Ratio Questions????

[alamski]

Hi there guys,

I am fairly knowledgeable in the conversion side of things but i just dont seem to understand much about ratios (16:9 and 4:3) and was wondering if someone here can help me understand it better? I have a Wintv PVR 250 capture card and have done a few captures from my digital satellite and the quality has been excellent. I however am not sure what aspect ratios they were captured at and dont know if the ratios are correct at all.

OK, for example, I capture a short program from the telly, now this program is ORIGINALLY transmitted in 4:3 format, will the capture card record at 4:3 or will I need to convert it using programs such as TMPGenc or Canopus?

Is it possible to get a 16:9 ratio from a 4:3 capture?

Any help will be good, furthermore if anyone knows a site or anything where info like this available, I would greatly appreciate it if you can post it here.

Thanks again.

Alam

[EddyH]

Well if it's come in letterboxed or anamorphically squashed as 16:9 to fit in a 4:3 screen you're in luck, especially if you have a decent deinterlacer for the first case. Just crop to fit if neccessary and then resize to your desired size & shape.

If it's full frame, normal unsquishedness then you can get 16:9, but you'll have to cut some bits off the top and bottom (1/8th each end, IIRC) and resize or whatever.

If you'd like to explain exactly what bit about the ratios you don't understand...

equivalent pixel widths...

         NTSC (square vga equiv)     VCD (unsquare)    PAL (s.v.e.)

4:3       320 (acutally 352)               352                      384 (ac. 352)

16:9      427                                   469                      512
equivalency                                   (squished into 352 with same vertical height for anamorphic)
                                                    (reduced into 352 with corresponding 25% height reduction for letterboxed)

blaaa :P

[DJRumpy]

Here's a more thorough description of what your asking for. Don't be intimidated by the length of the post. Just read through. It should hopefully make sense by the end....

Aspect ratios are created by dividing the width, by the height. Lets start with a television signal. It is broadcast with a 4:3 aspect ratio. if you divide 4 by 3 you get 1.33. This means your video is 1.33 times wider than it is tall. This is the aspect ratio of that video. To convert that signal to 16:9 (16/9 = 1.77, hence the almost 2 times as wide), you would need to make the video almost twice as wide, as it is tall. You do this by cropping from the top and bottom of your video to increase the width, compared to the height.

A 4:3 ( 4/3 = 1.33 ) aspect ratio video is almost square (it's only 1.33 times as wide as it is tall). This means you have to cut a LARGE amount of video off the top and bottom to make it 16:9. How do you figure out how much to crop? Since we're keeping all of our width (our width will not change) and cropping our height, you use the WIDTH as part of your formula of Width / height = Aspect ratio. We do not yet know our target HEIGHT. The only other part of the math is Aspect Ratio, and that we CAN figure out.

Lets say you captured/downloaded a video at 640x480. This is a common enough capture resoution. If you do the math (width / height), you find that this resolution has a 4:3 aspect ratio (640 / 480 = 1.33 or 480 * 1.33 = 640 ). Too convert a 640x480 capture to 16:9 (1.77 aspect ratio), you would need to figure out the new height value. Since we do not know our new height, but we DO know what aspect ratio we want, you do this by simply dividing our width by the desired aspect ratio:

WIDTH / ASPECT RATIO = HEIGHT

Since I can't even remember how to do fractions (I'm getting a little long in the tooth), I use the decimal value for the desired aspect ratio instead of 16:9.

16 / 9= 1.77

Now you might be thinking to yourself, that you've heard of 1.85:1, and 2.35:1, but 1.77:1 ?? This is simply widescreen 16:9. It is not anamorphic widescreen. Anamorphic widescreen takes an image that is even wider than 16:9, and squeezes it into a 1.77 aspect ratio. This makes it look tall because the left and right sides have been squeezed in horizontally like so: -->[ ]<--
It's stretched back out to it's full width during playback in case your wondering why they don't appear tall on televsion.

Now you need to make a choice. What aspect ratio do you need? That depends on your source material. If your source material is letterboxed, then you can crop off the letterboxing, and resize according to it's true aspect ratio. Usually, once you crop off letterboxing, you can simply look at the cropped video resolution to figure out what it's aspect ratio is. A quick search on google will also net you the movies original aspect ratio. If your movie is NOT letterboxed, you will have to chop out the top and bottom of your video. Usually a 4:3 to 16:9 chop job results in almost 1/2 of your image area being lost. You can see why this isn't a good idea. That said...on to the rest...

Divide your video's current width (640 in our example) by your desired aspect ratio. Lets assume for this disucssion that you want to convert it to an anamorphic wide screen movie with an aspect ratio of 2.35:1. You would divide 640 by 2.35 (640 / 2.35 = 272 ). So a movie that is 640 pixels wide (2.35 times as wide as it is tall), would be 272 pixels high. (put another way, since we now know our target height:

640 / 272 = 2.35

272 * 2.35 = 640

640 / 2.35 = 272

See how interchangle these values are. You only need two of them to figure out the third. Hopefully this is starting to make sense around now...

272 will be your new HEIGHT value. You started with 480, and you want 272, so you need to subtract 272 from 480 to get the AMOUNT TO CROP
( 480 - 272 = 208 ).

If we cropped 208 pixels from the top of our video, we'd end up with a mess, with everything showing up from the neck down , so we want to take equally from the top and bottom of our video. To do this, simply take half of our CROP value from the top, and half from the bottom ( 208 / 2 = 104 ). So you would crop 104 pixels from the top, and 104 pixels from the bottom.

Just remember. Get two of your values, to get the third. Use your width. Divide by your target aspect ratio, to get your new height.