johnb4467
Junior Member
"That which does not kill us, makes us stronger." -Nietzsche
Posts: 54
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Post by johnb4467 on Nov 29, 2004 2:29:50 GMT -8
I forget, we got a basic rundown of how DV pixel ratios differ from the 'norm', and how that will result in stretched/distorted results...did we ever get around to covering how to avoid & remedy those problems? Just curious, as I'll be using some DV footage in my AAU commerical project....
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Post by Garrett on Nov 29, 2004 20:37:52 GMT -8
Here you go homeboy:
In computer graphics, pixels are square. In video, pixels are rectangular. This is equivalent to saying that video has different spatial sampling in the horizontal and vertical directions.
A square pixel has a pixel aspect ratio (or sample aspect ratio [SAR]) of 1:1. A non-square pixel has an aspect ratio W:H where W and H are not equal.
Do not confuse this with the display aspect ratio [DAR], which describes the shape of the physically displayed image. This describes the shape of the frame as measured with a ruler.
The other two parameters describing video frame are the width and height measured in pixels. These parameters are related to each other as follows: width DAR -------- = ----- height SAR
Quick Facts
The industry standards (i.e. as practiced) for digital video sampling are derived from ITU-R BT.601 (also known as "CCIR-601" or "Rec.601"), and presume a horizontal sampling rate of 13.5 MHz and an active line of 720 samples. The pixel aspect ratio for 525-line video (NTSC) is 10:11. The pixel aspect ratio for 625-line video (PAL) is 59:54. Exactly. (Well, not exactly; see "What is square anyway?" below.)
An image with a standard display aspect ratio of 4:3 (i.e. TV) with 480 lines (i.e. NTSC) would thus have:
640 square pixels per line, or 704 non-square 10:11 pixels per line Note that a 720 pixel scanline has thus captured an extra 8 pixels on either side! A 720x480 frame is not actually a 4:3 image.
An image with a standard display aspect ratio of 4:3 (i.e. TV) with 576 lines (i.e. PAL) would thus have:
768 square pixels per line, or 702+54/59 non-square 59:54 pixels per line Note that a 720 pixel scanline has thus captured an extra 17+5/59 pixels! (Too bad it doesn't work out to an even 8 per side...) A 720x576 frame is not actually a 4:3 image.
DV Frame Sizes
Standard (non-widescreen) provides a frame size of 720x480 (DV-NTSC) or 720x576 (DV-PAL) using appropriate Rec.601 pixel aspect ratios. Thus, according to the previous calculations, these frames contain a visual image which is slightly wider than TV's 4:3 display aspect ratio. To be displayed properly in a 4:3 screen or window, the frames should be cropped by 8 pixels on each side.
Widescreen Format
Standard TV has a 4:3 aspect ratio. The new widescreen format (HDTV, anamorphic DVD's, wide DV, etc.) has a 16:9 display aspect ratio. Frames are encoded at the same size, but with an appropriately scaled pixel aspect ratio:
525-line (NTSC): 10:11 / 4:3 * 16:9 = 40:33 625-line (PAL): 59:54 / 4:3 * 16:9 = 118:81
VCD Requirements
An NTSC VCD contains an MPEG-1 video stream with 352x240 frames. The NTSC-VCD pixel aspect ratio is just Rec.601's standard 10:11. The frames implicitly are downsampled from a source containing 704x480 pixels. This is equivalent to a truly 4:3 source containing 640x480 square pixels.
Similarly, a PAL VCD contains on MPEG-1 video stream with 352x288 frames. The PAL-VCD pixel aspect ratio is just Rec.601's standard 59:54. The frames implicitly are downsampled from a source containing 704x576 pixels. This is equivalent to a source containing approximately 768x576 square pixels.
DV source material (with 720x480 or 720x576 sized frames) must be cropped by 8 pixels on each side (16 total) before scaling and encoding for VCD.
Exact 4:3 source material with Rec.601 pixels (i.e. 704x480 or 704x576 sized frames) or square pixels (i.e. 640x480 or 768x576 sized frames) does not require any cropping or padding before scaling and encoding for VCD. The actual MPEG-1 specification specifies the following pixel aspect ratios:
1.0950 for "CCIR Rec. 601, 525-line" images with frames of "711x487 at 4:3" 0.9157 for "CCIR Rec. 601, 625-line" images with frames of "702x575 at 4:3" These numbers are self-consistent, but otherwise rather contrived. They should have just said "10:11 for CCIR Rec.601, 525-line" and "59:54 for CCIR Rec.601, 625-line" since that is apparently what they meant and that is how they are used.
