Shader for overlaying textures

Hello! I need some help. I try to understand the way shaders work and how to apply it to my problem.

Here is my problem:
I have a dark NGUI texture. I want to draw it with transparent areas in fixed positions and with fixed sizes. I try to resolve this with overlaying textures and custom shader. But I can’t find a proper shader for this purpose. I start to learn ShaderLab but it seems to me, that it needs much time.

Does anyone had such problem and managed to find an easy solution? I’d really appreciate any help on this issue.

Thanks for any advice.

You can determine the screen space position of a pixel in the fragment shader using screenPos in a surface shader or the ComputeScreenPos macro.

There’s examples here and here.