feat: Add Lanczos Frag

src/video_core/host_shaders/present_lanczos.frag

@@ -0,0 +1,86 @@
+// SPDX-FileCopyrightText: Copyright 2025 citron Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#version 460 core
+
+layout(location = 0) in vec2 frag_tex_coord;
+layout(location = 0) out vec4 color;
+
+layout(binding = 0) uniform sampler2D color_texture;
+
+// This block defines the variable that will hold our setting value.
+// It uses a preprocessor directive to switch between the OpenGL and Vulkan way of doing things.
+#ifdef VULKAN
+layout(push_constant) uniform LanczosPushConstant {
+	layout(offset = 128) int u_lanczos_a;
+} lanczos_pc;
+#else // OpenGL
+layout(location = 0) uniform int u_lanczos_a;
+#endif
+
+const float PI = 3.14159265359;
+
+float sinc(float x) {
+	if (x == 0.0) {
+		return 1.0;
+	}
+	return sin(PI * x) / (PI * x);
+}
+
+float lanczos_weight(float x, float a) {
+	if (abs(x) < a) {
+		return sinc(x) * sinc(x / a);
+	}
+	return 0.0;
+}
+
+vec4 textureLanczos(sampler2D ts, vec2 tc) {
+	// Get the 'a' value from the correct uniform based on the renderer
+	#ifdef VULKAN
+	const int a_val = lanczos_pc.u_lanczos_a;
+	#else
+	const int a_val = u_lanczos_a;
+	#endif
+	const float a = float(a_val);
+
+	// If 'a' is 0 (which it will be by default since we aren't sending a value yet),
+	// just do a basic sample to avoid errors.
+	if (a_val == 0) {
+		return texture(ts, tc);
+	}
+
+	vec2 tex_size = vec2(textureSize(ts, 0));
+	vec2 inv_tex_size = 1.0 / tex_size;
+	vec2 p = tc * tex_size;
+	vec2 f = fract(p);
+	vec2 p_int = p - f;
+
+	vec4 sum = vec4(0.0);
+	float weight_sum = 0.0;
+
+	const int MAX_A = 4;
+	for (int y = -MAX_A + 1; y <= MAX_A; ++y) {
+		if (abs(y) >= a_val) continue;
+		for (int x = -MAX_A + 1; x <= MAX_A; ++x) {
+			if (abs(x) >= a_val) continue;
+
+			vec2 offset = vec2(float(x), float(y));
+			float w = lanczos_weight(f.x - offset.x, a) * lanczos_weight(f.y - offset.y, a);
+
+			if (w != 0.0) {
+				sum += texture(ts, (p_int + offset) * inv_tex_size) * w;
+				weight_sum += w;
+			}
+		}
+	}
+
+	if (weight_sum == 0.0) {
+		return texture(ts, tc);
+	}
+
+	return sum / weight_sum;
+}
+
+void main() {
+	color = textureLanczos(color_texture, frag_tex_coord);
+}