An Up Close Look At Green Screen Shots

[BobDiaz]

In a perfect world, we would all shoot green screen videos with 4:4:4 color. Still, the world is not perfect and we must deal with the realities of cost. Yet, after looking at the green screen images up close, the imperfections do show under magnification, but are really small when shown on the screen.

http://i46.photobucket.com/albums/f103/KQ6WQ/Shots-4.jpg


What follows is a 10x magnification from sections of the above images. Looking at the DV25 image, you can clearly see the problem created by 4:1:1 color. While the limits in the color are in a horizontal direction, the errors appear in a vertical direction.

http://i46.photobucket.com/albums/f103/KQ6WQ/DV25-Green.jpg


Without question, the DVCPRO image has a much cleaner edge to it. At high magnification you can see some imperfections, BUT at a normal viewing distance, it will look very good or in non-technical terms, it kicks a**!!!

http://i46.photobucket.com/albums/f103/KQ6WQ/DV50-Green_Zoom.jpg


The 720p image surprised me, because it looked so good. This image made me believe that I was looking at a native 960x720 CCD, because I can't spot any glitch appearing every 4 scan lines. I reasoned that IF the CCD was 960x540, every 4 scan lines there should be a minor imperfection. Well, know we know that my logic was flawed and a 960x540 Native CCD DOES produce a clean 720p image.

Like the DVCPRO50 image, you can see the green appear along the edge every 2 columns. I do see a slight vertical error that occurs, but because it is vertical, it tells me that this is the limit in the horizontal color resolution.

http://i46.photobucket.com/albums/f103/KQ6WQ/HVX200_Key_720_Zoom_2.jpg


http://i46.photobucket.com/albums/f103/KQ6WQ/HVX200_Key_720_Zoom.jpg


The one mystery that I can NOT answer is why there is a vertical glitch that appears every 2,4,2,4,2,4,... pixels in 1080 mode. The glitch appears as vertical lines, so this has something to do with limits in the horizontal color resolution. This is odd, because with 1280x1080 at 4:2:2, we only need 640 color samples and 960 color samples is more than enough.

Could this somehow be connected to 1920 pixels being converted to 1280 and later on back to 1920? If so, 3 pixels would be converted to 2 pixels and later on back to 3 pixels. This might create a 6 pixel pattern that I see. Then again, it could be something else...

http://i46.photobucket.com/albums/f103/KQ6WQ/Panasonic_Green_Emery_1080_30p_Zoom.jpg


For comparison sake, here's a green screen shot from the Canon. The file name says it was shot in 1080i. I wish I had a shot taken in 24F or 30F mode, but this is what I have:

http://i46.photobucket.com/albums/f103/KQ6WQ/h1greenscreen1080i8bituncomp39.jpg


You can see the glitches appear in the horizontal direction. This is due to the lack of color information in the vertical direction.

http://i46.photobucket.com/albums/f103/KQ6WQ/h1greenscreen1080i8bituncomp39_Zoom.jpg



CONCLUSION:

There's no question that the DVCPRO50 is really clean for green screen video and the 720p footage offers a reasonably clean edge too. I wish I had more 1080 green screen stills to look at for comparison. It does bug me that I can't fully explain the funny 4, 2, 4, 2... glitch pattern and I don't know if it's normal or just something that only appears in that one green screen clip. Oh well, another mystery in life...



Bob Diaz

[Ralph Oshiro]

You should post more often, Bob!

[dolph2000]

Once again. the advantage of dvcpro 50 is huge. In most countries dvcpro or digibeta is highest you can get in broadcast and now we have it for under $6000. I find that amazing. Bob you're amazing to

[philnerd]

Bob, I'm reaching deep into my memory banks, but I think the 1080P HVX image there had... issues. I don't remember the original threads, but I think there was some discussion about the Apple codec maybe having problems there. It would indeed be nice if someone could post some quality 1080P green screen footage in raw MXF format.

www.philipwilliams.com

[vidled]

I don't doubt your conclusions, but do have a question regarding the first comparison: it "appears" that the first still of the girl's hand is slightly out of focus, possibly because of movement of the fingers?

[BobDiaz]

Bob, I'm reaching deep into my memory banks, but I think the 1080P HVX image there had... issues. I don't remember the original threads, but I think there was some discussion about the Apple codec maybe having problems there. It would indeed be nice if someone could post some quality 1080P green screen footage in raw MXF format.

www.philipwilliams.com (http://www.philipwilliams.com)


It's easy to see an image and jump on it saying there is a problem, but when I tried to recall the source, I couldn't remember it. IT may well be a different codec that caused the glitch and not hte HVX. It would be nice to get another green screen shot at 1080 to compare.



I don't doubt your conclusions, but do have a question regarding the first comparison: it "appears" that the first still of the girl's hand is slightly out of focus, possibly because of movement of the fingers?

This shot came from Barry, so maybe he can shed some light on it. There were other still shots from Berry that showed the 4:1:1 color problem. Right now, I need to go to El Camino College, so later on I'll look up the other DV25 shots and add them to this thread.


