Wrapmode clamp still has the white line.

Hi

In my project, I had place 16 png images(no alpha) align into a 4x4 grid. All textures are saved as 2048x2048 and place on a plane (made in maya UVed). However, when I compress the texture format into PVRTC the ugly line appear even if I enabled clamp wrapmode or disable mip map. It looks fine if change compression format to RGB 16 bits mode, but the file size is 5x larger. Is there a way to uses the compression but get rid of that line?

Do anyone have any idea how may fix this? Thanks!

Jason.C