Merge pull request #71 from Monroe88/gb_palette

Add Game Boy palette shader

handheld/gb-palette-dmg.slangp

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+shaders = 1
+shader0 = shaders/gb-palette/gb-palette.slang	//colorize image
+
+scale_type0 = source
+filter_linear0 = false
+
+textures = COLOR_PALETTE
+COLOR_PALETTE = shaders/gb-palette/resources/palette-dmg.png
+COLOR_PALETTE_linear = false

handheld/gb-palette-light.slangp

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+shaders = 1
+shader0 = shaders/gb-palette/gb-palette.slang	//colorize image
+
+scale_type0 = source
+filter_linear0 = false
+
+textures = COLOR_PALETTE
+COLOR_PALETTE = shaders/gb-palette/resources/palette-light.png
+COLOR_PALETTE_linear = false

handheld/gb-palette-pocket.slangp

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+shaders = 1
+shader0 = shaders/gb-palette/gb-palette.slang	//colorize image
+
+scale_type0 = source
+filter_linear0 = false
+
+textures = COLOR_PALETTE
+COLOR_PALETTE = shaders/gb-palette/resources/palette-pocket.png
+COLOR_PALETTE_linear = false

handheld/shaders/gb-palette/gb-palette.slang

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+#version 450
+
+layout(push_constant) uniform Push
+{
+	vec4 SourceSize;
+	vec4 OriginalSize;
+	vec4 OutputSize;
+	uint FrameCount;
+} params;
+
+layout(std140, set = 0, binding = 0) uniform UBO
+{
+	mat4 MVP;
+} global;
+
+#pragma stage vertex
+layout(location = 0) in vec4 Position;
+layout(location = 1) in vec2 TexCoord;
+layout(location = 0) out vec2 vTexCoord;
+
+void main()
+{
+   gl_Position = global.MVP * Position;
+   vTexCoord = TexCoord * 1.0001;
+}
+
+#pragma stage fragment
+layout(location = 0) in vec2 vTexCoord;
+layout(location = 0) out vec4 FragColor;
+layout(set = 0, binding = 2) uniform sampler2D Source;
+layout(set = 0, binding = 3) uniform sampler2D COLOR_PALETTE;
+
+void main()
+{
+     //sample the input textures
+
+     vec4 out_color = texture(Source, vTexCoord.xy);
+
+     //input grayscale values:
+     //0.000 black - 0.333 medium gray - 0.667 light gray - 1.000 white
+     //multiply grayscale value by 3 to obtain int value in range [0, 3], acts as index for arrays storing custom palette UV coordinates and alpha value
+
+     vec2 palette_coordinate = vec2(0.5, (abs(1 - out_color.r) * 0.75) + 0.125);	//directly map input grayscale value to color palette y coordinate
+     out_color = vec4(texture(COLOR_PALETTE, palette_coordinate).rgb, ceil(abs(1.0 - out_color.r)));	//sample color from palette, alpha = 1 if color is not transparent
+
+     FragColor = out_color;
+}