Client: Fix alpha compositing for translucent water surfaces

After some investigation, it's become clear that the reference client seems to incorrectly mix sRGB and linear-space colors. While this is a bit of a guess, it would explain why the contrast correction was needed in the first place; it makes up for the underexposure introduced by the lack of gamma-decompression steps after loading sRGB-encoded textures. Indeed, screen blending is used for this exact purpose in digital photo editing (apparently). At least that's the best explanation I could come up with so far.

In order to reproduce the resulting outputs, neither sRGB nor linearized colors can be used. The "in-between"  method used so far, however, trips up the alpha blend stage while the underlying texture view is in sRGB format: WebGPU automatically converts the fragment colors - first to sRGB and then back to linear as part of the blend stage. Unfortunately, the color space used is actually neither of the two by this point, and so the results can't possibly match expectations unless a non-sRGB output format is used.

Originally, sRGB was selected because it's the preferred surface format. Using a different one grants full control of the color space used and eliminates the undesirable conversion, but may incur a performance hit. However, conversions between RGBA and sRGB should be hardware-accelerated on any reasonably modern device. Switching to RGBA also simplifies the shader logic as the added linearization step is no longer needed. So overall it's probably a net gain? Either way, for now that's a secondary concern.

Author: rdw-software

Core/NativeClient/Renderer.lua

@@ -99,10 +99,7 @@ local Renderer = {
 
 function Renderer:InitializeWithGLFW(nativeWindowHandle)
 	Renderer:CreateGraphicsContext(nativeWindowHandle)
-
-	-- Need to compute the preferred texture format first
-	self.backingSurface:UpdateConfiguration()
-	Renderer:CompileMaterials(self.backingSurface.preferredTextureFormat)
+	Renderer:CompileMaterials(self.backingSurface.textureFormat)
 
 	Renderer:CreateUniformBuffers()
 
@@ -134,6 +131,8 @@ function Renderer:CreateGraphicsContext(nativeWindowHandle)
 
 	printf("Creating depth buffer with texture dimensions %d x %d", viewportWidth, viewportHeight)
 	self.depthStencilTexture = DepthStencilTexture(device, viewportWidth, viewportHeight)
+
+	self.backingSurface:UpdateConfiguration()
 end
 
 function Renderer:CompileMaterials(outputTextureFormat)

Core/NativeClient/WebGPU/Pipelines/GroundMeshDrawingPipeline.lua

@@ -46,6 +46,11 @@ function GroundMeshDrawingPipeline:Construct(wgpuDeviceHandle, textureFormatID)
 							dstFactor = ffi.C.WGPUBlendFactor_OneMinusSrcAlpha,
 							operation = ffi.C.WGPUBlendOperation_Add,
 						},
+						alpha = {
+							srcFactor = ffi.C.WGPUBlendFactor_One,
+							dstFactor = ffi.C.WGPUBlendFactor_OneMinusSrcAlpha,
+							operation = ffi.C.WGPUBlendOperation_Add,
+						},
 					}),
 					writeMask = ffi.C.WGPUColorWriteMask_All,
 				}),

Core/NativeClient/WebGPU/Pipelines/WaterPlaneDrawingPipeline.lua

@@ -46,6 +46,11 @@ function WaterPlaneDrawingPipeline:Construct(wgpuDeviceHandle, textureFormatID)
 							dstFactor = ffi.C.WGPUBlendFactor_OneMinusSrcAlpha,
 							operation = ffi.C.WGPUBlendOperation_Add,
 						},
+						alpha = {
+							srcFactor = ffi.C.WGPUBlendFactor_One,
+							dstFactor = ffi.C.WGPUBlendFactor_OneMinusSrcAlpha,
+							operation = ffi.C.WGPUBlendOperation_Add,
+						},
 					}),
 					writeMask = ffi.C.WGPUColorWriteMask_All,
 				}),

Core/NativeClient/WebGPU/RenderTargets/ScreenshotCaptureTexture.lua

@@ -10,7 +10,7 @@ local Device = require("Core.NativeClient.WebGPU.Device")
 local format = string.format
 
 local ScreenshotCaptureTexture = {
-	OUTPUT_TEXTURE_FORMAT = ffi.C.WGPUTextureFormat_RGBA8UnormSrgb,
+	OUTPUT_TEXTURE_FORMAT = ffi.C.WGPUTextureFormat_RGBA8Unorm,
 }
 
 function ScreenshotCaptureTexture:Construct(wgpuDevice, width, height)

Core/NativeClient/WebGPU/Shaders/TerrainGeometryShader.wgsl

@@ -167,10 +167,5 @@ fn fs_main(in: VertexOutput) -> @location(0) vec4f {
 
 	// Should be a no-op if fog is disabled, since the fogFactor would be zero
 	let foggedColor = mix(fragmentColor.rgb, uPerSceneData.fogColor.rgb, in.fogFactor);
-
-	// Gamma-correction:
-	// WebGPU assumes that the colors output by the fragment shader are given in linear space
-	// When setting the surface format to BGRA8UnormSrgb it performs a linear to sRGB conversion
-	let gammaCorrectedColor = pow(foggedColor.rgb, vec3f(2.2));
-	return vec4f(gammaCorrectedColor, diffuseTextureColor.a + DEBUG_ALPHA_OFFSET);
+	return vec4f(foggedColor.rgb, diffuseTextureColor.a + DEBUG_ALPHA_OFFSET);
 }

