Apply camera image atifacts + Alternatives

Using the project image method, I made a texture for the front view of my room. When I pprojected the image using the ‘apply camera image’ method, I noticed the image quality of the new texture had square pixely artifacts. Any idea what these are and how to get rid of them?

Also I really like this method of working for my texturing, but from what I have read it is a bit of an old technique. Does any one have any alternatives that you like texturing with.

Texture paint - Stencil mode does pretty much the same.