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HVX color key question
I’m trying to find out the best settings on the hvx for pulling a key later. Right now we’re shooting for 720 24p. The DP is suggesting we shoot in 60p for higher quality. This will no doubt add to my work load. Is there truly any benefit to shooting in 60p? And what would you suggest is the best settings for green screen work.
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10 Replies
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It all depends on your format for delivery. There is no advantage in 60p regarding chroma keying. Maybe your DOP is referring to motion blur, but he can always reduce exposure time for 24p as well.
Sometimes it makes sense to reduce blur for keying and add some to the composite with vectorized filters like RSMB.Regards,
Uli
Author of “DVDs gestalten und produzieren”, a book on professional DVD-authoring in German.
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Too much motion blur is right. The tests I worked with apparently had a slow shutter which made it difficult to key. I hope to convince him to change the shutter speed. I’d rather do that than 60 though I think he’s going to do both. One more question… would I get a better key with 1080 24p than the 720 24p? Thanks again.
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Yes you will get a better key with 1080/24p than you will with 720/24p. There’s more detail retained in the frame (about 20% more), but the big benefit is that the HVX’s 1080 image is much less compressed than its 720 image. The 1080 frame has twice as much bandwidth, but only about 20% more detail to deal with, so it has a lot more bandwidth per pixel available and accordingly it delivers less-compressed footage. You have 20% more detail and 100% more bandwidth.
The downside is, of course, that you’ll be taking up 2.5x as much space on the P2 cards. But assuming that ultimate quality is your goal, 1080 should be your choice.
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Hi,
I’m the DP in question here.
The 2 main problems we have are these:
1. The motion blur- what would our optimal shutter speed be shooting 24pa? We are shooting a rock band that jumps around alot and makes fast moves. We are just going for a good clean image.
2. The stair-stepping/compression-aliasing you get with the HVX. From what I understand the 1080 will go the farthest to minimize this. And of course a good backlight aswell. I’m trying turning the detail levels down on the camera too. Any thoughts on this?
Thanks in advance,
Dave
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Barry,
How does it have twice as much bandwidth when they are both at 100mbit
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Well, shooting in 24p would normally mean shooting with 1/48th of a second exposure to have a good amount of motion blur. Otherwise you’ll get jerky motion when something (or somebody) is moving fast.
But for chroma keying you’ll want clean edges: use the shortest time you can afford in your lighting situation. Also turn down sharpness until you don’t see any double edges on a good monitor any more, they can also spoil your key. After compositing you may need to introduce some motion blur again, I’d recommend RSMB from RE:VisionFX.Regards,
Uli
Author of “DVDs gestalten und produzieren”, a book on professional DVD-authoring in German.
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I would shoot 1080 30p.
Aloha
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[Brooks] “How does it have twice as much bandwidth when they are both at 100mbit
Thanks”I’m talking about at equal frame rates. At 24p you have effectively 40 megabits in 720/24p mode, and 80 megabits in 1080/24p mode.
Put a simpler way: every frame of 720p is allocated exactly the same amount of space, 1.67 megabits (100/59.94 = about 1.67 megabits). 720 stores up to 60 frames per second, and the amount of space allocated per frame is constant.
And every “frame” of 1080 is allocated a fixed amount of space, being about 3.34 megabits (100 / 29.97 = 3.34 megabits). 3.34 is twice as much as 1.67, so on a frame per frame basis, 1080 has twice as much bandwidth allocated per frame. And in the matrix of pixels, 1080 has twice as many pixels to encode per frame too (each frame of 1080 encodes 1280×1080 pixels = 1,382,400 pixels; each frame of 720 encodes 960×720 pixels = 691,200 pixels per frame). And 1,382,400 = 2 * 691,200.
So you have twice as many pixels in half as many frames, so it all nets out at the same 100 megabits. And, because the same number of pixels per second are being encoded in 100 megabits (1,382,400 x 29.97 = 691,200 x 59.94) you end up with the same compression ratio, 6.67:1.
Which would be all fine and dandy, except that the HVX doesn’t deliver enough pure resolution to fully saturate the 1080 frame size. It does on the horizontal, but on the vertical it only musters about 800 lines of detail. It’s more than enough to fully saturate the 720 frame, but not quite enough to fully saturate the 1080 frame.
So you end up with twice as much bandwidth (3.34 megabits per frame) but only having to encode about 30% more detail. The result is that a 1080 frame is less “taxing” to the compression engine, and you get milder compression and fewer compression artifacts. We’ve demonstrated it over on our other forum, search for posts by Jim Arthurs; 1080 delivers more resolved detail than 720 with milder compression.
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Get the most from your DVX camera. The DVX Book and DVX DVD are now available on ebay and at Amazon (https://www.fiftv.com/db) -
Hi, Barry,
I did a search and found nothing. Can you direct me to the “other forum”, or elaborate please?
thx,
HarryD
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I’d love to, but every time I post the name or a link, the post gets held in a queue for approval… I guess they don’t like that other forum over here all that much, or don’t like redirecting COW browsers to look at a competing forum.
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Get the most from your DVX camera. The DVX Book and DVX DVD are now available on ebay and at Amazon (https://www.fiftv.com/db)
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