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Green / blue screen
I am in need of a Blue and Green screen. I've seen lots of videos on how to light and photography with one of these screens but I can't find anything on editing with one of these screens. Any suggestions? Mar 17 18 05:36 pm Link Mar 17 18 05:53 pm Link Buzz Photography LLC wrote: It depends on what you are actually trying to do. Blue or green screens are usually used for motion picture chroma keying (tons of info about it) but that doesn't always work well for still photography. Mar 18 18 02:08 am Link chromakey is so cool   Mar 18 18 11:52 am Link Technically speaking it's called channel difference keying. Chroma keying is not very good for extraction. Of course the best way to work would be using a compositor but you could replicate the basic workings of it in photoshop. The approach shown below I discovered on my own just for curiosity sake so it might not be the de facto method but hopefully will be useful to someone. Plate taken from here: https://s3.amazonaws.com/fxhome-static/ … screen.jpg  BG from my personal library:  It's a two step process. Matte extraction and Screen subtraction. To extract the matte you need to subtract the red channel from your screen color channel. In photoshop it can be done real easy by using Channel mixer with these settings:  Set the levels to get rid of holes and garbage. Use black layer to paint out the remaining holes. Invert everything. Stamp the layers and use the result as a mask (just in case: Ctrl click on RGB in channels panel) Next is "screen subtraction". Clip channel mixer to the plate with the mask. Go to Green output channel and set Red +50, Green 0, Blue +50 Blending mode - Darken. That way you could suppress any green spill in general not just from green screens. The result:  Of course it's just a start. Compositing doesn't end there. Mar 18 18 02:10 pm Link Andrey, what you are showing is not really chroma keying as you are not working with chromaticity but with additive color (RGB) which tightly coupled with luminance. For a more efficient chromatic extraction you could try using HSL or LAB color model. Then you won't need to create so many layers, manually paint out holes etc. In fact if the shot is good you could easily do with just a blend-if slider. FWIW (re. our previous discussion about memory): it is more efficient (computing-wise) to have layer masks turned off by default and add them only when needed rather than have adjustment layers create fully filled white masks automatically. Mar 18 18 04:51 pm Link You might find something interesting here:https://www.youtube.com/watch?v=3Z1ROw4fhnQ Mar 18 18 05:46 pm Link  Based on discussion above, I did some regenerating, but still can not find a simple solution. A little lost with Andrey's method.... Finally have to go back to a normal method - instead of "refine edge", I start with Andrey's channel mixer to delete green, then in channel I deal with those grey parts(some calculation, some overlay brushing), with that perfect b&w result I get a selection, but stray hair is still green ( that's good, you have to keep most stray hairs), I use a blank layer, make it clipping, make it color mode, brush the green stray hair(avoiding inner hair) with adjacent hair color Mar 26 18 05:34 am Link findart wrote: You don't seem to understand that RGB, painting masks and selections is not chroma keying. Also the source image is of very low quality - you can never get good result with such data because in JPG chromatic data is always damaged due to the 422 compression which is used almost always. Even if you convert the image to 16bpc in order to reduce the artifacts of the toolchain itself it still lacks the necessary data: Mar 26 18 10:21 am Link anchev wrote: Color data is useful for masks but for extraction you need good edges and that data just isn't there in chroma channels, be that YCbCr or LAB. Holes and outside garbage is the easy stuff. Mar 26 18 11:20 am Link Andrey Bautin wrote: As I said in the beginning - there is a reason why it is not very suitable for photography. There is also a reason why making a good blue/green box is expensive. It needs quality paint, quality light etc. You can't simply do quality chroma separation without quality chromatic data. And FWIW Photoshop is not the proper tool for such things. Mar 26 18 03:09 pm Link I watched a Creative Live online class where the instructor was Renee Robyn who does a lot of still image compositing and illustrative work. Her background color preference was gray and you can see her using it in this video: https://www.youtube.com/watch?v=UblxncmkDRI Couple more where it shows her gray background and subjects. https://www.youtube.com/watch?v=GLRofDdXdcw https://www.youtube.com/watch?v=cSHVhN3EkxI I forget why she didn't like blue or green and preferred gray, but she does change the gray brightness depending on the final mood which she doesn't get with the chroma colors which seem suited for video editing. Mar 26 18 06:47 pm Link GRMACK wrote: Yeah, I remember someone famous said that, too, hard light can delete that gray bg easily. Mar 27 18 12:39 am Link anchev wrote: Exactly, never heard of chroma keying, or extracting.... don't know what's talked about here. Your LAB curve seems a neat method. Mar 27 18 12:44 am Link For photography grey is ok because you don't have to have consistent edges, you could just erase or mask out stuff you don't like. People avoid green screens because photoshop doesn't have a toolset to work with them and people afraid of the color spill. I looked through some tutorials on youtube and yep. All working around and not using the screen itself, thinking that it's a problem that you need to remove. findart, extraction is a compositing term for extracting edges using green/blue/red screen for subsequent compositing. Keying is a broad term. Chroma keying, Luma keying, Difference keying, House keying (probably not that one)... all have their uses. Mar 27 18 02:30 am Link Thanks for the info, Andrey. Mar 27 18 06:02 am Link findart wrote: https://en.wikipedia.org/wiki/Chroma_key Mar 27 18 07:26 am Link |
