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Thread started 04 Jul 2009 (Saturday) 12:04
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Trying Chroma Key using a technique used for gray backgrounds

 
bwolford
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Jul 04, 2009 12:04 |  #1

I took a shot at trying an "overlay" technique for replacing a gray background with another background as outlined in the glamour thread and summarized here in this PDF (external link). No nudes appear in the PDF. The technique was introduced by TooManyHobbies. Not sure if it's his originally, but I wouldn't know how to do it if he hadn't shared it.

My objective was to take this greenscreen image and replace it with a different background using the technique outlined in the PDF. Frankly, I've found a few lessons learned along the way that I'll share. One of the most important is the original technique works best with grey background images that are properly lit... read on for more.

IMAGE: http://thewolfords.com/POTN/greenscreenorig.jpg

The final image is:
IMAGE: http://thewolfords.com/POTN/greenscreen5.jpg

Lesson 1 - You'll notice a couple of important items. The green screen was not lit properly and put a green cast onto my subject along the legs and most of edges parallel to the ground, including her hair. This is one reason why this works with gray better. You won't have this artifact to deal with if your image is properly lit, but this list of a problem if the original background was gray.

Step 1 - Download a texture from http://www.cgtextures.​com/ (external link)
My original intent was to use two textures, but my PS2 skills are such that this proved problematic so I punted and used one texture.
IMAGE: http://thewolfords.com/POTN/Cliffs0109_2_Mbaw.jpg

Step 2 - Turn the green to gray. I masked the gymast using Select->Color Range and applied a hue saturation layer reducting the saturation of green to gray.

Step 3 - Won't be necessary if lesson 2 wasn't learned.

Lesson 2 - If you look at the original image, I had the foreground of the greenscreen too hot. There was a large difference between the floor and the background which posed a problem. I had to solve this problem.

I created a levels adjustement layer, with the same mask for the gymnast and then used trial and error to apply a gradiant to the mast to make the grey as uniform as possible throughout the image. This drove me crazy getting the gradiant right so I elected the "Close enough for a tutorial" option. I just adjusted midtones.

IMAGE: http://thewolfords.com/POTN/greenscreen2.jpg

Step 4 - To apply your new background.

  1. Simply copy the texture/background over your original image
  2. Change the layer mode to overlay.
  3. Mask the gymast.
You should have something like this:
IMAGE: http://thewolfords.com/POTN/greenscreen3.jpg

It looks ok. I liked the way the shadows blended nicely, but I didn't like the lack of DOF in the background so I applied one more step.

Step 5 - Apply some DOF

In this case I used a simple gaussian blur with a gradiant applied to the mask to give the impression of depth. I'm SURE there are better ways.

IMAGE: http://thewolfords.com/POTN/greenscreen4.jpg

You can also see the mask I made of the gymnast that was used repeatedly in layer as a mask. Does anyone know how to copy a mask from one layer to another. I just created an invisible layer to store the mask and had to copy paste it to the mask every time. There's got to be a way to copy masks...

If it wasn't for the green color cast (that wouldn't be there if i had a) used a gray background and b) lit it right), this look pretty good IMHO.

If anyone has any thoughts or suggestions for the technique, share them. I'm specifically interested in:


  1. Ideas about how to light the seen evenly to minimize color cast when shooting with green screen
  2. Copy a mask from layer to layer
  3. Improve DOF
Hope you found this helpful.

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DozerLYP
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Jul 04, 2009 13:27 |  #2

to Copy a mask from layer to layer, Just hold "Alt" and drag the mask to the other layer.
I don't think their is a "right way" to lit a green screen to prevent the color cast from it when your subject is that close to it. The only way to prevent this, is to have your subject far enough from it that the reflection is not an issue.


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René ­ Damkot
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Jul 05, 2009 12:37 |  #3

Getting rid of the green shine is about a one minute job: (Down and uploading takes most time ;))

Use a new layer, set blend mode to "Hue". Sample a skin tone. Brush.
Sample a hair tone. Brush again.

You probably should also fix the green cast on her outfit. Same method: Sample, brush.

I probably hit the background a few times here, as well as a part of her sleeve. On a high res image, with the subject on a separate layer, it's easy to clip the hue layer to the subject layer, so you don't color the background.


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Jul 05, 2009 13:30 |  #4

René Damkot wrote in post #8227475 (external link)
Getting rid of the green shine is about a one minute job: (Down and uploading takes most time ;))

Use a new layer, set blend mode to "Hue". Sample a skin tone. Brush.
Sample a hair tone. Brush again.

