Replacing a background using gray or green backgrounds (chromakey).

[brice]

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. No nudes appear in the PDF. The technique was introduced someone on a another forum. Not sure if it's kosher to point folks to other forums, so I'll post a link here. Not sure if it's his originally, but I wouldn't know how to do it if he hadn't shared it. I wrote this tutorial based on what he described in the PDF to show others how it's done. It's not a perfect tutorial, but it gets the job done and points out some lesson learned.

My objective was to take this green screen image and replace it with a different background using the technique outlined in the PDF normally used with gray backdrops. 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 gray background images that are properly lit... read on for more.

Original image:



The final image is:


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 [CG Textures] - The worlds largest free texture site

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.


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.



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:


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.



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.

[brice]

I found a way to solve the green screen color cast problem:

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

Here are two images: the screen shot of the PS layer while working it and the end result. It looks like I missed a spot or two on her left side, image right.

[oceanofapathy]

Hi, I just wanted to say thanks for posting this. Though I do background manipulation quite a bit, I've never shot green screen, and have been considering it. I think I'll get a dark gray BG instead, and just deal with any issues of gray "leaking" into the image of the person. Should be easy cleanup. Right now I do white or black, and it's challenging sometime to "push" the background really far away (like white to black or the other way around). Using gray should provide some balance.

Thanks!
-Porter

[dicksallee]

I think you will find that if you hold down the ALT key you can drag (copy) a layer mask to a different layer.

[cesarke]

try FXhome photokey 2 a great programm


FXhome.com :: Special effects, keying and compositing software

[brice]

Thanks for the input folks. The technique here works if you have photoshop and want the "free" method. There are countless other options for green screen addons and stand alone programs.

[racingmoose]

Thanks for your insight Brice. I've also read the information on other forum you mention but it's always great to get first hand input and suggestions from someone trying it for the first time.