Do the black letterboxes take up much encoding data?

[broaddd]

Hi folks,

I would like to convert a 2.35:1 DVD movie I have to avi. In a few scenes it has a brief subtitle appearing underneath the movie frame, which is 'burned in' (ie. can't be switched on or off). For simplicity, I was going to encode the movie to avi and only crop out the top letterbox. I am assuming it doesn't take a signficant amount of encoding data to encode the bottom letterbox, given it's completely black for 99.9% of the movie.

However, if it does waste a significant amount of space in the avi file, I'd consider trying to make my own subtitle .srt file to go with the avi; and crop the bottom letterbox as well.

What do you recommend?

Cheers,
B

[jagabo]

Black borders take almost no bitrate with codecs like MPEG2, MPEG4, h.264, etc.

For example, I took a 90 second 720x480 MPEG2 file (movie trailer) and encoded it with Xvid using its default settings (constant quantizer = 4). In this mode every frame is encoded with the same quality regardless of the content. Each frame consumes whatever number of bits is needed to maintain the requested quality level. The file turns out whatever size is necessary.

I encoded the video normally first. Then I filled half the frame with black and ecoded again. Then I filled the other half of the frame instead (in case one half was more difficult than the other) and encoded. Here's the results:

Full frame: 15.6 MB
half black frame: 7.65 MB
other half black frame: 8.00 MB

There was no audio in these tests.