When in the config.txt file a frame buffer size is less than the screen resolution set with hdmi modes, and the scaling_kernel != 6, the result is some pixel are used and other not. it seems not really stable by different boot with the same config, some times one over 4 pixel is used, sometimes one over 4 is black.
With HDMI groupe 2, mode 4 (640x480) and framebuffer of 320x240 and scaling_kernel = 8 , nothing is displayed.
Image with hdmi groupe 2, mode 9 (800x600) and scaling_kernel = 8 :
Image with1920x1200 and scaling_kernel = 8 :
Another Image with 800x600 and scaling_kernel = 8 : there the upscaling is wrong
I've reopened #104 as that is not resolved. Please continue there.