Core/NativeClient/WebGPU/Shaders/WaterSurfaceShader.wgsl

@@ -170,9 +170,5 @@ fn fs_main(in: VertexOutput) -> @location(0) vec4f {
 	let materialColor = vec4f(uMaterialInstanceData.diffuseRed, uMaterialInstanceData.diffuseGreen, uMaterialInstanceData.diffuseBlue, uMaterialInstanceData.materialOpacity);
 	let finalColor = in.color * diffuseTextureColor.rgb * materialColor.rgb;
 
-	// Gamma-correction:
-	// WebGPU assumes that the colors output by the fragment shader are given in linear space
-	// When setting the surface format to BGRA8UnormSrgb it performs a linear to sRGB conversion
-	let gammaCorrectedColor = pow(finalColor.rgb, vec3f(2.2));
-	return vec4f(gammaCorrectedColor, diffuseTextureColor.a * materialColor.a + DEBUG_ALPHA_OFFSET);
+	return vec4f(finalColor.rgb, diffuseTextureColor.a * materialColor.a + DEBUG_ALPHA_OFFSET );
 }

Core/NativeClient/WebGPU/Surface.lua

@@ -16,6 +16,7 @@ local Surface = {
 		BACKING_SURFACE_OUTDATED = "The backing surface is outdated and can't be used (window resized or moved?)",
 		BACKING_SURFACE_TIMEOUT = "The backing surface couldn't be accessed in time (CPU or GPU too busy?)",
 	},
+	textureFormat = ffi.C.WGPUTextureFormat_BGRA8Unorm,
 }
 
 function Surface:Construct(wgpuInstance, wgpuAdapter, wgpuDevice, glfwWindow)
@@ -33,23 +34,16 @@ function Surface:Construct(wgpuInstance, wgpuAdapter, wgpuDevice, glfwWindow)
 end
 
 function Surface:UpdateConfiguration()
-	local preferredTextureFormat = webgpu.bindings.wgpu_surface_get_preferred_format(self.wgpuSurface, self.wgpuAdapter)
-	self.preferredTextureFormat = preferredTextureFormat -- Required to create the render pipeline
-	assert(
-		preferredTextureFormat == ffi.C.WGPUTextureFormat_BGRA8UnormSrgb,
-		"Only sRGB texture formats are currently supported"
-	)
-
 	local textureViewDescriptor = self.wgpuTextureViewDescriptor
 	textureViewDescriptor.dimension = ffi.C.WGPUTextureViewDimension_2D
-	textureViewDescriptor.format = preferredTextureFormat
+	textureViewDescriptor.format = self.textureFormat
 	textureViewDescriptor.mipLevelCount = 1
 	textureViewDescriptor.arrayLayerCount = 1
 	textureViewDescriptor.aspect = ffi.C.WGPUTextureAspect_All
 
 	local surfaceConfiguration = self.wgpuSurfaceConfiguration
 	surfaceConfiguration.device = self.wgpuDevice
-	surfaceConfiguration.format = preferredTextureFormat
+	surfaceConfiguration.format = self.textureFormat
 	surfaceConfiguration.usage = ffi.C.WGPUTextureUsage_RenderAttachment
 
 	-- The underlying framebuffer may be different if DPI scaling is applied, but let's ignore that for now
@@ -62,12 +56,14 @@ function Surface:UpdateConfiguration()
 
 	webgpu.bindings.wgpu_surface_configure(self.wgpuSurface, surfaceConfiguration)
 
+	local preferredTextureFormat = webgpu.bindings.wgpu_surface_get_preferred_format(self.wgpuSurface, self.wgpuAdapter)
 	printf(
 		"Surface configuration changed: Frame buffer size is now %dx%d (preferred texture format: %d)",
 		viewportWidth,
 		viewportHeight,
 		tonumber(preferredTextureFormat)
 	)
+	self.preferredTextureFormat = preferredTextureFormat
 end
 
 function Surface:AcquireTextureView()

Core/NativeClient/WebGPU/Texture.lua

@@ -10,7 +10,7 @@ local new = ffi.new
 local math_floor = math.floor
 
 local Texture = {
-	DEFAULT_TEXTURE_FORMAT = ffi.C.WGPUTextureFormat_RGBA8Unorm, -- TBD: WGPUTextureFormat_BGRA8UnormSrgb ?
+	DEFAULT_TEXTURE_FORMAT = ffi.C.WGPUTextureFormat_RGBA8Unorm,
 	MAX_TEXTURE_DIMENSION = 4096,
 	ERROR_DIMENSIONS_NOT_POWER_OF_TWO = "Texture dimensions should always be a power of two",
 	ERROR_DIMENSIONS_EXCEEDING_LIMIT = "Texture dimensions must not exceed the configured GPU limit",