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How To Minimize Interlace Jitter In Video

When trying to minimize interlace jitter in a still frame of video, this method has better vertical resolution with less aliasing (less jaggies) than the deinterlace filter in Photoshop (which throws away half of the vertical resolution).

STILL FRAME OF VIDEO: Take your still frame that came from video (BMP, TGA, JPG, or whatever) into Photoshop or similar paint program (as long as it does sub pixel bicubic scaling) & scale the 486 NTSC vertical resolution up to 494 (8 pixels taller)(don't scale the horizontal resolution)(constrain proportion turned off), no need to save yet. Now scale it back down to 486 & "save as" a new name. Put this new file on the timeline & compare with the original. If the scaling didn't eliminate enough jitter (it won't get rid of all the jitter), then take the "saved as" version back into Photoshop, but this time only scale it up 1 line (to 487 from 486)(don't scale the horizontal resolution)(constrain proportion turned off), then scale it back down to 486 & "save as" another new name & compare on the timeline.

This sub pixel process does not throw away one of the fields, but instead it blends the fields together a bit to help minimize the interlace jitter, while still maintaining much of the vertical resolution. For whatever reason, scaling 486 up by 8 & or 1 lines, works better than most other line combinations for NTSC. I'd love to hear from some PAL folks or NTSC 480 line users to know if the same combination (of 8 & 1 lines) works better for them than other line combinations.

MOTION VIDEO: It is also possible to apply this same interlace jitter reduction principal to motion video (using sequential frames) in Photoshop by creating a batch action. To batch process in Photoshop, you first need to create an "action" script by clicking on the "Actions" tab on the right side of the screen (if it isn't visible, menu under Window/Show Actions), then go to the small icon just to the left of the garbage can (if you hold your mouse over it, the name will say "Create new action") & click on it. Give your action a name (like "scale up to 494 then back down to 486 to reduce jitter") & click on record. Next open up the first frame of your sequential movie & then under image/image size, resize it to the size you need (probably 486 to 494 vertically)(constrain proportions off). Now resize it back to it's original vertical height. Now go over to the "actions" box again & click on the square black "Stop" button to stop recording this action (don't save the frame you just resized). Now to actually do the batch scaling, menu under File/automate/batch & pick the correct action, set the source & destination folders & click on ok. Photoshop will now automatically dual scale each frame in the sequence. You might get a better explanation of how to batch resize in the Photoshop help file.

Note that when doing an automated batch of sequential files where the virtual files of the P drive on a DPS NLE card are the source, it seems to miss the first file (perhaps I am doing something wrong). This bug doesn't seem to occur when converting sequential file from a systems drive.

Also note that when applying this method to interlaced motion video, it will destroy some of the temporal resolution or smoothness over time (much like a film look), because you are effectively changing the video from 60 discrete fields per second (for NTSC) to 30 frames where both fields are very similar.

By Doug Hembruff.
Last updated March 30/2001.

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