Why do we crop?

[cornetet]

Why does it seems to me that in the Resolution Tab in Gknot takes a 720x480 DVD, then resizes it in the Crop section to 708x462, then to resizes it again to 608x320 in the output resolution?

What does all this mean?

Here's my thinking... if a DVD is about as clean edge-to-edge as it gets, why should I want to crop anyway? Is it a requirement for Gknot to do some kind of calculation?

Please explain why I want to crop...

[RadicalEd]

Originally posted by cornetet
if a DVD is about as clean edge-to-edge as it gets, why should I want to crop anyway?

Thatsa thing, DVDs are hardly ever clean edge-to-edge. In fact they shouldn't be. There should always be at least 8 pixels of black on either side to account for the 16 extra pixels in the 720 x 480 resolution. These extra pixels are used to reduce overscan without affecting AR which on a TV should be 704 x 480 for 4:3 (1.333 * 11/10 = 1.4666) etc etc. Not to mention the fact that the actual source can often have a slightly off AR to begin with and often times has unnecessary borders on either side or even the top and bottom.
Basically Gknot's cropping system tries to get the most correct aspect ratio out of the source for the final encoded file.

[Suzahara]

Originally posted by cornetet
Why does it seems to me that in the Resolution Tab in Gknot takes a 720x480 DVD, then resizes it in the Crop section to 708x462, then to resizes it again to 608x320 in the output resolution?

What does all this mean?

Here's my thinking... if a DVD is about as clean edge-to-edge as it gets, why should I want to crop anyway? Is it a requirement for Gknot to do some kind of calculation?

Please explain why I want to crop...

Here's why I crop. No, it's not a requirement, you don't have to crop if you don't want to, that's why the disabled radio button is there. However, to me, if when I put the video at fullscreen and the video isn't edge to edge on the screen, even just the tiniest bit, it takes away from the movie. I've never seen a dvd that was clean edge to edge though. Mostly because of the reasons that RadicalEd posted. Cropping is mostly an asthetic thing though, most people want their video to be edge to edge when they fullscreen it. Although there's more bits for the actual video if you do crop off the black. It's also nice if there's lines on the side (mostly in anime) or some kind of disturbance near the edges of the video to take that off as well.

[N_F]

It will also increase the compressability. Those "sharp" edges between the picture and the black bars don't compress very well, so by cropping a pixel or so in the picture the compressability will increase.

Don't ask me by how much, I've never compared... but it would be interesting to know if anyone else have done this. How big difference does it make? 0.5% ?

[hakko504]

There can also be a speed increase, i.e. the less pixels we have to use when resizing or filtering the faster it will be.
And for resizing, the best is to crop 8 pixels at each side to 704 wide and then only correct aspect ratio by changeing height. This will completely remove the horizontal resizing, thus making it faster.
Typical scenario:Mpeg2source(film.d2v)
Crop(8,72,-8,-72) #PAL source
#Crop(8,60,-8,-60) #NTSC source
Convolution3D(preset="MovieHQ")
BicubicResize(704,304,0,0.5)

[N_F]

I did a quick test last night:

no cropping (~55 pixels black bars on both vertical sides): ~68%
bad cropping (4 pixels black bars on both vertical sides): ~69%
good cropping (no black pixels): ~71%

2% difference isn't that bad considering that we don't need the black bars anyway.

[theReal]

Don't ask me by how much, I've never compared... but it would be interesting to know if anyone else have done this. How big difference does it make? 0.5% ?

That was what I thought until I tried it with a copy of Die Hard 3 (ntsc, but it wasn't the official release, it was a Chinese copy, I think).

Leaving one single black line at the top resulted in a compress test that was 8% less than when I cropped this line off. It really amazed me how much this one single line could do...

...and btw you'll always get mosquito noise around the sharp edges if there are any leftovers of black lines at the top or bottom.