Partial repeat of 0001947.

Observed in DOM 2.16.59 on Mac OS 10.14.6 and Ubuntu 20.04.

The preview in DOM GUI often shows a 1-pixel black line either on right or along bottom of preview pane. Whether it appears or not depends on the window size (which affects the scaling of the preview pane). But by adjusting the size of the window, it's possible to produce this in both flat and scope aspects. On Ubuntu, occasionally you can get a black line both down right-hand side and along bottom simultaneously.

Some example images below. These are screenshots is from an iMac with a retina screen, so you'll probably need to zoom the images to see it clearly, as the lines are only 1 pixel wide.

In this example I've used a TIFF file (attached below), but same effect is observable with any input e.g. ProRes or DCP.

This effect was first seen in around 2.14.47, probably due to changes related to cropping of sub-sampled sources. It was fixed by 2.14.56, possibly by 0001872. But it seems to have reappeared in 2.16.x.

A guess at the cause: Perhaps the size of preview pane is calculated based on window size x aspect ratio. But then actually rendering the image uses a different calculation, which rounds up/down in the opposite direction - so there's a 1 pixel discrepancy between the two and the black background shows through in the gap.

Just to say, this is more important than it may sound, a least for my workflow. My standard procedure for cropping off letterboxing/pillarboxing is to set "Scale" to 1.85, crop until the picture is filling the whole preview window exactly (i.e. all black is gone) and then reverting "Scale" to "No stretch".

This method is useful because: (1) The black bars will be pure black rather than possibly containing some small noise present in the original source due to imperfect compression and (2) DCP-o-matic then displays the exact active aspect ratio e.g. "Cropped to 1440x1080 (1.33:1)" and I can put that in the DCP name "F-133".

The presence of the black line makes this difficult to do. But I don't know if anyone else would be using this slightly convoluted method, so maybe no-one else is really affected.