Getting the overlapping pixels between a UI mask and a UI image

Hey there, I'm not sure if I fully understand what you're wanting - but to me it sounds like you have a mask image which you want to use and cut out only what is relevant from the user image?

Here is some code that may help - you would need to correctly set up your mask image prior with alpha however this should do the job... Let me know how you go!

 Texture2D userImage;
 Texture2D mask;
         
 // check image size, resize if required
 if (userImage.width != mask.width || userImage.height != mask.height)
 {
     userImage.Resize(mask.width, mask.height);
 }
 
 // Set alpha from mask into userImage        
 for(int i = 0; i < userImage.width * userImage.height; i++)
 {            
     int y = i / mask.width;
     int x = i - (y*mask.width);
     Color col = userImage.GetPixel(x, y);
     col.a = (mask.GetPixel(x, y)).a;
     userImage.SetPixel(x, y, col);
 }

Have you try using a camera with transparent color as clear flag, and render it to a render texture? i guess it should keep the transparency of the ui elements.

 RenderTexture.active = renderTexture;
 texture2D.ReadPixels (new Rect(0,0, renderTexture.width, renderTexture.height),0,0);
 texture2D.Apply ();
 RenderTexture.active = null;