What do dt and RT consider “1:1 preview”? I.e. is it 1:1 wrt to the full raw size, or 1:1 wrt the embedded image? And is it the same for both programs?
There’s no direct relation between a “full screen” view and a “1:1” (100% zoom) view. Which do you want? And what are the sizes of yuor embedded jpegs and your screen? Those artifacts look like you get an embedded jpeg blown up to fill your screen, with the jpeg being smaller than your screen… For information, for me an embedded jpg is 1616×1080 pixels, my screen is 1920×1080, so a decent fit. If you use a 4k screen, the poor program has no choice when you ask it to show the image full screen while using the embedded preview
If dt wants to show a 1:1 of raw size, it has to process the raw data, for at least some cameras. The speed difference compared to darkroom mode is caused by dt using a quick and dirty demosaicing when you’re at zoomed-out view (like on opening; see configuration → darkroom → demosaicing for zoomed out darkroom mode).
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Btw, Editing a post that has replies isn’t really that comfortable for the other users. For one, it becomes impossible to decide what’s replied to…