Is there a way in photoshop to convert k values to alpha values?

Question

Basically, I would like to make it so that the lighter a pixel is (the more white it has) the more transparent it becomes. Black pixels would be opaque, white pixels would be transparent.

I currently use the slider in blending options on the first tab to get rid of the white and make it transparent but find it works slightly better if I use the magic wand tool with delete.

I figure that there is some blending option out there but there are so many and I'm not sure which to use to achieve my goal.

(Using Photoshop CS6)

Answer 1

You can load the luminosity of an image as a selection, then create a mask and invert the mask. It sounds to me like you really want luminosity, not "white" data.

1. Command/Ctrl the RGB Channel Thumbnail in the Channels Panel. To load a selection. (This essentially loads the L channel from the L*A*B data)
2. Highlight the image layer in the Layer Panel
3. Click the Add Mask button at the bottom of the Layers Panel
4. Choose Image > Adjustments > Invert to invert the mask

Done. You can then use some Levels or Curves adjustments to fine tune the mask.

_{Image from http://www.saltartists.com/goats-on-trees/}

Answer 2

How to: Convert K values (Black intensity %) to Alpha values (0-255)

Duplicate the original layer.

Deselect the original layer and select the copy.

Convert it to Black and White. (Image --> Adjustments --> Black and White)

Open channels

Duplicate one of the color channels (they should all have the same pixelvalue)

Rename the new channel Alpha.

Return to Layers, delete the black and white layer.

Done.

You now have a new Alpha channel, based on the K value (black intensity) of each pixel. The relation between Alpha and K is: (100% - K value) * 255 = Alpha pixel value.

Answer 3

Set the layer blending mode to multiply to make white transparent and to screen mode to make black transparent.