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It seems to be a bug. I can confirm that on my computer TVPaint crashes when I do the same (import footage with 12 fps and stretch to frame rate). But I think all you have to do is import the footage with 24fps and then select all the frames and click on this button in the handy custom panel. exp2....

I create a project,and set "Frame Rate" to 24, then in the "Import Footage" dialog, set the "Frame Rate" to 12, and select "Stretch to Project Frame Rate". When I click the "Import" button, the Tvpaint told me "Unknown file format", and can...