Another Letterboxing Question

[rumplestiltskin]

Let's say I have a 640x480 video but there are black bars that provide what we call "letterboxing". Let's also assume I like wide-screen videos so I expect to see black bars no matter what. I can do one of the following:

1. Live with the black bars (because they'll be there on my 4:3 TV no matter what and I expect black bars).

-or-

2. Crop the top and bottom. Let's say I find that 100 pixels at the top and 100 pixels at the bottom should be removed in order to have a wide-screen video (so on my Mac it has no black bars). So the next question is: What pixel size should I use? 640x280 or 640x480?

Final "bonus " question: Can I use the iPod H264 setting and still crop or must I do the cropping in Xvid or Divx? Where will I maintain the best quality?

Thanks!

[Case]

2: While black bars don't require much of the available bitrate, including unneeded black areas does waste some resources, which could have been used to improve the video of the main area. So I would go with cropping.
Bonus: Best quality is with all the alterations and encoding in one step. Check the Crop section in the Filters tab.

[rumplestiltskin]

<snip> Best quality is with all the alterations and encoding in one step. Check the Crop section in the Filters tab.

...and leave the pixel size alone? So a 640x480 setting with 100 pixels cropped off the top and 100 off the bottom will yield a video of 640x280?

Thanks.

[Case]

On the website it says to scale to 320 width for H.264 iPod encodings, with whatever height corresponds to the aspect ratio of the movie. The cropping is done before scaling, so the crop values are applied to the source material. Does that answer your question?

[rumplestiltskin]

For wide-screen videos I do the math and, following the rule of 16's, keep the proportions correct while maintaining a total pixel qty of no more than 76,800. Usually this is 368x208 or 400x192 or 416x176 depending on the proportions of the source.

So, if I understand you, I can calculate the desired size by -first- subtracting what I am cropping (ie, 480-200=280) and then use the 640x280 as the source size; I'm using the 640x480 source I mentioned in an earlier post. This would yield a 416x182 calculated reduction - 416x176 being the closest legal size.

Nothing to do but test it, eh?

Thanks!

[DeusExMachina]

As I understand it, since cropping takes place prior to resizing, the target size is independent of the other values.
So the process is this:

640x480 source / 100 pixel bars -> cropped to 640x280 -> resized to chosen size (416x176) -> encoded to final encoding scheme.

i.e. decode -> crop -> resize -> encode.

So it does not matter how many pixels you cropped, the final size will be what you set it to in the video tab. So for instance, if you have 640x480 mpeg and you set it to cvonvert to a 640x480 h264 with a crop 240 from top and bottom, you would end up with a blank 640x480 clip.

[rumplestiltskin]

Well, using the H264 for iPod setting, cropping does nothing. Guess I'll have to do a two-step conversion from AVI to AVI to do the cropping and then transcode & resize the cropped AVI to H264.

Another item for the wishlist, I guess.

[Case]

416x176 being the closest legal size.

The ffmpegX webpage suggests that a 320 px width is needed when using H.264:
"If you use h264: don't change the image width"

[rumplestiltskin]

I believe the warning relates to the 320x240=76,800 total pixels limitation. Many of the apps for converting to H264 make the (erroneous) assumption that 320 wide is the issue whereas it's the total number of pixels. However, I'll give it a try again and report back. Thanks.

[rumplestiltskin]

I was able to do the crop while using the AVI/Divx encoder. It looks like H264 ignores the crop settings. Okay, I'll have to do two encodes. Doesn't take too long on an Intel iMac.