Darktable not working well with preview downsampling set to less then 1.0
I started the mailing thread above because of my finding that on my opencl enabled system rendering of the preview pixelpipe (processed on CPU device) is terribly slow comparing to rendering of the main pixelpipe (on GPU opencl device). Typical timing of single image edit is as follows: main: 0.2-0.4sec, preview: 1.0-1.5sec. This considerably slows down any image editing. It also negatively affects the responsiveness of some modules which depends on preview such as histogram.
After discussion with jo, it was discovered that lowering of the downsampling ratio value in src/develop/develop.c helps to improve the preview performance a lot. Value of some 0.4 makes the preview to be processed as fast as main. On the other hand lowering of the thumbnail size (from 2560x1440 to 2112x1408) has hardly noticeable effect to performance.
The problem is that with downsampling set to less then 1.0, darktable doesn't work well. There are visual problems while zooming and panning the image (zoom jumps out and in) and what is more important, the crop and rotate module is not usable at all.
The goal is to resolve those problems caused by the not default downsampling value and to allow user to set this value in config file.
System configuration is as follows: Ubuntu 13.04, AMD Phenom II X6 1090T, AMD Radeon R9 270, fglrx driver.