What is the mathematical formula to figure out the right resolution when resizing?

[Newtype]

I've been doing some reading for days and I'm so confused over this. I have a source that's 1440x1080 ( 1.33 ) in resolution when I check it with MediaInfo. However, if I check it in Media Player Classic (File => Properties => Details), it says that it's 1920x1080 ( 1.78 ). Why? Does it have something to do with DAR and PAR? Because I read about those things a million times and I still don't understand.

In MeGUI, it asks me for the Input DAR and it automatically detects it as 16:9. Does that mean that the DAR is 1920x1080 and the PAR is 1440x1080?

Then, I want to resize it to a 720p resolution. I saw some resolution table located here (scroll down a bit):
http://ribolabs.wordpress.com/2009/05/23/guide-for-re-encoding-blu-ray-to-x264-mkv-megui

After seeing it, I'm guessing there's some kind of mathematical formula to find the right resolution for my video, just like the person who wrote that blog did. What is that formula? And do I use 1440x1080 or 1920x1080 to calculate it?

Thanks for reading.

[redwudz]

Try that without the code tags: http://ribolabs.wordpress.com/2009/05/23/guide-for-re-encoding-blu-ray-to-x264-mkv-megui/

[deadrats]

I've been doing some reading for days and I'm so confused over this. I have a source that's 1440x1080 ( 1.33 ) in resolution when I check it with MediaInfo. However, if I check it in Media Player Classic (File => Properties => Details), it says that it's 1920x1080 ( 1.78 ). Why? Does it have something to do with DAR and PAR? Because I read about those things a million times and I still don't understand.

In MeGUI, it asks me for the Input DAR and it automatically detects it as 16:9. Does that mean that the DAR is 1920x1080 and the PAR is 1440x1080?

Then, I want to resize it to a 720p resolution. I saw some resolution table located here (scroll down a bit):

http://ribolabs.wordpress.com/2009/05/23/guide-for-re-encoding-blu-ray-to-x264-mkv-megui/

After seeing it, I'm guessing there's some kind of mathematical formula to find the right resolution for my video, just like the person who wrote that blog did. What is that formula? And do I use 1440x1080 or 1920x1080 to calculate it?

Thanks for reading.

it can be very confusing, but here's the deal:

your file is 1440x1080 with non-square pixels, specifically 4:3 pixels. if you solve the following equation:

(1440*4)/(1080*3) you get 1.78.

1920x1080 and 1280x720 use square pixels, (1:1), and when you divide 1920 by 1080 or 1280 by 720 you likewise get 1.78.

what some encoders and players (improperly) do is read a 1440x1080 4:3 file as 1920x1080 1:1 file, since they both have the same aspect ratio.

as far as megui is concerned, it is seeing the input display aspect ratio as 16:9 because 16/9 equals 1.78.

so, did i manage to confuse you even more?

[Khaver]

Your source file is what is called anamorphic widescreen. There is a block of numbers in the header of the file that tells the decoder to stretch the pixels in the vertical direction by 1.333333333 times when it's displayed. Thus your 1440 wide pixels get stretched by 1.333333 turning your squished 4:3 video to a correctly displayed 16:9 video.

If you want to resize 16:9 video without it being anamorphic just use these formulas. If you want the final width to be say 640 use 640 / 1.7777777 = 360. 360 would be your height. If you want the final height to be 360 use 360 * 1.7777777 = 640. So devide the width by 1.777777 to get the height or multiply the height by 1.77777777 to get the width.

[jagabo]

DAR = PAR * SAR

DAR = Display Aspect Ratio -- the final shape of the picture
PAR = Pixel Aspect Ratio -- the shape of individual pixels
SAR = Storage Aspect Ratio -- the frame dimensions

For example 704x480 16:9 DVD (PAR = 40:33):

16/9 = 704/480 * 40/33
1.7778 = 1.4667 * 1.2121
1.7778 = 1.7778

Or 704x480 4:3 DVD (PAR = 10:11)

4/3 = 704/480 * 10/11
1.3333 = 1.4667 * 0.9091
1.3333 = 1.3333

Your 1440x1080 source has 4:3 PAR pixels:

16/9 = 1440/1080 * 4/3
1.7778 = 1.3333 * 1.3333
1.7778 = 1.7778

[Newtype]

Aaahhh, thanks guys, that was really helpful! There's just one thing I don't understand now. How do you find the PAR? For example, jagabo wrote:

704x480 16:9 DVD (PAR = 40:33)

704x480 4:3 DVD (PAR = 10:11)

How do you calculate what the pixel dimensions are?

