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Chroma key (Tai Farnsworth)

Chroma-key
Chroma-keying is a special effect invented as an analog technique in the 1930s and made much easier by the advent of digital manipulation of pixels. In video and film production, chroma keying on green or blue screen allows filmmakers to  create impossible effects. It allows teachers to use standard off-the-shelf applications (Fireworks, for example) to combine graphic information from a variety of sources.

The subject can be shot on camera and then placed in an entirely different background scene. "Chroma" refers to the purity, brilliance or richness of a color, and the “key” refers to cutting a “keyhole” or outlined image. Blue and green are the primary colors used as backgrounds because those are the least likely human skin tones.

Teachers can use chroma keying through Adobe Photoshop, Fireworks or Pixlr, as we have done in class. In this case, the digital image, shot against a green screen, can be extracted from the green background.

This is done with a pixel range isolation tool like the Magic Wand, seen to the left.

To chroma-key in Pixlr you can follow the following instructions - 

1: upload photos to Pixlr from your desktop. You can do this when you open the app by selecting 'open image from computer.' 

2: Once you've selected an image it will show up on the screen. You need to crop your images so only the main image and the background show. For example, in a picture that will just be a bust shot it is recommended to crop from the shoulders to the top of the head. That way, the variety of green in the background is minimal. 

3: Next, select the Magic Wand Tool as pictured above.

4: Click on the green color that is most prevalent in the picture. If the color green is too varied it may select an incomplete sample from the image. In that case you can undo the application by selecting cntrl + z and either a: select a different shade of green in the picture or b: move onto step 5.

5: The tolerance of the image can be changed to help more accurately remove the background. The tolerance slide, in the upper left hand corner, is automatically set to 21. (If you cannot see it, it's because you have not selected the Magic Wand Tool.) Set the tolerance around 40 and retry step 4.

6: Once the selection is as close to accurate as possible you can remove the green by hitting the delete key. Again, if it didn't work to your liking, or, it took off too much, you can undo by hitting cntrl + z and then start again at a different tolerance.

7: If you need to delete some stray green, maybe behind your hair, you can do so with the erase tool. 

8: Now, you must delete the white background so that you are left with only the main image (in our example this is the bust). To do that you make sure the image is unlocked. In the layer box on the right hand side is the title of your layer and a small lock. Double click the lock to open it and then repeat steps 4 through 6 while selecting the white background. You'll know you've been successful when the checkered background shows up. Yay!

9: Next we need to upload the background picture of your choice. You can do that by selecting 'File' > 'Open Image' and selecting your picture. The picture will open up as a new box in the same work space and will be added to the layers list on the right hand side. 

10: Crop the picture to the appropriate size using the crop tool that looks like two triangles overlapping to make a square and clicking enter. You can also change the pixel size by selecting 'Image' > 'Image Size' and adjusting the height of the pixels (the width will adjust accordingly). 

11: When you have the background picture to your desired size you can drag your chroma-keyed picture to your background. You do this by selecting the chroma-key picture and dragging the picture label from the layer box on the right hand side to the actual background. 

12: If the chroma-keyed picture is the right size in relation to your background picture and you've placed it in an area that suits you - success! If not, you can ctrl + z to undo and then adjust the size of the chroma-keyed image the same way as you adjusted the background image in step 10, through the 'Image' tab and redo step 11. And then - success!

13: If you would like to write words on your chroma-keyed image you can do that with the text tool that is a little 'A.'

14: When you've finished editing your image to your liking you can save it onto your desktop by using 'File' > 'Save' > 'My Computer.' Make sure to save it somewhere where it will be easy to locate and by an easily identifiable name, as you won't be able to get it on Pixlr again (unless you pay for the pro version). 

15: Now go out and chroma key everything!!! With the magic of chroma-keying you can go to the Bahamas without even leaving your home.

Read more: The History of Chroma Keying | eHow.com

Read more: The History of Chroma Keying 2 | eHow.com 

Read more: Chroma Key at Wikipedia



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