Question about the size of the black borders on a 2.35:1 movie

[DrChair]

Hi,

I week or so ago, I was converting a dvd from letterbox to widescreen, and I noticed something strange about the black borders.

Almost all 2.35:1 dvds have a black borders on the top and on the bottom of the screen, both being 72 pixels in height. (in case of PAL)

But shouldn't the height of those borders be a multiple of 16??
Cause one of the first things i learned about encoding is, that you should never have a sharp crossing between a black border and the movie, in the middle of a 16x16 block, because it's hard to compress.

In other words, would a dvd encode better if I were to add

Crop(0,72,720,432)
AddBorders(0,64,0,80)

to my avisynth script?

Greetingz DrChair

[Kedirekin]

If you look closely at those black borders, you might notice that the border fades to black over 2 or 3 scan lines. That is an alternate approach to avoiding the sharp border transition within a macroblock, and one that allows precise aspect ratios.

You can certainly distort the aspect ratio slightly (which you'll probably never notice) to get the border to fall on a macroblock boundary, but because of the fade you won't gain much savings in bitrate.

[windtrader]

If you look closely at those black borders, you might notice that the border fades to black over 2 or 3 scan lines. That is an alternate approach to avoiding the sharp border transition within a macroblock, and one that allows precise aspect ratios.

That's really cool. I did not know that was happening. I curious; Which filters perform this function?

[Kedirekin]

I don't know what filters, or even what software, authoring studios use to do this.

You could use a combination of Crop, Levels, and StackVertical filters in AviSynth. You would probably need to Crop a number of different clips, some of which would only be one scan line high, apply varying amounts of Levels to each, then reassemble them. It wouldn't be pretty.

Or perhaps someone has developed a plugin filter that does this. I haven't been keeping up on AviSynth plugins lately.