crossfade command stretches image when movie recording with width specified

[omid.hajighassemi@…]

Begin forwarded message:

From: Omid Haji-Ghassemi via ChimeraX-users
Subject: [chimerax-users] crossfade issue
Date: May 12, 2022 at 12:16:20 PM PDT
To: ChimeraX Users Help <chimerax-users@…>
Reply-To: Omid Haji-Ghassemi <>

Dear ChimeraX community,

I am having some issues with the crossfade command while making a movie. For some reason the command stretches out the image while running the transition.
I am just doing a simple transition for a movie as part of a more complicated movie.

To illustrate the problem I made a simple script that reproduces the issue.

movie record supersample 3 size 640,480
wait 60
crossfade; cartoon hide #1/D-F; wait 60
movie encode framerate 60 output test.mov

Be great if someone can let me know what I am doing wrong.

Best,
Omid

[Tom Goddard]

I believe this is a bug in the crossfade command where it is using the screen graphics window size instead of the size specified in the movie record command width/height options.

[Tom Goddard]

The crossfade command works by capturing an image of the scene in the graphics window and then blending that image with later images. It won't work correctly if you record a movie with a different width and height in pixels from the graphics window. If the the movie aspect ratio differs then the initial captured image will have the wrong aspect ratio for blending with later movie frames. If the movie aspect ratio matches the graphics window aspect ratio but is at higher resolution then the initial image is captured at too low a resolution and will probably cause a noticeable flash when the resolution drops at the start of the crossfade.

For these reasons the "movie crossfade" command should be used if you intend to record a movie at a different size in pixels than the graphics window. It does not have these problems because it captures the initial image using the movie size.

It is not very feasible to fix the crossfade command because it does not know if a movie is being recorded or not. So I don't plan on fixing the crossfade command.

[Tom Goddard]

Elaine, could you document that the "movie crossfade" command should be used instead of the crossfade command if the movie will be recorded at a different size in pixels than the graphics window on the screen.

[omid.hajighassemi@…]

Hi Tom,

Thanks for clarifying. Nice to know for the future. Appreciate all the hard work you, Elaine and the rest of the team put into this.

All the best,
Omid