You probably should also fix the green cast on her outfit. Same method: Sample, brush.

I probably hit the background a few times here, as well as a part of her sleeve. On a high res image, with the subject on a separate layer, it's easy to clip the hue layer to the subject layer, so you don't color the background.


Missed some spots under the arms ;)

While chroma key can be used as a last resort, two things should be cautioned:
When possible, try to use the same color as the background you will use.
When possible, avoid letting the subject get too close to the key background. Resting on the key is about the worst possible thing, but as Rene showed, it's not the end of the world. Just better not to have to do it and waste time right ?


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bwolford
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Jul 05, 2009 21:38 |  #5

René Damkot wrote in post #8227475 (external link)
Getting rid of the green shine is about a one minute job: (Down and uploading takes most time ;))

Use a new layer, set blend mode to "Hue". Sample a skin tone. Brush.
Sample a hair tone. Brush again.

You probably should also fix the green cast on her outfit. Same method: Sample, brush.

I probably hit the background a few times here, as well as a part of her sleeve. On a high res image, with the subject on a separate layer, it's easy to clip the hue layer to the subject layer, so you don't color the background.

WOW! Thanks. So simple. I've tried so many ways to fix this and never considered this! Rene', you are a life saver. This wasn't a production image, but I have a few others that I had to scrap because I couldn't solve this very problem.

Took my own stab at it too. Incredibly simple!!!!


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Jul 06, 2009 00:06 |  #6

Missed a bit on the waist, left arm/hand, right side of chest and stomach, and left foot ;)


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Jul 06, 2009 05:37 as a reply to  @ basroil's post |  #7

Great thread. I'm bookmarking this one! Thanks so much for posting.


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Jul 06, 2009 06:32 |  #8

OK, now someone show me how to do it with Elements, which doesn't have layer mask. Although I've downloaded an action that does layer masks, I haven't figured out how to make it work.




  
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wizeguy4
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Jul 06, 2009 10:27 |  #9

how do you create a gradient mask like you did when you created the DOF?

I also bookmarked this thread


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Jul 06, 2009 10:30 |  #10

Won't help with broad areas of skin resting directly on the green material (her shins/knees) but you could have greatly mitigated the green hair and arms with some well flagged (to avoid flare and spill) backlighting.


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Jul 06, 2009 13:00 |  #11

wizeguy4 wrote in post #8232419 (external link)
how do you create a gradient mask like you did when you created the DOF?

I also bookmarked this thread

I assume you mean in Photoshop? Simple


  1. create the mask on the layer that has beeb blurrred above the layer without the blur.
  2. select the gradient tool. Make sure it's a linear gradient.
  3. set background and foregound color selections to black and white
  4. hold the shift key and drag the mouse from the starting point of where you want out of focus to begin, down (in my case) to where you want the image to be tack sharp.
Does that help?

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bwolford
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Jul 06, 2009 13:01 |  #12

FlyingPhotog wrote in post #8232435 (external link)
Won't help with broad areas of skin resting directly on the green material (her shins/knees) but you could have greatly mitigated the green hair and arms with some well flagged (to avoid flare and spill) backlighting.

Agreed. I had over lit the background and didn't provide enough separation.


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Jul 06, 2009 13:06 |  #13

subscribing! thanks for posting!




  
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René ­ Damkot
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Jul 06, 2009 13:18 |  #14

Totally missed the blur part.
Better way to "fake" background blur is to use a depth map & lens blur:
This results in less blur instead of less opaque blur ;)

Thread.
Here's a step by step video:Link (external link)

If you have the girl on a seperate layer on top of the background, there's no need to mess about with a complicated depth map here. Simply use a gradient, and you can set the "focus" where you want by dragging the slider (or by clicking in the image in the lens blur dialog window):

(Not color managed screenshot, sorry about the garish colors)

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wizeguy4
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Jul 06, 2009 14:34 |  #15

bwolford wrote in post #8233340 (external link)
I assume you mean in Photoshop? Simple

  1. create the mask on the layer that has beeb blurrred above the layer without the blur.
  2. select the gradient tool. Make sure it's a linear gradient.
  3. set background and foregound color selections to black and white
  4. hold the shift key and drag the mouse from the starting point of where you want out of focus to begin, down (in my case) to where you want the image to be tack sharp.
Does that help?

I am guessing that I also have to have clicked on the mask? Otherwise i would just fill the layer with the chosen colors?


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Trying Chroma Key using a technique used for gray backgrounds
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