MVKSampler: Support LoD bias.

Metal actually supports this, but this is a hidden property of the
`MTLSamplerDescriptor`. It's not part of the public API. Guard our use
of it appropriately.

Docs/Whats_New.md

@@ -21,6 +21,7 @@ Released TBD
 - Add `MVK_USE_METAL_PRIVATE_API` build setting to allow **MoltenVK** to be built with access to _Metal_ private API calls.
 - Add support for `VkPhysicalDeviceFeatures::wideLines` feature when `MVK_USE_METAL_PRIVATE_API` is enabled in a **MoltenVK** build.
 - Add support for the `VkPhysicalDeviceFeatures::logicOp` feature when `MVK_USE_METAL_PRIVATE_API` is enabled in a **MoltenVK** build.
+- Add support for the `VkPhysicalDevicePortabilitySubsetFeaturesKHR::samplerMipLodBias` feature when `MVK_USE_METAL_PRIVATE_API` is enabled in a **MoltenVK** build.
 - Fix potential crash when using multi-planar images.
 - Ensure buffers available for buffer addresses in push constants.
 - Support `libMoltenVK.dylib` for _iOS Simulator_ architecture.

MoltenVK/MoltenVK/GPUObjects/MVKDevice.mm

@@ -366,7 +366,7 @@ void MVKPhysicalDevice::getFeatures(VkPhysicalDeviceFeatures2* features) {
 				portabilityFeatures->multisampleArrayImage = _metalFeatures.multisampleArrayTextures;
 				portabilityFeatures->mutableComparisonSamplers = _metalFeatures.depthSampleCompare;
 				portabilityFeatures->pointPolygons = false;
-				portabilityFeatures->samplerMipLodBias = false;
+				portabilityFeatures->samplerMipLodBias = getMVKConfig().useMetalPrivateAPI;
 				portabilityFeatures->separateStencilMaskRef = true;
 				portabilityFeatures->shaderSampleRateInterpolationFunctions = _metalFeatures.pullModelInterpolation;
 				portabilityFeatures->tessellationIsolines = false;
@@ -2523,6 +2523,10 @@ void MVKPhysicalDevice::initLimits() {
 
 	_properties.limits.maxImageDimension3D = _metalFeatures.maxTextureLayers;
 	_properties.limits.maxImageArrayLayers = _metalFeatures.maxTextureLayers;
+	// Max sum of API and shader values. Bias not publicly supported in API, but can be applied in the shader directly.
+	// The lack of API value is covered by VkPhysicalDevicePortabilitySubsetFeaturesKHR::samplerMipLodBias.
+	// Metal does not specify a limit for the shader value, so choose something reasonable.
+	_properties.limits.maxSamplerLodBias = getMVKConfig().useMetalPrivateAPI ? 16 : 4;
 	_properties.limits.maxSamplerAnisotropy = 16;
 
     _properties.limits.maxVertexInputAttributes = 31;
@@ -2799,11 +2803,6 @@ void MVKPhysicalDevice::initLimits() {
 #endif
 	}
 
-	// Max sum of API and shader values. Bias not supported in API, but can be applied in shader directly.
-	// The lack of API value is covered by VkPhysicalDevicePortabilitySubsetFeaturesKHR::samplerMipLodBias.
-	// Metal does not specify limit for shader value, so choose something reasonable.
-	_properties.limits.maxSamplerLodBias = 4;
-
     _properties.limits.minTexelOffset = -8;
     _properties.limits.maxTexelOffset = 7;
     _properties.limits.minTexelGatherOffset = _properties.limits.minTexelOffset;

MoltenVK/MoltenVK/GPUObjects/MVKImage.mm

@@ -2198,6 +2198,11 @@ MTLSamplerDescriptor* MVKSampler::newMTLSamplerDescriptor(const VkSamplerCreateI
     mtlSampDesc.mipFilter = (pCreateInfo->unnormalizedCoordinates
                              ? MTLSamplerMipFilterNotMipmapped
                              : mvkMTLSamplerMipFilterFromVkSamplerMipmapMode(pCreateInfo->mipmapMode));
+#if MVK_USE_METAL_PRIVATE_API
+	if (getMVKConfig().useMetalPrivateAPI) {
+		mtlSampDesc.lodBiasMVK = pCreateInfo->mipLodBias;
+	}
+#endif
 	mtlSampDesc.lodMinClamp = pCreateInfo->minLod;
 	mtlSampDesc.lodMaxClamp = pCreateInfo->maxLod;
 	mtlSampDesc.maxAnisotropy = (pCreateInfo->anisotropyEnable

MoltenVK/MoltenVK/OS/MTLSamplerDescriptor+MoltenVK.h

@@ -41,4 +41,13 @@
  */
 @property(nonatomic, readwrite) /*MTLSamplerBorderColor*/ NSUInteger borderColorMVK;
 
+/**
+ * Replacement for the lodBias property.
+ *
+ * This property allows support under all OS versions. Delegates to the lodBias
+ * property if it is available. otherwise, returns 0 when read and
+ * does nothing when set.
+ */
+@property(nonatomic, readwrite) float lodBiasMVK;
+
 @end

MoltenVK/MoltenVK/OS/MTLSamplerDescriptor+MoltenVK.m

@@ -21,6 +21,15 @@
 #include "MVKCommonEnvironment.h"
 
 
+#if MVK_USE_METAL_PRIVATE_API
+/** Additional methods not necessarily declared in <Metal/MTLSampler.h>. */
+@interface MTLSamplerDescriptor ()
+
+@property(nonatomic, readwrite) float lodBias;
+
+@end
+#endif
+
 @implementation MTLSamplerDescriptor (MoltenVK)
 
 -(MTLCompareFunction) compareFunctionMVK {
@@ -45,4 +54,15 @@
 #endif
 }
 
+#if MVK_USE_METAL_PRIVATE_API
+-(float) lodBiasMVK {
+	if ( [self respondsToSelector: @selector(lodBias)] ) { return self.lodBias; }
+	return 0.0f;
+}
+
+-(void) setLodBiasMVK: (float) bias {
+	if ( [self respondsToSelector: @selector(setLodBias:)] ) { self.lodBias = bias; }
+}
+#endif
+
 @end

README.md

@@ -329,6 +329,7 @@ included in any of the `make` command-line build commands [mentioned above](#com
 Functionality added with `MVK_USE_METAL_PRIVATE_API` enabled includes:
 - `VkPhysicalDeviceFeatures::wideLines`
 - `VkPhysicalDeviceFeatures::logicOp`
+- `VkPhysicalDevicePortabilitySubsetFeaturesKHR::samplerMipLodBias`
 
 
 <a name="demos"></a>