
         SDL_DrawGPUPrimitives(pass2, 3, 1, 0, 0);
 
         // UI overlay (alpha-blended, uses sprite pipeline)

source/engine.hpp

@@ -163,6 +163,9 @@ class Engine {
         Uint64 last_frame_time_ = 0;
         float delta_time_ = 0.0f;
 
+        // PostFX uniforms (passed to GPU each frame)
+        PostFXUniforms postfx_uniforms_ = {1.5f, 0.0f, 0.0f, 0.0f};
+
         // Sistema de zoom dinámico
         int current_window_zoom_ = DEFAULT_WINDOW_ZOOM;
 

source/gpu/gpu_pipeline.cpp

@@ -85,7 +85,12 @@ vertex PostVOut postfx_vs(uint vid [[vertex_id]]) {
 
 // ---------------------------------------------------------------------------
 // PostFX fragment shader
-// Samples the offscreen scene texture and applies a subtle vignette.
+// Effects driven by PostFXUniforms (uniform buffer slot 0):
+//   - Chromatic aberration: RGB channel UV offset
+//   - Scanlines: sin-wave intensity modulation
+//   - Vignette: radial edge darkening
+// MSL binding for fragment uniform buffer 0 with 1 sampler, 0 storage:
+//   constant PostFXUniforms& u [[buffer(0)]]
 // ---------------------------------------------------------------------------
 static const char* kPostFXFragMSL = R"(
 #include <metal_stdlib>
@@ -96,14 +101,32 @@ struct PostVOut {
     float2 uv;
 };
 
-fragment float4 postfx_fs(PostVOut         in    [[stage_in]],
-                           texture2d<float> scene [[texture(0)]],
-                           sampler          samp  [[sampler(0)]]) {
-    float4 color = scene.sample(samp, in.uv);
+struct PostFXUniforms {
+    float vignette_strength;
+    float chroma_strength;
+    float scanline_strength;
+    float time;
+};
 
-    // Subtle vignette: darkens edges proportionally to distance from centre
+fragment float4 postfx_fs(PostVOut                   in    [[stage_in]],
+                           texture2d<float>           scene [[texture(0)]],
+                           sampler                    samp  [[sampler(0)]],
+                           constant PostFXUniforms&   u     [[buffer(0)]]) {
+    // Chromatic aberration: offset R and B channels horizontally
+    float ca = u.chroma_strength * 0.005;
+    float4 color;
+    color.r = scene.sample(samp, in.uv + float2( ca, 0.0)).r;
+    color.g = scene.sample(samp, in.uv                   ).g;
+    color.b = scene.sample(samp, in.uv - float2( ca, 0.0)).b;
+    color.a = scene.sample(samp, in.uv                   ).a;
+
+    // Scanlines: horizontal sine-wave at ~360 lines (one dark band per 2 px at 720p)
+    float scan = 0.85 + 0.15 * sin(in.uv.y * 3.14159265 * 720.0);
+    color.rgb *= mix(1.0, scan, u.scanline_strength);
+
+    // Vignette: radial edge darkening
     float2 d = in.uv - float2(0.5, 0.5);
-    float vignette = 1.0 - dot(d, d) * 1.5;
+    float vignette = 1.0 - dot(d, d) * u.vignette_strength;
     color.rgb *= clamp(vignette, 0.0, 1.0);
 
     return color;
@@ -215,7 +238,7 @@ bool GpuPipeline::init(SDL_GPUDevice* device,
     SDL_GPUShader* postfx_vert = createShader(device, kPostFXVertMSL, "postfx_vs",
                                               SDL_GPU_SHADERSTAGE_VERTEX, 0, 0);
     SDL_GPUShader* postfx_frag = createShader(device, kPostFXFragMSL, "postfx_fs",
-                                              SDL_GPU_SHADERSTAGE_FRAGMENT, 1, 0);
+                                              SDL_GPU_SHADERSTAGE_FRAGMENT, 1, 1);
     if (!postfx_vert || !postfx_frag) {
         SDL_Log("GpuPipeline: failed to create postfx shaders");
         if (postfx_vert) SDL_ReleaseGPUShader(device, postfx_vert);

source/gpu/gpu_pipeline.hpp

@@ -2,6 +2,18 @@
 
 #include <SDL3/SDL_gpu.h>
 
+// ============================================================================
+// PostFXUniforms — pushed to the fragment stage each frame via
+//   SDL_PushGPUFragmentUniformData(pass, 0, &uniforms, sizeof(PostFXUniforms))
+// MSL binding: constant PostFXUniforms& u [[buffer(0)]]
+// ============================================================================
+struct PostFXUniforms {
+    float vignette_strength;   // 0 = none, 1.5 = default subtle
+    float chroma_strength;     // 0 = off, 1 = full chromatic aberration
+    float scanline_strength;   // 0 = off, 1 = full scanlines
+    float time;                // accumulated seconds (for future animations)
+};
+
 // ============================================================================
 // GpuPipeline — Creates and owns the graphics pipelines used by the engine.
 //
@@ -9,6 +21,7 @@
 //                     Vertex layout: GpuVertex (pos float2, uv float2, col float4).
 //  postfx_pipeline_ : full-screen triangle, no vertex buffer, no blend.
 //                     Reads offscreen texture, writes to swapchain.
+//                     Accepts PostFXUniforms via fragment uniform buffer slot 0.
 // ============================================================================
 class GpuPipeline {
 public: