Loss of quality when rate stretching video clips 1

[jabbason]

Has anyone else noticed this problem? when i import a captured video or an avi and perform any kind of rate change (even just a 99% rate reduction) the clip pixelates and flickers when exported out of premiere as an avi. I understand that a certain loss of quality is to be expected when slowing any kind of footage, but this problem seems to be happening with even the smallest rate change, surely i, or premiere is doing something it shouldnt be?! Would be glad of any help that might be offered on this problem! cheers, john.

 

[Akribie]

Non-integer rate changes require that Premiere blends frames when rendering. Results can be less than impressive with rapid movement or fine detail.

Better results come from selecting integer multipliers or divisors (eg 200%,67%,50%,25%).

Or use After Effects or PPro CS3, which have better time remapping capabilities.

 

[jabbason]

Thankyou for that information Akribie, will try altering the rate changes by the percentages you have given. I wondered if the lack of quality might be affected by using different percentage changes and did a number of tests which produced a variety of changes, but none that were satisfactory.
I will try and get ppro cs3 as soon as i can as i would like to try to maintain as high a quality as possible in my slow-motion footage, especially when it is to be projected on large screens. Aside from the adobe premiere programs, are there any other programs that offer higher quality renderings? Does quality also depend on hardware specifications?

 

[Akribie]

The Twixtor plugin gets good reports, but I haven't used it myself because After Effects has given good enough results for my needs so far.

The percentages I gave are all OK except for 67%, which is an approximation to 2/3. To get best results, Premiere needs to be able to drop frames without manufacturing any new ones for slow motion. 1/2, 1/3, 1/4 etc are the ones to choose, not 2/3.

 

[jabbason]

Using a section of an avi, i performed a simple 50 percent rate change, to slow it by half. But i still experienced the same amount of distortion that i have been having up till now. I think i understand what you mean, if i am just doing a 50 percent rate change all premiere really needs to do is duplicate each frame to make them last 2fps. So why such a drop in quality should be occuring is quite frustrating. Shall i do a couple of screen grabs whilst playing the normal and stretched versions for you to take a look at?

 

[Akribie]

I am not sure exactly how Premiere works out the intermediate frames.

If, at 50%, they take each field and make it into a frame, you will seem to have somewhat less vertical resolution.

Taking de-interlaced stills from normal speed and half speed should, if my guess is correct, give very similar results (depending on which field is used).

 

[jabbason]

Yeah thats exactly what happened! i extracted a frame from the normal speed and half speed and the frames appeared to be indentical. The half speed version however plays very differently which not only accentuates the pixelation but also creates what i can only describe as a 'shimmer' which i think is occuring on the vertical axis.

 

[Akribie]

Shimmer can be due to interlacing artifacts where a narrow horizontal line jumps up/down a line due to very slight instabilities in the frame.

A small amount of vertical directional blur (around 1 pixel?) can reduce this effect.

 

[jabbason]

thanks for that option, i have given it a go doing a 1 pixel directional blue on 0 and 180 degrees, but it does not seem to help very much. how might these instabilities in the frame arise in the first place? is it due to the quality of the recorded footage being used?? is this instability something which exists in normal speed footage but does not show itself until the frames are limited in a slow-motion rendering?

Its all quite frustrating really, because i have seen slow-motion footage used by other people which does not seem to be affected in this manner. I have seen some footage slowed to well belowe 50 percent and yet retaining very high quality with no shimmer.

 

[Akribie]

Interlace flicker, if that is what you are suffering, is due to the use of horizontal scan lines to generate the picture. If there is a narrow, contrasted edge lying horizontally in the scene, then it is likely to flicker from field to field.

You might also want to investigate the effect of applying the various options available under Field Options from the right-click menu of a clip on the timeline. Unfortunately, these cannot be copied and pasted onto other clips, and cannot (as far as I am aware) to nested sequences.

 

[jabbason]

Thanks alot Akribie, the problem has been solved!! It turned out that i should have deinterlaced my original stock footage before applying and rendering the rate change! this has totally eliminated the flicker, which was as you thought due to the interlace flicker. Thanks alot!!

 

[Akribie]

Glad it's OK now. Thanks for the feedback.

 

[jabbason]

just wondering about another little issue i have been having with premiere, the preview window. I know it is not the best method for previewing your edited footage because of the small size which gives inconsistant quality and contrast etc, so i usually export as an avi to preview full screen.
I always work up to the edges of the visable preview screen, and when exporting as avi's this is fine. However, when converting into a dvd to play on a television there is alot of clipping of vertical lines. I know about the safe area markings you can turn on to the preview screen, but i have found these to be somewhat inaccurate. Through lots of testing i found that working with a altered scale of 93 percent on whichever avi i wanted to convert to dvd (through premiere) gave a suitable aspect for the tv to display effectively.

Just wondering if i am doing something wrong, which could make this all alot easier!? is it cos i am using premiere to export the dvd? should i be using encore or something?

 

[Akribie]

If you connect to your camera/DV deck via FireWire, and simultaneously connect the analogue output from the camera to a TV, you should be able to preview your edits (video and audio) straight off the timeline on the TV if you have the relevant Program Window output settings in place. There should be no need to make an AVI first.

Normal TVs do not show the overscan area. Most projectors and all computer replay shows right to the edge of the full frame.

TVs vary in the amount they obscure - hence the safe areas. What might look good on your TV might be clipped on someone else's.

There's nothing magic about DVDs. The area displayed should be virtually the same as when the program is played back off tape.

The moral to the story is to frame your shots and make your titles etc in such a way that you do rely on elements very close to the edge of the frame.

 

[jabbason]

Thankyou that information Akribie. I understood that the problem was not with the dvd itself but the difference in the viewable area provided by the television. I thought however that there might be better or more accurate way of cropping for replay using a television.

For example when i project films i have made for an installation that might be playing all day long, it is more convenient to use a dvd player as input for the projector. I have found that the laptops i have been using to play films continuously from avi suffer from periodic glitches. Running from a dvd player seems to provide a clipped image that is found when viewing on a television.

It would seem logical as you say to frame shots in the future to avoid problems that might occur if cropping happens, however when i edit i like to visualise exactly what will be projected, i dont like things i have no control over, if you know what i mean?

 

[Akribie]

I understand your frustration, but unless you are prepared to edit for a specific output system, a bit of flexibility remains essential if some viewers are not to be frustrated by having important elements clipped at the edges.