Calculating Proper Scale According to Aspect

[WARnux]

This question is in regard to what proper scaling size is for certain aspect ratios. I'm asking because I notice that mplayer is not consistent in the way it scales videos that are 4:3 and those that are 16:9. Here is what I mean:

16:9:
Original Size: 720x480
Scaled To: 854x480

4:3:
Original Size: 720x480
Scaled To: 720x540

Notice that for the 16:9 content, the width is changed, while for 4:3, the height is changed. I have heard that it is proper to keep the original height but scale the width, but mplayer is not always following that. Can anyone explain this?

[guns1inger]

mplayer is scaling correctly. DVD or DV material (which this appears to be) uses non-square pixels in order to maintain the correct aspect ratio inside a single resolution. The pixel aspect ratios determine which way the image needs to be scaled to get the correct display aspect ratio. What you are seeing here is how it is meant to be. In the case of 16:9 material, the pixels are wider than they are tall, so the height remains the same and the width changes on playback. For 4:3 material, the pixels are taller than they are wide, so the opposite is true.

[WARnux]

How does the player know that it needs to divide the width by the aspect rather than the multiplying height for DVD content? Is it part of the header information?

[poisondeathray]

It's a flag in the container or encoded into bitstream that tells the player to display as 16:9 or 4:3

Scaling behaviour is player dependent; for example kmplayer will display 4:3 NTSC as 640x480

There's arguments for either side being "proper" or not. For example, some people say it's not right to lose the 720=>640 pixels, because that 720width was actually encoded into the frame dimension.

[jagabo]

How does the player know that it needs to divide the width by the aspect rather than the multiplying height for DVD content?

It's an arbitrary decision on the part of the mplayer authors. You can adjust the width or the height or both. All that matters is that the final frame dimensions are a 4:3 or 16:9 ratio. Other than that there is no right or wrong way.

There is one little twist: interlaced video looks better if you scale the horizontal axis rather than the vertical axis. Ie, 853x480 and 640x480.

[edDV]

There is one little twist: interlaced video looks better if you scale the horizontal axis rather than the vertical axis. Ie, 853x480 and 640x480.

Yes, this is important for interlace video which suffers greatly when resized vertically. Any resize visibly disturbs the interlace structure and makes post deinterlace or inverse telecine nearly impossible. Among other issues, TV or HDTV display is greatly compromised. Horizontal resize has few artifacts.

Normal 4:3 resize would be 640x480. Wide 16:9 resize in decimal math results in 853.333x480 but digital display cards work in multiples of 16. 640/16=40, an interger. 853.333 does not so the closest width divisable by 16 is 848. The aspect distortion is about 1%. But since horizontal resize results in manageable artifacts 854 is usable.

[WARnux]

In case anyone is interested, my decision is to scale 4:3 video horizontally, making it 640x480.