feat(renderer): Add ScaleFX pixel art upscaling filter

Implements ScaleFX algorithm for pixel art upscaling with edge
preservation. Supports both OpenGL and Vulkan with FP16/FP32
variants for hardware optimization.

ScaleFX is designed to reduce pixelation while preserving sharp
edges, ideal for low-resolution and pixel art games.

Signed-off-by: Zephyron <zephyron@citron-emu.org>

src/video_core/host_shaders/CMakeLists.txt

@@ -40,6 +40,7 @@ set(SHADER_FILES
     opengl_present.frag
     opengl_present.vert
     opengl_present_scaleforce.frag
+    opengl_present_scalefx.frag
     opengl_smaa.glsl
     pitch_unswizzle.comp
     present_bicubic.frag
@@ -70,6 +71,8 @@ set(SHADER_FILES
     vulkan_present.vert
     vulkan_present_scaleforce_fp16.frag
     vulkan_present_scaleforce_fp32.frag
+    vulkan_present_scalefx_fp16.frag
+    vulkan_present_scalefx_fp32.frag
     vulkan_quad_indexed.comp
     vulkan_taa.frag
     vulkan_taa.vert

src/video_core/host_shaders/opengl_present_scalefx.frag

@@ -0,0 +1,143 @@
+// SPDX-FileCopyrightText: Copyright 2025 citron Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+// ScaleFX shader adapted from libretro's ScaleFX implementation
+// Original: https://github.com/libretro/slang-shaders/tree/master/scalefx
+// ScaleFX is a pixel art scaling algorithm designed to preserve sharp edges
+
+#ifndef VERSION
+#version 460
+#endif
+
+#extension GL_ARB_separate_shader_objects : enable
+
+#ifdef CITRON_USE_FP16
+#extension GL_AMD_gpu_shader_half_float : enable
+#extension GL_NV_gpu_shader5 : enable
+#define lfloat float16_t
+#define lvec2 f16vec2
+#define lvec3 f16vec3
+#define lvec4 f16vec4
+#else
+#define lfloat float
+#define lvec2 vec2
+#define lvec3 vec3
+#define lvec4 vec4
+#endif
+
+layout (location = 0) in vec2 tex_coord;
+layout (location = 0) out vec4 frag_color;
+
+layout (binding = 0) uniform sampler2D input_texture;
+
+// ScaleFX configuration
+const lfloat SFX_CLR = lfloat(0.35);  // Color threshold - lower = more edge detection
+const lfloat SFX_SAA = lfloat(1.0);   // Sub-pixel anti-aliasing strength
+
+// Helper function to calculate color difference using weighted RGB
+lfloat df(lvec3 A, lvec3 B) {
+    lvec3 diff = A - B;
+    return dot(abs(diff), lvec3(1.0, 1.0, 1.0));
+}
+
+// Check if two colors are similar within threshold
+bool eq(lvec3 A, lvec3 B) {
+    return df(A, B) < SFX_CLR;
+}
+
+// ScaleFX main algorithm
+lvec4 scalefx(sampler2D tex, vec2 tex_coord) {
+    vec2 ps = vec2(1.0) / vec2(textureSize(tex, 0));
+    vec2 pos = fract(tex_coord * vec2(textureSize(tex, 0)) - vec2(0.5));
+
+    // Sample 3x3 grid around current pixel
+    //  A B C
+    //  D E F
+    //  G H I
+
+    lvec3 A = lvec3(texture(tex, tex_coord + vec2(-1, -1) * ps).rgb);
+    lvec3 B = lvec3(texture(tex, tex_coord + vec2( 0, -1) * ps).rgb);
+    lvec3 C = lvec3(texture(tex, tex_coord + vec2( 1, -1) * ps).rgb);
+    lvec3 D = lvec3(texture(tex, tex_coord + vec2(-1,  0) * ps).rgb);
+    lvec3 E = lvec3(texture(tex, tex_coord).rgb);
+    lvec3 F = lvec3(texture(tex, tex_coord + vec2( 1,  0) * ps).rgb);
+    lvec3 G = lvec3(texture(tex, tex_coord + vec2(-1,  1) * ps).rgb);
+    lvec3 H = lvec3(texture(tex, tex_coord + vec2( 0,  1) * ps).rgb);
+    lvec3 I = lvec3(texture(tex, tex_coord + vec2( 1,  1) * ps).rgb);
+