[jagabo]

Sometimes you can get the PAR or DAR from MediaInfo or GSpot. Sometimes you have to guess. For DVD:

720x480 and 704x480 NTSC 16:9 PAR = 40:33
720x480 and 704x480 NTSC 4:3 PAR = 10:11
720x576 and 704x576 PAL 16:9 PAR = 16:11
720x576 and 704x576 PAL 4:3 PAR = 12:11

Note that with 720 pixel wide DVD the 4:3 or 16:9 frame is formed by the inner 704 pixels (with 8 pixels of padding on each side) so the total DAR is a little wider than 4:3 or 16:9.

[Newtype]

Ah, I understand now. Thank you. I have one last question. When I'm resizing my 1440x1080 video, I choose 1280x720 and if I check "Suggest Resolution (mod16)", it resizes it to 1280x704. Why? From what I read, mod16 is when the width and height are divisible by 16 and 1280x720 are both divisible by 16. Why does it need to change the 720 to 704?

[jagabo]

Your are right, 720 and 704 are both mod16. I wonder if it decided to use 704 because MPEG encoders usually use the YV12 colorspace internally. In YV12 the luma channel (grayscale image) is encoded at full resolution but the chroma channels (the colors) are encoded at half resolution (each dimension). So encoding at 1280x720 results in chroma channels that are 640x360 -- and 360 is mod8, not mod16. Where as 1280x704 has 640x352 chroma channels -- 352 is mod16. Or maybe the program decided to trim away a small black border at the top and/or bottom? Some programs automatically do that.

[Newtype]

What you said about YV12 is interesting so I checked two other 1280x720 files that I had on my PC and checked "Suggest Resolution (mod16)" for both of them and they didn't change. They remained 1280x720, so I don't think it's the YV12 thing, unless there's something I don't know. As for the trimming, you mean cropping, right? Well, it didn't automatically trim any borders on the other videos either. However, the other videos are MPEG-4 and the problematic video that we've been talking about is MPEG-2. Could it be that one of the things you've mentionned is only happening with MPEG-2 videos?

And what about videos that have a height of 1080? It's a popular resolution but it's not mod16. How come? I think I remember reading somewhere that it's because it truly encodes at 1088 or something like that. If that's so, do you have to set it at 1088 yourself or does it secretly do it on its own?

[jagabo]

What you said about YV12 is interesting so I checked two other 1280x720 files that I had on my PC and checked "Suggest Resolution (mod16)" for both of them and they didn't change. They remained 1280x720, so I don't think it's the YV12 thing, unless there's something I don't know... However, the other videos are MPEG-4 and the problematic video that we've been talking about is MPEG-2. Could it be that one of the things you've mentionned is only happening with MPEG-2 videos?

It's possible your MPEG2 decoder outputs YV12 (or other YUV 4:2:0 encoding) and your MPEG4 decoder outputs YUY2 (or other YUV 4:2:2 encoding). I doubt that's it though. It's a mystery why MeGUI would want to resize that video to 1280x704 rather than 1280x720.

As for the trimming, you mean cropping, right? Well, it didn't automatically trim any borders on the other videos either.

Sorry. Yes, I meant cropping the frame.

And what about videos that have a height of 1080? It's a popular resolution but it's not mod16. How come? I think I remember reading somewhere that it's because it truly encodes at 1088 or something like that. If that's so, do you have to set it at 1088 yourself or does it secretly do it on its own?

I don't really know why 1080 lines was chosen for broadcast and Blu-ray HD. You are correct it's mod8, not mod16. This is a source of problems with some encoders/decoders that want to make it 1088.