Vulkan HDR: unified scanline bloom to prevent colour shifts

Use luminance-based envelope for scanline bloom instead of per-channel
bloom, preventing hue shifts caused by independent beam width calculations
per phosphor channel. Per-channel bloom retained behind #ifdef.

Author: MajorPainTheCactus

gfx/drivers/vulkan_shaders/hdr.frag

@@ -15,8 +15,12 @@ const float kPi                    = 3.1415926536;
 const float kEuler                 = 2.718281828459;
 const float kMax                   = 1.0;
 
+#define UNIFIED_SCANLINE_BLOOM
+
 const float kBeamWidth             = 0.5;
 
+const vec3 kLuminanceWeights       = vec3(0.2126, 0.7152, 0.0722);
+
 const uint kChannelMask           = 3u;
 const uint kFirstChannelShift     = 2u;
 const uint kSecondChannelShift    = 4u;
@@ -249,19 +253,42 @@ vec3 ScanlineColour(const vec2 tex_coord,
 
    vec3 result = vec3(0.0);
 
+   float beam_width_adjustment      = (kBeamWidth / scanline_size);
+   float raw_distance               = abs(distance_to_line) - beam_width_adjustment;
+   float distance_adjusted          = max(0.0, raw_distance);
+   float effective_distance         = distance_adjusted * 2.0;
+
+#ifdef UNIFIED_SCANLINE_BLOOM
+   /* Unified bloom: compute a single horizontal interpolation from luminance
+    * so the scanline envelope is shared across all channels in the phosphor
+    * triad, preventing colour shifts from per-channel bloom differences. */
+   float lum_0                      = dot(signal_0, kLuminanceWeights);
+   float lum_1                      = dot(signal_1, kLuminanceWeights);
+   float lum_horiz_interp           = Bezier(narrowed_source_pixel_offset, BeamControlPoints(beam_attack, lum_0 > lum_1));
+   float beam_intensity             = clamp(mix(lum_0, lum_1, lum_horiz_interp), 0.0, 1.0);
+
+   float beam_width                 = mix(scanline_min, scanline_max, beam_intensity);
+   float scanline_dist              = clamp(effective_distance / beam_width, 0.0, 1.0);
+   vec4 control_points              = vec4(1.0, 1.0, beam_intensity * scanline_attack, 0.0);
+   float envelope                   = Bezier(scanline_dist, control_points);
+
+   for(int ch = 0; ch < 3; ch++)
+   {
+      float horiz_interp            = Bezier(narrowed_source_pixel_offset, BeamControlPoints(beam_attack, signal_0[ch] > signal_1[ch]));
+      float signal_channel          = mix(signal_0[ch], signal_1[ch], horiz_interp);
+
+      result[ch] = envelope * signal_channel;
+   }
+#else
+   /* Per-channel bloom: each channel computes its own scanline envelope based on
+    * its individual brightness (models independent electron gun bloom). */
    for(int ch = 0; ch < 3; ch++)
    {
       float horiz_interp               = Bezier(narrowed_source_pixel_offset, BeamControlPoints(beam_attack, signal_0[ch] > signal_1[ch]));
       float signal_channel             = mix(signal_0[ch], signal_1[ch], horiz_interp);
 
       float signal_strength            = clamp(signal_channel, 0.0, 1.0);
 
-      float beam_width_adjustment      = (kBeamWidth / scanline_size);
-      float raw_distance               = abs(distance_to_line) - beam_width_adjustment;
-      float distance_adjusted          = max(0.0, raw_distance);
-
-      float effective_distance         = distance_adjusted * 2.0;
-
       float beam_width                 = mix(scanline_min, scanline_max, signal_strength);
 
       float channel_scanline_distance  = clamp(effective_distance / beam_width, 0.0f, 1.0f);
@@ -271,6 +298,7 @@ vec3 ScanlineColour(const vec2 tex_coord,
 
       result[ch] = luminance * signal_channel;
    }
+#endif
 
    return result;
 }