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float4 EncodeRGBM(float3 in)
{ 
    float4 rgbm; 
    rgbm.a = max3(in.r, in.g, in.b) / RGBM_Range;
    rgbm.rgb = in / (rgbm.a * RGBM_Range);
    return rgbm;
}

float3 DecodeRGBM(float4 sample)
{ 
    const float RGBM_Range = 6.0f;
    float scale = sample.a * RGBM_Range;
    return sample.rgb * scale;
}

// Remap the X and Y channels to red and green channels for normal maps 
compressed with ASTC.
 
MTLTextureDescriptor* descriptor = [[MTLTextureDescriptor alloc] init];

MTLTextureSwizzle r = MTLTextureSwizzleRed;
MTLTextureSwizzle g = MTLTextureSwizzleAlpha;
MTLTextureSwizzle b = MTLTextureSwizzleZero;
MTLTextureSwizzle a = MTLTextureSwizzleZero;

descriptor.swizzle = MTLTextureSwizzleChannelsMake( r, g, b, a );

// Reconstruct z-axis from normal sample in shader code.

float3 ReconstructNormal(float2 sample)
{
    float3 normal;

    normal.xy = sample.xy * 2.0f - 1.0f;
    normal.z  = sqrt( saturate( 1.0f - dot( normal.xy, normal.xy ) ) );

    return normal;
}