Created attachment 140962 [details]
images gamma

I have a PC with:
MB: Asrock AM1B-ITX
CPU: AMD Kabini Athlon 5350 APU
iGPU: Radeon HD 8400 / R3 Series
Displays: DVI Asus, HDMI Samsung

kernel-4.15/4.16/4.17
xorg-server-1.19.5
xorg-drivers-1.19
xf86-video-amdgpu-18.0.1
xf86-video-ati-18.0.1
libdrm-2.4.92

When I increase the gamma to check the quality of images (xgamma -gamma 3),
some pixels are darker/colorful, it seems like gamma is not applied uniformly
for all pixels. This happens on all kernels with any video driver: radeon,
amdgpu, amdgpu-dc.

I thought there's something wrong with the images so I've edited them on Gimp
by  increasing brightness or change hue/saturation. This was fine until the
kernel 4.17 and the amdgpu-dc driver upgrade, images have again the issue. They
are fine with amdgpu.

Is there something I can do about this?
Thanks.