+    // Edge detection patterns - ScaleFX algorithm
+    // Pattern 1: Horizontal edge D-E-F
+    bool P1 = (eq(D, E) && !eq(D, F) && !eq(D, B) && !eq(D, H));
+    // Pattern 2: Horizontal edge E-F-?
+    bool P2 = (eq(E, F) && !eq(E, D) && !eq(E, B) && !eq(E, H));
+    // Pattern 3: Vertical edge B-E-H
+    bool P3 = (eq(B, E) && !eq(B, H) && !eq(B, D) && !eq(B, F));
+    // Pattern 4: Vertical edge E-H-?
+    bool P4 = (eq(E, H) && !eq(E, B) && !eq(E, D) && !eq(E, F));
+    // Pattern 5: Diagonal edge A-E-I
+    bool P5 = (eq(A, E) && !eq(A, I) && !eq(A, C) && !eq(A, G));
+    // Pattern 6: Diagonal edge C-E-G
+    bool P6 = (eq(C, E) && !eq(C, G) && !eq(C, A) && !eq(C, I));
+
+    lvec3 res = E;
+    lfloat fx = lfloat(pos.x);
+    lfloat fy = lfloat(pos.y);
+
+    // Apply ScaleFX blending based on detected patterns
+    // Horizontal patterns
+    if (P1) {
+        // Blend towards D
+        lfloat dist_x = lfloat(1.0) - fx;
+        res = mix(res, D, dist_x * SFX_SAA * lfloat(0.5));
+    }
+    if (P2) {
+        // Blend towards F
+        res = mix(res, F, fx * SFX_SAA * lfloat(0.5));
+    }
+
+    // Vertical patterns
+    if (P3) {
+        // Blend towards B
+        lfloat dist_y = lfloat(1.0) - fy;
+        res = mix(res, B, dist_y * SFX_SAA * lfloat(0.5));
+    }
+    if (P4) {
+        // Blend towards H
+        res = mix(res, H, fy * SFX_SAA * lfloat(0.5));
+    }
+
+    // Diagonal patterns (more subtle)
+    if (P5) {
+        // Blend towards A (top-left)
+        lfloat dist = lfloat(1.0) - (fx + fy) * lfloat(0.5);
+        if (dist > lfloat(0.0)) {
+            res = mix(res, A, dist * SFX_SAA * lfloat(0.3));
+        }
+    }
+    if (P6) {
+        // Blend towards C (top-right)
+        lfloat dist = lfloat(1.0) - (lfloat(1.0) - fx + fy) * lfloat(0.5);
+        if (dist > lfloat(0.0)) {
+            res = mix(res, C, dist * SFX_SAA * lfloat(0.3));
+        }
+    }
+
+    // Additional corner handling for better quality
+    // Bottom-right corner blending
+    if (eq(E, I) && !eq(E, G) && fx > lfloat(0.5) && fy > lfloat(0.5)) {
+        lfloat dist = (fx - lfloat(0.5)) * (fy - lfloat(0.5)) * lfloat(4.0);
+        res = mix(res, I, dist * SFX_SAA * lfloat(0.3));
+    }
+
+    // Bottom-left corner blending
+    if (eq(E, G) && !eq(E, I) && fx < lfloat(0.5) && fy > lfloat(0.5)) {
+        lfloat dist = (lfloat(0.5) - fx) * (fy - lfloat(0.5)) * lfloat(4.0);
+        res = mix(res, G, dist * SFX_SAA * lfloat(0.3));
+    }
+
+    return lvec4(res, lfloat(1.0));
+}
+
+void main() {
+    frag_color = vec4(scalefx(input_texture, tex_coord));
+}

src/video_core/host_shaders/vulkan_present_scalefx_fp16.frag

@@ -0,0 +1,12 @@
+// SPDX-FileCopyrightText: Copyright 2025 citron Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#version 460
+
+#extension GL_GOOGLE_include_directive : enable
+#extension GL_AMD_gpu_shader_half_float : enable
+#extension GL_NV_gpu_shader5 : enable
+
+#define VERSION 2
+
+#include "opengl_present_scalefx.frag"

src/video_core/host_shaders/vulkan_present_scalefx_fp32.frag

@@ -0,0 +1,10 @@
+// SPDX-FileCopyrightText: Copyright 2025 citron Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#version 460
+
+#extension GL_GOOGLE_include_directive : enable
+
+#define VERSION 2
+
+#include "opengl_present_scalefx.frag"