Compute between two textures with different size

I think i have to update my questions again.

Actually, it seems that i need to calculate between two textures with different size. The details are listed below;

The first Texture, _imageTexture, is generated from the original image; The second texture, _maskTexture, is generated from the subpart of the original image and some computation has been done to this texture later;

I need to use the second Texture, _maskTexture, to update the corresponding area in the first texture, _imageTexture. How could i do this? I’m really puzzled.