1. Why do these black borders appear in doodles?
Short Answer: The create video/editing resolution is smaller than the exporting resolution.
Technical Answer: Your screen and videos are sized in pixels. Pixels, in the basic discussion, are a measurement just like millimeters or inches. When you create a doodle the creation resolution must match the exporting resolution.
If you create something that is 360 pixels (The p does not stand for pixels. That would be px.) and then put it on a 1920 pixel canvas you are going to get an image the size of a stamp on a blank canvas. Doodly essentially makes everything outside of that video transparent around that 360 pixel video creation. A .mp4 file does not support transparency (known as alpha channels) so it replaces the transparency with a black background during the rendering process so what you see is “black borders” or “black bars” within your doodle.
2. Why isn’t this automatic?
I have long since complained about this issue and have been very vocal about it.
Doodly requires us to manually choose the resolution upon creation. This is essentially your drawing on an art canvas but then instead of automatically setting your canvas to the same size as your drawing it requires you to choose the resolution again during the export process but this time you are choosing the size of your canvas.
It is an awful methodology and makes no sense but neither does having the resolution at the bottom of the create video window and making you select a custom image first and then when you change your resolution, Doodly removes your image.
3. How do I fix this issue?
Ensure your creation resolution matches your exporting resolution or use a custom resolution. The only time Doodly uses the canvas size chosen at creation time during the export process is when a custom resolution is selected.
Note: If there are black lines or borders appearing in a different video software after your export your doodle even though the resolution is the same, the correction for this may be to stretch your video. Researching why this happens is beyond this Doodly Tip of the Day but perhaps the export from Doodly is not quite the custom resolution that is stated.