720x480 -> 640x480 or 704x480, crop vs resize vs keep it all

[stunted]

Hi everybody!

I'm encoding an NTSC source off a DVD with AGK 2.27, one pass @ 75%, (not so fused about HD space) I've been leaving the Hidden Options at defaults and setting the width to "Fixed width" 640 in Advanced Options, this resizes the line of 720 pixels to 640 (there is a lot of debate round here asto what this does to the aspect ratio).

I'm getting something I'm going to call block jitter where the image is generally excellent quality but some blocks flash, this particularly happens when you have a panning shot with fairly low light levels and high brightness static text (credits or whatever) over the top, it also happens (though less so) on lines of high contrast in the video itself.

I think this artifact is something to do with the resizing as I'm pretty sure I've never seen it on a wide screen move which AGK will tend to leave at source resolution, 720x320 or whatever.

Soooo what to do,
1. leave the video as is, disable "Detect and force 4:3" and enable "Auto width" so I'll get a video file of 704x480 or 720x480 (not 4:3)

2. force a crop of 40 on each side to produce 640x480 while retaining aspect ratio but losing a fair slice of image on either side

3. set "Fixed width" to 640, get a very slightly squashed 4:3 image with the full screen and live with the artifacts (which the girlfriend doesn't see anyway)

I've done some searching and read a lot of frequently contradictory advice, most of which is a bit, old so I thought I'd start a new thread.

What think you?

[unskinnyboy]

First a bit of basics - if a 720x??? picture gets resized by AutoGK (or any intelligent encoding application, for that matter) to 640x???, it shouldn't do anything to the aspect ratio since it will resize the width *and* the height to attain a proper AR which resembles the source AR (there are a few other variables here like AR errors due to MOD16 resizing, ITU-R BT.601 resizing etc, but for the sake of simplicity let's put all those aside for the moment).

Resizing will not cause "flashes", the kind you have seen. Since you are using a quality based encode, the resizer used here is LanczosResize, and it definitely won't cause any artifacts like these. It must be due to something else, maybe even the source. Have you checked the source?

If the source doesn't have any issues, but your encode did, what you need to do is to give us samples (of the untouched vob and the avi, or just the vob alone), so we can try and reproduce the error. Also, post your AutoGK log file.

Your solutions (1, 2 & 3), don't make any sense to me. Like forcing a different aspect ratio and all that.

[stunted]

Well except that by default in the "Hidden Options" the setting "Detect and force 4:3 AR if close to that aspect" is set to ON, now I don't know how 'close' it has to be but 4/3=1.333... recurring and 720/480=1.5 while after auto crop of the 8 blank pixels each side 704/480=1.4666.... recurring, I suspect all of these are close enough to get forced.

I have checked the source and I don't get the problem, I'm looking for a VOB thats under the creative common licence so I can post samples. I don't think that would count as a derivative work would it?

solution 1. doesn't alter the AR.

Thank for taking the time to reply

[unskinnyboy]

What exactly is the issue? "Flashes", as you said, right? Then why are we even talking about aspect ratios and such? That has nothing to do with the issue.

I believe, even if it is a copy-righted DVD, cutting out a sample VOB for troubleshooting an issue would fall under the fair use clause. People do that here all the time.

[stunted]

You're probably right but I don't want to be the poster boy or the test case right now, anyway since the OP I have seen the artifact in a non-scaled movie, it's much less pronounced and more infrequent though.

Sorry about the slow response, RL has stopped me having time to experiment for the moment, I'm downloading the Elephants Dream DVD now, I'll Burn, rip and encode it to see if I get the artifacts (it's PAL tho) then post again.