Shader.metal

#include <metal_stdlib>
using namespace metal;
#include <SceneKit/scn_metal>

#define GREEN_SCREEN_R 71.0/255.0
#define GREEN_SCREEN_G 255.0/255.0
#define GREEN_SCREEN_B 21.0/255.0

struct VertexInput {
    float3 position  [[attribute(SCNVertexSemanticPosition)]];
    float2 texCoords [[attribute(SCNVertexSemanticTexcoord0)]];
};

struct NodeBuffer {
    float4x4 modelViewProjectionTransform;
};

typedef struct {
    float4 renderedCoordinate [[position]];
    float2 textureCoordinate;
} TextureMappingVertex;


vertex TextureMappingVertex mapTexture(VertexInput          in       [[ stage_in ]],
                                       constant NodeBuffer& scn_node [[ buffer(0) ]]) {
    TextureMappingVertex out;
    out.renderedCoordinate = scn_node.modelViewProjectionTransform * float4(in.position, 1.0);
    out.textureCoordinate = in.texCoords;
    
    return out;
}

//float4 over( in vec4 a, in vec4 b )
//{
//    return a + b*(1.0-a.w);
//}

fragment half4 displayTexture(TextureMappingVertex mappingVertex [[ stage_in ]],
                              texture2d<float, access::sample> texture [[ texture(0) ]]) {
    constexpr sampler s(address::clamp_to_edge, filter::linear);

//    return half4(1.0);
    
    float4 videoTexture = texture.sample(s, mappingVertex.textureCoordinate);
    videoTexture.a = videoTexture.g;
//    greenColor.sample(s, mappingVertex.textureCoordinate)
//    float4 diff = videoTexture - greenColor;

    return half4(videoTexture);
}