SVCD Requirements
SVCD's are, like everything in the video world, just a little bit different.
An NTSC SVCD contains on MPEG-2 video stream with 480x480 frames. The frames implicitly are scaled from a source containing 720x480 Rec.601 (10:11) pixels. This is equivalent to a source containing approximately 654.5x480 square pixels. One could even say that the NTSC-SVCD pixel aspect ratio is: 10 720 480 10 * 3 15 Rp = ---- * ----- * ----- = -------- = ---- 11 480 480 11 * 2 11
Similarly, a PAL SVCD contains on MPEG-2 video stream with 480x576 frames. The frames implicitly are scaled from a source containing 720x576 Rec.601 (59:54) pixels. This is equivalent to a source containing approximately 786.67x576 square pixels. One could even say that the PAL-SVCD pixel aspect ratio is: 59 720 576 59 * 3 59 Rp = ---- * ----- * ----- = -------- = ---- 54 576 480 54 * 2 36
Since it uses MPEG-2, SVCD also allows for encoding/specifying 16:9 material. Scaling appropriately, the requisite pixel aspect ratios for such sources are:
NTSC 16:9 source: 15:11 / 4:3 * 16:9 = 20:11 PAL 16:9 source: 59:36 / 4:3 * 16:9 = 59:27
DV source material (with 720x480 or 720x576 sized frames) can be directly encoded to SVCD --- with 2:3 scaling, of course, but no padding or cropping.
Exact 4:3 source material (with Rec.601 pixels, i.e. 704x480 or 704x576 sized frames) needs to be padded to a width of 720 before scaling and encoding for SVCD.
Square-pixel 4:3 source material (640x480 or 768x576 sized frames) likewise needs to be padded (to widths of 654 and 786 respectively) before scaling and encoding for SVCD. According to the actual MPEG-2 specification, the video stream sequence header specifies the intended display aspect ratio rather than the pixel aspect ratios, from one of these choices:
4:3 16:9 2.21:1 (Oh, and you can also specify "1:1 pixel aspect ratio".) This is supposed to be an improvement over MPEG-1's contrived pixel aspect ratio descriptions. It's not, because in practice, MPEG-2 sources are 720x480 or 720x576 with CCIR-601 pixels, which are slightly wider than 4:3 or 16:9. So, as before, the actual numbers in the specification don't mean very much.
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Post by Garrett on Nov 29, 2004 20:47:00 GMT -8
Oh.. I almost forgot.. Apple has a really nice new (virtually lossless) compression codec coming out called H.264 ( I know this has nothing to do with DV footage, but it something to be aware of, since it has been adopted by the MPEG (Motion Picture Experts Group) as the new standard for new High Definition DVD compression), coming soon to a DVD disc near you! And Shake can export in this format of course... I actaully also seem to recall After Effects being able to read this footage? Ah.. who knows?! Oh yeah.. it will be standard in OS X 10.4 Tiger! Here's some links if you want to learn more: www.apple.com/macosx/tiger/h264.htmlwww.apple.com/pr/library/2004/jun/23quicktime.htmlAnd.... If you want to get really dorky: www.vcodex.com/h264.htmlThere you go.. as I push up my glasses and adjust my pocket protector
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johnb4467
Junior Member
"That which does not kill us, makes us stronger." -Nietzsche
Posts: 54
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Post by johnb4467 on Dec 3, 2004 3:27:35 GMT -8
It is literally amazing how far codecs have come. I remember using the original indeo codec...man did it look crappy! Blocks of stuff everywhere...and every pixel changed every frame! A near-lossless codec...man we have gone a long way!
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Post by Vince De Quattro on Dec 3, 2004 13:59:05 GMT -8
A near-lossless codec...man we have gone a long way! there are lossless codecs available now. as far as pixel ratio go, AE will adjust the display qualities of your media (square vs non-square) as long as you define them properly when imported. make sure that you render out a non-square pixel aspect ratio if your bg plate is dv. if you're still a bit confused, check out the link under the Compositing (After Effects) FAQ.
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