Metal Core Image Filter does not Compile

Question

In an application, I am using a Metal filter to remove the background from a playing video. Here is the filter's complete code:

Code Block Metal#include <metal_stdlib>
#include <CoreImage/CoreImage.h> // includes CIKernelMetalLib.h
extern "C" {
    namespace coreimage {
        float4 alphaFrame(sampler source, sampler mask) {
            float4 color = source.sample(source.coord());
            float opacity = mask.sample(mask.coord()).r;
            return float4(color.rgb, opacity);
        }
    }
}

The problem is that, when compiling in Xcode 12 for iOS 14, two errors pop up about this file preventing the build from compiling: "/air-lld:1:1: symbol(s) not found for target 'air64-apple-ios13.0.0'" and "air-lld command failed with exit code 1 (use -v to see invocation)". If I comment out the filter, the build is able to compile as normal. How can I use this filter in my app and have it compile as well?

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Posted by

ggogis

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Answer 1

Thank you for the detailed question.

It looks like your build does not use the required -fcikernel compile option and -cikernel link option. You can set up the project to include those with the instructions provided in WWDC20 session, "Build Metal-based Core Image kernels with Xcode". For its first custom build rule, please use -I MTL_HEADER_SEARCH_PATHS only if your build setting for MTL_HEADER_SEARCH_PATHS is non-empty.

Also note that, despite use of namespace, all functions marked extern "C" link as if in a common namespace. You would not be able to have any other extern "C" function with same name, "alphaFrame". Your particular syntax puts your function in a system namespace, which may result in other name collisions, as the project grows.

The following version, without namespace, compiles correctly using the directions from the session, sans -I:

Code Block #include <metal_stdlib>
#include <CoreImage/CoreImage.h> // includes CIKernelMetalLib.h
extern "C" {
		float4 alphaFrame(coreimage::sampler source, coreimage::sampler mask) {
				float4 color = source.sample(source.coord());
				float opacity = mask.sample(mask.coord()).r;
				return float4(color.rgb, opacity);
		}
}

Posted by

Graphics and Games Engineer

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Answer 2

Thank you for the detailed question.

It looks like your build does not use the required -fcikernel compile option and -cikernel link option. You can set up the project to include those with the instructions provided in WWDC20 session, "Build Metal-based Core Image kernels with Xcode". For its first custom build rule, please use -I MTL_HEADER_SEARCH_PATHS only if your build setting for MTL_HEADER_SEARCH_PATHS is non-empty.

Also note that, despite use of namespace, all functions marked extern "C" link as if in a common namespace. You would not be able to have any other extern "C" function with same name, "alphaFrame". Your particular syntax puts your function in a system namespace, which may result in other name collisions, as the project grows.

The following version, without namespace, compiles correctly using the directions from the session, sans -I:

Code Block #include <metal_stdlib>
#include <CoreImage/CoreImage.h> // includes CIKernelMetalLib.h
extern "C" {
		float4 alphaFrame(coreimage::sampler source, coreimage::sampler mask) {
				float4 color = source.sample(source.coord());
				float opacity = mask.sample(mask.coord()).r;
				return float4(color.rgb, opacity);
		}
}

Posted by

Graphics and Games Engineer

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Answer 3

It's also worth noting that there is a built-in filter that does exactly that: CIBlendWithRedMask

Uses values from a mask image to interpolate between an image and the background. When a mask red value is 0.0, the result is the background. When the mask red value is 1.0, the result is the image.

Posted by

FrankSchlegel

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Metal Core Image Filter does not Compile

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