fogbound.net




Sat, 24 Sep 2011

Ffun ffmpeg ffunctionality

— SjG @ 2:59 pm

I’ve been processing a collection of product videos which came to me in a huge variety of sizes, aspect ratios, and qualities. I need to re-encode them to work in HTML 5, but, more importantly, I need to make them fit into a common player space on the web page.

It turns out that newer versions of ffmpeg support not only cropping, but also padding, and you can even do both operations at once!

For example, I had a source video that was originally 16:9, but had been letterboxed to 4:3, and then had two different sets of labels added. I needed to crop out the letterboxed portion and the top set of labels, and make the result fit nicely into 16:9. So I used VLC, a screen capture utility, and Photoshop to get the measurements. Then I used ffmpeg to crop the relevant section and pad it out to fit into my space (in this case, I’m left aligning the video in the padded output):


ffmpeg -i original/converted.wmv -vf crop=394:295:6:0,pad=524:295:0:0:0xFFFF00 -sameq converted.mov

That’s cropping a 394 x 295 piece out of the original video (with the origin at 6 pixels from the left, and 0 pixels from the top), and then padding it out to 524 x 295 filling the padded area with bright yellow. The 524 x 295 is really close to 16:9 — and in a later process, it gets resized to the more standard 480 x 2721.

You can string together the padding and cropping in either order, depending on the effect you’re trying to achieve.

1I’m sure some educated person out there could tell me why video standards are so confused/confusing, down to the non-square pixels. While a true 16:9 would dictate 480 x 270 pixels, everybody seems to use 480 x 272. Why? The only thing I can figure out is that 272 is evenly divisible by a power of 2, which probably made display hardware cheaper to manufacture. As you can see, my resizing adds a bit of distortion, but at these resolutions, it doesn’t really matter.


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