GH3 1080p video stretched vertically?

[Shawn Storm]

While testing the differences between quality of video at 1080p modes and 720p modes, I noticed something that seems odd to me. All the 1080p footage seems slightly stretched vertically. I noticed this because I was comparing identical scenes and jumping back and forth to inspect image quality. At first I thought it was my imagination, but I did some tests to confirm what I was seeing.

From my tests, it appears that 1080p footage gets stretched roughly 6-7 pixels vertically (downwards). I also took some still photos at 16:9 ration and normal 4:3 and cropped them to confirm that it is just the 1080p video that is acting this way. So for example, if a square box is filmed, it will be square when filmed at 720p (or photographed), but becomes a rectangle when filmed at 1080p (being roughly 6-7 pixels taller than wide if the square was filling the frame).

Here is an image that illustrates what I am talking about (it is cropped to the center of the frame). As you can see the box in the 1080 footage (which I scaled down to 720p for comparison) is roughly 2 pixels longer up and down. The difference is only 2 pixels because the box is only taking less than half the frame. If it was taking up the whole frame it would show an approximate 6 pixel difference.



Anyways, I thought this is strange and find it slightly annoying. Does anyone know the reason why this would be? Is there some logical explanation that I am unaware of?

Thanks,
Shawn

 

[TheTrickster]

I've also noticed (via a Ninja2) that on hitting record (on the GH3) the picture slightly changes height on the Ninja screen (from monitoring to recording)...Have you tried different codec's ?

 

[Shawn Storm]

...Have you tried different codec's ?

I tried both 720p modes (MP4 and MOV) and they lined up perfectly. With this test I only used the 1080p30 MOV (50mbps), but I think I saw the same behavior on other codecs (but don't have visual proof).

Here is another image taken from that same test. This time I included the full vertical length (720px) to show the 6 pixel difference. Notice that the top is lined up perfectly, and by the bottom of the frame there is a 6 pixel difference.

 

[TheDingo]

...Just out of curiosity, what NLE are you using ?

 

[Shawn Storm]

...Just out of curiosity, what NLE are you using ?

I use FCPX, but that doesn't seem to be a factor. Most of the tests I did were straight off the camera without going into my NLE. I was comparing the videos with Quicktime (both old and new versions). I also opened the videos with VLC just to make sure it isn't some weird Quicktime thing.

I am actually certain this is happening in the camera and isn't a software thing. To prove this, I just looked on my camera at the two videos (which luckily were still on my SD Card) and the difference is clearly visible there as well.

So I am still curious as to why this happens. I assume it isn't just my camera (perhaps someone else would be willing to test this). A test only takes a minute; simply set up the camera on a stable tripod and film a flat scene with plenty of identifiable details. Be sure to have the same reference point in the same spot on all videos. Then film at 1080p and 720p and compare the two files either in camera or on a computer. Pay attention to specific details at the bottom of the frame and see if they shift (as in my examples).

-Shawn

 

[Shawn Storm]

..Have you tried different codec's ?

Hello again,

I had some spare time today so just for curiosity sake I did some more testing. I tried a number of codecs and confirmed that this "issue" seems present on various codecs. I tested the following: 1080p30 AVCHD, 1080p30 MP4, 720p30 MP4, 1080p30 MOV (IPB), 1080p30 MOV (All-Intra), 720p60 MOV (All-Intra)

All of the 1080p videos were stretched in a manner consistent with my above findings. Both 720p videos were not. I also took extra care to make sure frames lined up by minimally touching the camera and using my iPhone as a wireless remote to change settings and trigger recording.

So the question remains: Why does this happen. Also, do other people see this as an issue? I feel the 720p footage is more spatially accurate, where as the 1080p footage makes vertical subjects slightly "skinnier" (such as a person).

-Shawn