Update all shaders to treat colours as premultiplied

osu.Game.Resources/Shaders/sh_ArgonBarPath.fs

@@ -31,7 +31,7 @@ lowp vec4 glowColAt(highp float absoluteTexturePos, highp float absoluteGlowPort
     mixValue *= mixValue;
     mixValue *= mixValue;
     mixValue *= mixValue; 
-    return vec4(glowColour.rgb, glowColour.a * mixValue);
+    return glowColour.rgba * mixValue;
 }
 
 lowp vec4 getColour(highp float absoluteTexturePos)

osu.Game.Resources/Shaders/sh_ArgonBarPathBackground.fs

@@ -21,13 +21,19 @@ layout(location = 0) out vec4 o_Colour;
 lowp vec4 bgColAt(highp float absoluteTexturePos, mediump float pathRadius)
 {
     highp float relativeTexturePos = clamp(absoluteTexturePos / pathRadius, 0.0, 1.5);
-    return mix(vec4(vec3(0.0), 0.2), vec4(vec3(1.0), 0.8), relativeTexturePos / 1.5);
+    vec4 result = mix(vec4(0.2), vec4(vec3(1.0), 0.8), relativeTexturePos / 1.5);
+    result = vec4(result.rgb * result.a, result.a);
+    return result;
 }
 
 lowp vec4 getColour(highp float absoluteTexturePos, mediump float pathRadius)
 {
     if (absoluteTexturePos > pathRadius - 1.0)
-        return mix(vec4(1.0), vec4(vec3(1.0), 0.0), absoluteTexturePos - (pathRadius - 1.0));
+    {
+        vec4 result = mix(vec4(1.0), vec4(vec3(1.0), 0.0), absoluteTexturePos - (pathRadius - 1.0));
+        result = vec4(result.rgb * result.a, result.a);
+        return result;
+    }
 
     if (absoluteTexturePos > pathRadius - 2.0)
         return mix(bgColAt(absoluteTexturePos, pathRadius), vec4(1.0), absoluteTexturePos - (pathRadius - 2.0));

osu.Game.Resources/Shaders/sh_CircularFlashlight.fs

@@ -6,5 +6,5 @@ lowp vec4 getColourAt(highp vec2 diff, highp vec2 size, lowp vec4 originalColour
     highp float dist = length(diff);
     highp float flashlightRadius = length(size);
 
-    return originalColour * vec4(1.0, 1.0, 1.0, smoothstep(flashlightRadius, flashlightRadius * flashlightSmoothness, dist));
+    return originalColour * vec4(smoothstep(flashlightRadius, flashlightRadius * flashlightSmoothness, dist));
 }

osu.Game.Resources/Shaders/sh_CursorTrail.vs

@@ -22,7 +22,7 @@ void main(void)
     vec3 maskingPos = g_ToMaskingSpace * vec3(m_Position, 1.0);
     v_MaskingPosition = maskingPos.xy / maskingPos.z;
 
-    v_Colour = vec4(m_Colour.rgb, m_Colour.a * pow(clamp(m_Time - g_FadeClock, 0.0, 1.0), g_FadeExponent));
+    v_Colour = m_Colour.rgba * pow(clamp(m_Time - g_FadeClock, 0.0, 1.0), g_FadeExponent);
 
     v_TexCoord = m_TexCoord;
     v_TexRect = m_TexRect;

osu.Game.Resources/Shaders/sh_Flashlight.h

@@ -18,5 +18,7 @@ void main(void)
     // todo: workaround for a SPIR-V bug (https://github.com/ppy/osu-framework/issues/5719)
     float one = g_WrapModeS >= 0 ? 1 : 0;
 
-    o_Colour = mix(getColourAt(flashlightPos - v_Position, flashlightSize, v_Colour), vec4(0.0, 0.0, 0.0, v_Colour.a), flashlightDim) * one;
+    vec4 result = mix(getColourAt(flashlightPos - v_Position, flashlightSize, v_Colour), vec4(vec3(0.0), v_Colour.a), flashlightDim) * one;
+    result = vec4(result.rgb * result.a, result.a);
+    o_Colour = result;
 }

osu.Game.Resources/Shaders/sh_LogoAnimation.fs

@@ -18,7 +18,13 @@ void main(void)
     // todo: workaround for a SPIR-V bug (https://github.com/ppy/osu-framework/issues/5719)
     float one = g_BackbufferDraw ? 1 : 0;
 
-    vec4 colour = texture(sampler2D(m_Texture, m_Sampler), v_TexCoord, -0.9) * one;
+    vec4 texel = texture(sampler2D(m_Texture, m_Sampler), v_TexCoord, -0.9) * one;
 
-    o_Colour = colour.r < progress ? vec4(v_Colour.rgb, v_Colour.a * colour.a) : vec4(0);
+    // in the logo animation textures, progress information is stored in red channel, 
+    // and alpha information is stored in green channel. this is done this way
+    // to avoid alpha pre-multiplication causing data precision loss.
+    float current = texel.r;
+    float alpha = texel.g;
+
+    o_Colour = current < progress ? v_Colour.rgba * alpha : vec4(0);
 }

osu.Game.Resources/Shaders/sh_RectangularFlashlight.fs

@@ -6,5 +6,5 @@ lowp vec4 getColourAt(highp vec2 diff, highp vec2 size, lowp vec4 originalColour
 
     lowp float alpha = length(smoothstep(size, size * flashlightSmoothness, diff));
 
-    return originalColour * vec4(1.0, 1.0, 1.0, alpha);
+    return originalColour * vec4(alpha);
 }

osu.Game.Resources/Shaders/sh_TriangleBorder.fs

@@ -55,5 +55,5 @@ void main(void)
 
     lowp vec4 col = getRoundedColor(vec4(1.0), v_TexCoord);
 
-    o_Colour = vec4(col.rgb, col.a * alpha);
+    o_Colour = col.rgba * alpha;
 }

osu.Game.Resources/Textures/Intro/Triangles/LogoTextureProcessor.cs

@@ -0,0 +1,28 @@
+// Post-process for logo animation path textures, moving alpha information away from the alpha channel
+// for the shader to produce correct animation without losing information from textures being alpha-premultiplied.
+// Input: textures with colours in the form: (x, x, x, y), in which x represents current progress and y represents alpha.
+// Output: textures with colours in the form: (x, y, 0, 255).
+
+// Code is commented to not cause compile errors on osu-resources.
+
+/*
+const string path = "<osu-resources>/osu.Game.Resources/Textures/Intro/Triangles";
+processImage(Path.Combine(path, "logo-background.png"));
+processImage(Path.Combine(path, "logo-highlight.png"));
+
+void processImage(string filename)
+{
+    using var image = Image.Load<Rgba32>(filename);
+
+    image.ProcessPixelRows(p =>
+    {
+        for (int i = 0; i < p.Height; i++)
+        {
+            foreach (ref var pixel in p.GetRowSpan(i))
+                pixel = new Rgba32(pixel.R, pixel.A, 0, 255);
+        }
+    });
+
+    image.SaveAsPng(filename);
+}
+*/