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What about video source, which is for example 1920 x 818 pixel ?
There exists no output format to set it equal, so SVRT shows always red. Also in the user specific format settings one can't choose a free display ratio, only predefined ones.
Does this mean, that video sources with "not normal" pixel ratios never can make use of SVRT?
There are two different issues here. The first is the aspect ratio, and PD18 can only produce to 1:1, 4:3, 9:16, 16:9 and 360. 1920x818 is not a standard pixel size or aspect ratio, so you'll end up with 16:9 video with black bars along the top and bottom edges due to the missing 131 rows of pixels along each edge.
PD18 has a Profile Analyzer option on the Produce page that may be able to generate a custom profile that will let you use SVRT with your source clip, although it would still have the horizontal black bars.
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What can I do to make my source video meet the defined output display ratio?
I've heard of for example "DVD patcher", a little programm that patches every single frame to the desired pixelratio without rendering the video itself?
About a technical funded answer I#d be very happy.
There are many free apps that will let you either stretch the vertical axis to fill 1080 lines or let you trim the horizontal edge to 1454 pixels to get a full 16:9 frame. Handbrake, VirtualDub2 and VLC player can do that, and either will let you produce the clip in PD without any balck bars.
You can also use these same apps after producing to trim the bars and return to a 1920x818 clip, See this
post and/or search the forum for more info on nonstandard ARs.