Bob Diaz

[oneinfiniteloop]

Very good stuff...this is making DVCPRO50 a very attractive source for low-budget keying with the HVX!

[imgentertainment@mac]

Bob agin you answer the big questions in terms that I can under stand DVCPro 50 kicks ass. That is all You need to say. Thank You

[BobDiaz]

Here is the 720p green screen shot that someone keyed onto another background. I'm sorry, but I don't recall who did this image. This image was cropped to fit better inside this message.

http://i46.photobucket.com/albums/f103/KQ6WQ/720_Image_Keyed_Crop.jpg


The edge of the image appears clean and even under 10x magnification, the edge holds together reasonably well.

http://i46.photobucket.com/albums/f103/KQ6WQ/720_Image_Keyed_Zoom.jpg


As the old saying goes, "The proof in in the pudding!" or in this case, the proof in in the photos. DVCPRO50 has the cleanest color edge, but the 720 image looks really good too. I'm sorry that I don't have another 1080 green screen image to zoom into to show how the color looks at the pixel level. So all I can show right now is that 960x540 Native CCDs produce very good color edges with DV50 and 720p. We'll just have to wait for another 1080 green screen image...



Bob Diaz

[Jack_Felis]

Holy schmeck that looks great!!!

Who cares if the 1080p mode doesn't work right? This 720p green screen still looks pretty good as it is!

Really, I don't expect too much native 1080p until the next generation of cameras, but if it somewhat works right now for the HVX200 then I'm all for it. Why's everyone getting all over this? You've got HD! Go shoot a movie!

[cre8tive]

...but when I tried to recall the source, I couldn't remember it...


Bob Diaz

Bob,
I think I kept the MXF file from the first green screen sample you posted here (the young guy shrugging). Do you want it?

Regards,

Cre8tive

[BobDiaz]

Bob,
I think I kept the MXF file from the first green screen sample you posted here (the young guy shrugging). Do you want it?

Regards,

Cre8tive

Thank you for the offer, but my computer can't play the file, so it won't do me any good.

I've been digging through my collection of images, but have yet to find the ideal image of a 1080 green screen shot or even a good shot with a sharp contrast in the colors. I did find one image that might work, but the edges seems a bit soft and I'm not sure if it's just the limit of the detail of the camera or that part of the image was out of focus. I may post a blow-up of that image just to show the 1080 color contrast.

Bob Diaz

[bgundu]

I may have time tonight for a 1080 Greenscreen setup.

[BobDiaz]

I posted some 1080 images in the following message. Rather than copy the whole message here, just clink on the link below:


http://www.dvxuser.com/V6/showpost.php?p=454978&postcount=147




Bob Diaz

[dvxStephen]

Bob- thanks for posting these. One question: Your 720p and other HD enlargements are 10x the native pixel count, right? Your zoom-ins appear to have same-sized pixels in each case, indicating this.

Obviously 720p has way more pixels than DV50...

If my guess is right it would suggest that, for SD work, the 720p image might not have quite the "per-pixel" edge quality of DV50, but if one shot in 720p, and THEN down sized to DV50 afterwards, the result should have a much cleaner key than an image originally shot with DV50 as the source codec.

Does this match what you're seeing?

[Yuval Shrem]

I may have time tonight for a 1080 Greenscreen setup.


Would you try a blue-screen test as well (will be interesting to test the theory of getting better key with greenscreen vs. bluescreen).

Thanks!

[BobDiaz]

Bob- thanks for posting these. One question: Your 720p and other HD enlargements are 10x the native pixel count, right? Your zoom-ins appear to have same-sized pixels in each case, indicating this.

Obviously 720p has way more pixels than DV50...

If my guess is right it would suggest that, for SD work, the 720p image might not have quite the "per-pixel" edge quality of DV50, but if one shot in 720p, and THEN down sized to DV50 afterwards, the result should have a much cleaner key than an image originally shot with DV50 as the source codec.

Does this match what you're seeing?


When I say a 10x zoom, a single pixel is converted to a 10x10 pixel image so that each pixel can be seen clearly. (In a few cases, I screwed up and it ended up as an 11x11 pixel image, but it was close enough for seeing what is happening.) I had to crop the image before I blew it up, otherwise the final file would be WAY TOO big.

This makes DV50 look sharper per pixel than 720 and 1080 looks worse per pixel. This does give a distorted view of how things really are. If I had made 1080 10x10, 720 should be shown as 15x15 and DV50 should be shown as 23x23. Thiis would keep the Dots (or Pixels) Per Inch on the same and allow you to see that the "fuzzyness" does not really increase in the higher resolutions. If I can get the time, I'll try creating some images to show that... Tonight I have a class to teach, so my chance for trying it will need to wait... but it would generate an interesting view of the data.

Assume that we shoot a green screen at 1080, 720, and DV50. Then the final result is reduced to SD. I've never doen this, because I don't have a camera to play with, but my understanding is that going from a higher resolution to SD will produce results equal to or beter than starting with SD. Berry or someone else who has tried it should be the final word on that question.


Bob Diaz