I am rotoscoping a scene and it looks fine in the compositor: the mask is bordering all the figure as in the picture 1enter image description here

But when I render it the mask appears in a different location as shown in picture 2 (look left hand, up) enter image description here

Can somebody please tell me what am I doing wrong? Node in picture 3 enter image description here Thanks in advance.

Here is the file

  • $\begingroup$ In the current node settings, the size of the mask depends on the size of the settings for the resolution for the project. Is the video the same size as the project? Is the resolution set to 100%? Else you should switch the mask node from scene size to fixed/scene and input the size of the source manual. Also, you can evaluate the mask (or any other node) by plugging it directly the view node. And last question, what format are you using to save the image? make sure that is a format that supports alpha channel. $\endgroup$
    – susu
    Commented Jan 19, 2021 at 5:12
  • $\begingroup$ I made the clip on Iphone 11 on vertical position 4k, so as resolution I put 3840X2160 to 100% I also switched the mask node as you said but the problem still not fixed. Im saving the image as PNG RGBA 16. Please tell me how can I upload the file for you. $\endgroup$ Commented Jan 19, 2021 at 5:54
  • $\begingroup$ File has been added. Thank you. $\endgroup$ Commented Jan 19, 2021 at 6:13
  • $\begingroup$ Sorry. Here justbeamit.com/jq3zu $\endgroup$ Commented Jan 19, 2021 at 7:08
  • $\begingroup$ Can't reproduce the error. See answer. $\endgroup$
    – susu
    Commented Jan 19, 2021 at 8:10

1 Answer 1


If I render your file I get no problems with the mask being misaligned:

enter image description here

enter image description here

As for the rough edges by the armpit, those are part of the key from the green screen, they are not part of mask you are using as garbage matte.

The keying needs to be refined, so that you get better separation.

enter image description here

You might want to try blender.2.92 so that you don't have to convert from straight to premultiplied.

Read: How to put animated background to keyed video?

I suspect the issue could related to the kind of video compression, or how the image is being decoded by blender. Sometimes, depending on the codec used, blender fails to read the correct frame, and reads one frame ahead or one frame late.

Try prefetching the video, or rendering the video to an image sequence first, and use that image sequence to generate the key.

Please report back if the issue persists

See: Render is duplicating random frames when loaded into 3D tracker

  • $\begingroup$ Thank you very much for your interest. I just downloaded 2.93 version but was still having the missaligned mask. Then rendered it as image secuence as you said but still having the same problem. I also notice that it renders fine when I render only the current image but the problem appears when I render the whole animation. At last, I was thinking of what you said about "reads one frame ahead or a frame late", went to the dope sheet and moved the mask animation one frame ahead and that solved the issue. Thank you too for the keying advise. $\endgroup$ Commented Jan 19, 2021 at 14:34

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