PVRTC compressed lightmap color display incorrect

i got a lightmap issue on mac, baked uncompressed lightmap below

while the PVRTC compressed lightmap get bad quality.

how to handle this issue?

are you trying to pack a ~good quality image with smooth edges to 4 bit of depth? methinks you shpuld use 8 bits to at least get some same quality. 4 bits means just 16 values of color/channel power.

xyzw: What compression tool did you use? I just took a copy of your xx.jpg (cropping it to 512 wide and scaling the height to 512), and compressed it to PVRTC 4bpp with the IMG PVRTexTool. I found it fairly difficult to spot differences.