measuring image brighness

[burner-]

what would be most efficient way to measure image brighness?
I need some kind classification number or even better if I can get sum of pixel color values in each color channel.

[JimBobSquarePants]

The most common brightness supposition is Luma . We have pixel formats specifically designed to represent an image luma as calculated in accordance with the ITU-R Recommendation BT.709 specification.

Given an Rgba32 image you can convert and calculate the average luma of an image in the following manner.

long luma = 0;
double averageLuma = 0;
using (var image = Image<Rgba32>.Load(...))
{
    // Use memory pooling to allocate a buffer the length of one row 
    // to house our converted luma values.
    Configuration configuration = image.GetConfiguration();
    MemoryAllocator allocator = configuration.MemoryAllocator;
    using (IMemoryOwner<L8> lumaBuffer = allocator.Allocate<L8>(image.Width))
    {
        // Convert one row at a time adding the sum of each row to our total. 
        Span<L8> lumaSpan = lumaBuffer.Memory.Span;
        for (int y = 0; y < image.Height; y++)
        {
            Span<Rgba32> rowSpan = image.GetPixelRowSpan(y);
            PixelOperations<Rgba32>.Instance.ToL8(configuration, rowSpan, lumaSpan);

            // This loop could be vectorized for maximum performance.
            for (int x = 0; x < lumaSpan.Length; x++)
            {
                luma += lumaSpan[x].PackedValue;
            }
        }
    }

    // Finally calculate the average luma.
    averageLuma = luma / (double)(image.Width * image.Height);
}

If you don't need the image for any other operation you can skip the conversion part and use Image<L8>.Load(...) directly.

[antonfirsov]

I would declare luma as long, to make sure it doesn't overflow for large images.

[JimBobSquarePants]

Good catch. Will update. 👍