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Adam's Transparency Tutorial (Alpha channels and more)

Some time ago, Adam Crowley posted a superb tutorial on using Alpha Transparency in PowerPoint 97. We used it here in the PPT FAQ with Adam's permission. Adam has since done even more testing and has graciously supplied this updated version of his Alpha Channel Tutorial

Introduction: alpha channels - what they are/how they work:

Most RGB bitmap files are 24-bit, i.e. there are three 8-bit (256-level) channels, one each for Red, Green and Blue. The image combines 256 levels of each color to make a full color image containing up to 256*256*256 or 16,777,216 colors, often abbreviated to 16.7 million.

Some file types, TIFFs* and PNGs* notably, allow you to create and save an extra channel, making it a 32-bit image. (Targa (*.TGA) files can also include a fourth transparency channel, but PowerPoint doesn't recognize 32-bit TGA files.) This extra channel, the alpha channel, isn't visible but acts as a mask to make certain areas of the image transparent in software that recognizes alpha channels. And because this alpha channel is 8-bit, 256 levels, it can have 256 levels of transparency from black/100% transparent to white/100% opaque.
TIFF files can also be CMYK, also 32-bit, but with no transparency channel.

From PowerPoint 97 onwards PowerPoint recognizes alpha channels in TIFF and PNG files.

The following is intended as an introduction to using alpha channels to create transparency. An in-depth explanation of the methods you can use to improve the masking quality or to create interesting and useful effects would take up too much space here.

How to use Photoshop to make an image with an alpha channel:

The channels of an image are shown in the Channels tab (next to the layers tab) on the layers palette. A 24-bit RGB image shows three channels - red, green and blue, plus the composite RGB image (if you click the red layer, for example, you will see in grayscale the red content of your image).

There are many ways to use Photoshop to create an alpha channel. The simplest is to select what you want to be visible (here called the subject) and save the selection as a channel. How you select your subject depends largely on what you're starting with. Here are a few ways:

1. If you have a picture on a white background and you want to make the white around the subject transparent in PowerPoint: use the magic wand tool to select all areas of white that you want to be transparent and invert the selection (Select>Invert), leaving your subject selected.

2. If the background is more complicated than a single color, trace around your subject with the lasso tool.

3. You have an .EPS or .PSD file that already has transparency and you want to use this in PowerPoint. Right click on the thumbnail of the subject layer in the Layers palette. Select 'Select layer transparency' from the context menu.

Click on the Channels tab. There is a button at the bottom, second from the left, called 'Load selection as channel'. Click this and you will see you have created a new channel (named either #4 or Alpha 1, depending on the version of Photoshop you use) with your selected area white against black.

Save your image as a 32-bit TIFF with LZW compression or as a PNG (see below). In Photoshop 5 be sure not to check the 'Exclude Alpha' box, in Photoshop 6 be sure to check the 'Alpha channels' box.

Saving PNGs from Photoshop 5.5 or 6:

Confusingly, in Photoshop 5.5 onward the method for creating 32-bit PNGs is slightly different. You need to create the transparency on the layer (ignoring channels altogether and simply deleting the parts of the layer that you want to be transparent) and choose File>Save for Web. Choose PNG-24, check the transparency box and click OK to give the file a name.

Importing into PowerPoint

Bring your image into PowerPoint (Insert>Picture>From File). In PP 97 or 2000 it'll look relatively crude on screen in Slide mode, but you'll get a good idea how it works. To see the full effect, view the slide in Slide Show mode. In 97 the image will retain its rough appearance even in Show mode if you animate it, though this has been cured in the later versions.

If you're using PowerPoint XP things work slightly differently. The initial view of the imported TIFF shows the alpha channel in black 'masking' the image. In order to activate the transparency you need to use the transparency wand on the Picture toolbar (View>Toolbars>Picture) to select the mask. Once this is done the masked image is shown fully rendered even in Slide mode.

Alpha channel resources:

There are a number of third-party applications that simplify and improve the alpha channel creation process. These can be particularly effective on complicated subjects - even hair. A couple of examples are Mask Pro from Extensis (www.extensis.com) and Knockout from Corel (www.corel.com).

Two superb collections of 50,000 high quality, alpha channelled, photographs covering a huge range of subjects is available from Hemera Technologies (www.hemera.com) called Photo Objects vols I and II.

Steve's caveats:
The techniques Adam has explained here work beautifully on screen; the results may not be as pleasing when printed out, so I suggest you test carefully if printed output is important.

When I tested with PowerPoint 97 and 2000, I found that PowerPoint did NOT respect anything but pure black or white values in the alpha channel.

When I used intermediate gray values, PowerPoint converted them to pure B/W at print time. Anything over 50% gray converted to white/100% opaque masking; anything under went black/100% transparent masking.

If you need your presentations to look the same on paper/overheads/slides/PDF files as they do on screen, you may need to restrict yourself to pure B/W alpha masking effects.

And there's more
Indezine has a
PowerPoint And Alpha Channels tutorial by MVP Geetesh Bajaj that describes alpha channels, their usage and creation - as well as their role in the PowerPoint scheme of things.

Alpha transparency brings the MVPs out of the woodwork it seems! MVP TAJ Simmons
also has a PowerPoint transparency tutorial

O'Reilly has published this chapter on alpha transparency from Greg Roelofs' book on the PNG format. Greg's PNG Site has everything you'd ever want to know about PNG.


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Adam's Transparency Tutorial (Alpha channels and more)
http://www.pptfaq.com/FAQ00190_Adam-s_Transparency_Tutorial_-Alpha_channels_and_more-.htm
Last update 07 June, 2011
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