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That seems awfully complex… Is there a problem with just using Texture2D.SetPixels() to pack your alpha matrix into the alpha channel of your texture?
“Matrix” is not the appropriate word or concept for what you want. You said “2D array”. A texture is a 2D array. That’s exactly what you need.
Also, welcome to the forum, but please give your threads better titles. There are nearly 5,000 topics in this subforum, and I think you’d be hard-pressed to find one of them whose author didn’t want what your title commands, to happen.