Hi @bukkideme, thanks for opening this PR!

I spent way too much time looking into this topic 🤪 but I learned something System.Drawing can do that I didn't previously appreciate

https://learn.microsoft.com/en-us/dotnet/desktop/winforms/advanced/how-to-use-a-color-matrix-to-set-alpha-values-in-images?view=netframeworkdesktop-4.8

ColorMatrix cm = new() { Matrix33 = (float)Opacity }; // define opacity using a matrix
ImageAttributes att = new();
attr.SetColorMatrix(cm, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);

gfx.DrawImage( // draw the heatmap using the custom matrix
        image: BmpHeatmap,
        destRect: destRect,
        srcX: 0,
        srcY: 0,
        BmpHeatmap.Width,
        BmpHeatmap.Height,
        GraphicsUnit.Pixel,
        attr);

This is a shortcut that lets us control global opacity without actually calculating any Color values. By adding an Opacity property (not passing it into functions) we can keep the original API the same and also not add any logic to check whether multiple types of opacity settings are being used. I'll update this PR to use this simpler method and merge it in shortly 🚀