ASSIGNING ALPHA CHANNELS WITH PHOTOSHOP

This document touches on the following topics:

  • Establishing alpha channels by editing in Quick Mask Mode.
  • Assigning alpha channels using the Channel Palette.
  • To finely adjust an alpha channel and reduce annoying jagged haloes that might appear around images.

Once you have scanned original artwork (or created an image digitally) you can then attach an alpha channel to the file.

Making an alpha channel is the same as saving a selection. However, it is important to note that an alpha channel is actually an additional 8-bit greyscale channel that is attached to a 24-bit colour image (that is, 8 bits for each Red, Green, and Blue channel). There are only a few file types that we use at Emily Carr that will include an alpha channel, such as the following:

  • PSD (Photoshop document)
  • TIF (Tag Image File Format)
  • TGA (Truevision Targa Format)
  • PNG (Portable Network Graphic)

Sometimes, it is quite useful to scan animation drawings as 8-bit greyscale images (in the TIF format), in order to capture clear white areas in a drawing, regions that you will then remove when creating an alpha channel. However, you must remember to use Photoshop to convert the greyscale images to RGB files, by going to Image > Mode > RGB Colour.

By using Adobe Photoshop you can then take the following, fairly straightforward steps to save selections and make alpha channels.

USING EDIT IN QUICK MASK MODE TO MAKE AN ALPHA CHANNEL

You should start by using a PSD, TIFF, TGA, or PNG image. However, before you begin, you should also make sure that it is an RGB file. To do so, refer to the Photoshop’s main menu and go to Image > Mode > RGB Colour. Alternatively, you should check the Channels Palette (i.e., Window > Channels) to make sure that there is a composite channel for RGB, plus one each for Red, Green, and Blue channels.

  • Referring to Photoshop’s Tools (i.e., Window > Tools), take the Magic Wand tool and click on the area of artwork that you want to make digitally transparent. The region will then become active.
  • Go to the main menu and choose Select > Inverse.
  • Click the Quick Mask Button (found in the lower section of the Tools Palette). The selection will then turn pink.
  • In the Channels Palette (i.e., Windows > Channels) "Quick Mask" will appear as a channel (along with RGB, Red, Green, and Blue).
  • Save the document.
  • If you need to open the selection again, you should refer to the main menu and then go to Select > Load Selection.

If you want to view the Quick Mask (by hiding every channel but the Quick Mask), go to the Channels Palette and switch off the "eye" icons for RGB, Red, Blue, and Green. Leave the Quick Mask alone. As a result, you will find that the mask is represented as an 8-bit greyscale channel: what will be known as the transparent pixels are indicated as black and the foreground areas are “masked” as white. In a sense, black means "transparent" and white stands for "opaque."

You can edit the Quick Mask, provided that you reactivate the RGB channel (check the “eye” icon) and go back to the Layers Palette. You can then change the selection by using the brush, pencil, and eraser tools. For example, if you draw with the brush in this mode, the line will be viewed as the same pink, Quick Mask colour as the transparent selection in the Layers Palette. By drawing in this way, you will actually add opaque information to the alpha channel. These tools are useful if you are not able to make a proper selection with the Magic Wand Tool, for one reason or another.

You can also add and remove opaque information by working directly on the alpha channel itself. If you go back to the Channel Palette and activate Alpha (click the “eye” icon in the left column), while the RGB, Red, Green, and Blue channels are selected, the same pink color will appear around the mask area. As long as you select the Alpha channel, you can then use the brush and eraser tools to add and remove material from the matte.

If you happen to turn off the visibility of the RGB, Red, Green, and Blue channels but keep the Alpha channel visible, the changes that you make will be indicated in high-contrast black and white. You can still use the brush, pencil, and eraser tools to edit the mask.

USING THE CHANNELS PALETTE TO MAKE AN ALPHA CHANNEL

The following is a similar process to using the Quick Mask Mode (which was explained in the section above):

  • With the Magic Wand, click on the area of the artwork that you want transparent. The region will then become active.
  • Call up the Channels Palette (i.e., Windows > Channels).
  • Click the Save Selection Button located at the bottom edge of the Channels Palette (or go to the Main Menu > Select > Save Selection and choose Operation: New Channel).
  • "Alpha 1" will appear as a channel in the palette (along with RGB, Red, Green and Blue).
  • Save the document.
  • If you need to open the selection again, you should refer to the main menu and then go to Select > Load Selection.

By selecting Alpha in the Channels Palette, you can edit the image with the brush, pencil, and eraser tools. If you erase the black, you will add opaque information to the image file.

Again, see the above section, “Using Edit in Quick Mask Mode to Make an Alpha Channel,” for further information on how to edit an alpha channel.

FOR FINER CONTROL AT MAKING ALPHA CHANNELS

There are ways in which you can reduce or eliminate the jagged lines or haloes around images often seen with alpha channels.

First of all, once you have created an alpha channel (i.e., by saving a selection), you can refer to the Channel Palette (i.e., Window > Channels) and select the channel.

  • To be most effective, you should hide the visibility of the RGB, Red, Green, and Blue channels, but keep Alpha visible.
  • Once you have selected the channel, use the Navigator Palette's Zoom Slider to isolate a region of pixels to refer to.
  • Then, go to Filter > Blur > Gaussian Blur. From here, you can add a slight blur, between 0.5 and 1.0 for instance.
  • Save the document.
  • If you need to open the selection again, you should refer to the main menu and then go to Select > Load Selection.

Sometimes, you might want to expand a selection, so that the selection goes into the artwork a bit, to cover some of the soft, grey edge of a pencil line, for example.

  • When you choose to make a selection, take the Magic Wand Tool and click on an area of artwork (or areas with Shift + click), sections that you would like to turn transparent. The region will then become active.
  • Go to Select > Inverse.
  • Use the Navigator Palette's Zoom Slider to isolate a region of pixels to refer to.
  • From Select, choose Modify > Contract and then contract by 1 pixel. You should observe the edge of the lines in the artwork to ensure that the selection is suitable.
  • If you are satisfied, click the Quick Mask Button. The selection will then turn pink.
  • In the Channels Palette, "Quick Mask" will appear as a channel (along with RGB, Red, Green, and Blue).
  • Go to the Channels Palette and isolate the Quick Mask by switching off the "eye" icons for RGB, Red, Blue, and Green (leave the Quick Mask alone).
  • From the Main Menu, choose Filter > Blur > Gaussian Blur. While previewing the file, choose a suitable Radius of somewhere between 0.5 and 1.0.
  • Save the document.
  • If you need to open the selection again, you should refer to the main menu and then go to Select > Load Selection.

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