Tuesday, October 13, 2009

Notes to myself: advanced selection in Photoshop

In Photoshop there are many ways to select a desired area, which is then used to cut-and-paste to another picture. When it comes to marking out the hair or similarly intricate objects, it requires some special techniques to make the job easier. One such technique is to use channels. The procedure is as follows:

1. Open an image, then go to the Channels window. Select the channel (in grayscale) that gives the most contrast.
2. Go to Image>Calculations. Choose Source 1 and Source 2 to be the same selected channel and choose the Blending option that maximizes the contrast. Set the Result to New Channel (Alpha channel).
3. Hit Ctrl L (shortcut for Image>Adjustments>Levels) to make adjustments to further boost up the contrast.
4. Use the paintbrush to paint the image, or eraser to erase undesired parts of the image. The idea is to be able to paint in the area that will be used as a mask later on. The hairy parts, which are difficult to paint accurately, should be made automatically black by the preceding steps so that not much painting is involved.
5. Without making any selection, but with the Alpha channel still active, hit Ctrl I (shortcut for Image>Adjustments>Invert) to inverse the selection from black to white if necessary. White is what will be used for mask later on.
6. Hold Ctrl and click the Alpha channel to make the selection for mask. Now reactivate the RGB channel and deactivate the Alpha channel (i.e. the "eye" icon). Go back to Layers window and you will see the selection you have made.
7. Alternatively, you can click Select>Load Selection and choose the Alpha channel to reload the selection to be used as mask. Use Ctrl J to copy and make a new image minus the background (if desired).
8. Now you can copy and past the selection to another image. To further improve on the parts of the hair that is not well defined, use Burn tool to darken. (In newer Photoshop, use Refine Edge).
9. Alternatively, if the hair is light in colour, try converting to Lab colour mode and use the channels to paint the background black, instead of the subject black. The technique is the same.

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