This is a part of the PowerPoint and... series. You may have read a few of these articles and they have covered important PowerPoint elements like sound, autorun, text, fonts and narration. And now there's an article on 'alpha channels'. It's difficult to find the word instanced in the PowerPoint help file or the manual, and yet here's a full page of juicy, interesting information on alpha channels here for you to read. And for those of you who still don't know what alpha channels are, here's a basic definition.
'Alpha channel' is a graphic term for a simple concept. Most of us are aware of three visible color channels, which comprise RGB. RGB stands for red, green and blue. These are the colors of light and are mixed in specific values to output the TrueColor spectrum we're so used to seeing presently. Images and video files store the visibility components of color in these three RGB channels. In simple terms, this means that every tiny dot in a picture is composed of a blend of varying values. These values signify 256 possibilites each of red, blue and green.
In addition, some bitmaps can also store 256 levels of transparency; they have an ability to incorporate an 'A' channel. This A channel is basically a selection or mask represented in 256 colors of the grayscale spectrum. While white stands for 100% opaque, black represents 100% transparent and the shades of grey in between represent varying degrees of transparency. This A channel is the alpha channel.
File formats like PNG or TIF can support a single alpha channel. Proprietary file formats (with multi-layer capabilities) used by applications like Adobe Photoshop (PSD) and Corel PhotoPaint (CPT) can store multiple alpha channels.Back
Alpha Channels in PowerPoint
PowerPoint supports the alpha channels prevalent in PNG and TIF files. Of the two, PNG is more suited since PowerPoint's PNG support is better than its TIF support. Also, TIF files are usually bulkier than their PNG counterparts. That's not to say that TIF files are inferior in any way. It's just that TIF with its support for both RGB and CMYK spectrums is more suited for use in print rather than on screen.
PowerPoint imports TIF and PNG files with their alpha channels intact. Selections embedded in such alpha channels show up as transparent areas.Back
Between TIF and PNG
Before discussing differences between TIF and PNG, let's discuss similarities. Both TIF and PNG can store alpha channels, and both formats come in many variants.
The Tagged-Image File Format (TIFF or TIF) format is older than PNG, and was developed by Aldus Corporation (merged into Adobe Systems) along with Microsoft. As such, it's raison d'être was to fill a requirement for a cross-platform file format that worked across operating systems, color spectrums, and graphic applications. In addition, it allowed more control in print layouts. Resolution and colors are important factors in the print media and the TIF format excels at coping with these challenges. The TIF format offers LZW compression, the same compression used by the GIF file format. Whereas the GIF format has limitations of 8-bit support, the TIF format offers complete 32-bit support. Newer standards (TIF version 6) also allow JPEG compression algorithms within the format. TIF's greatest disadvantage is it's wide range of variants, called 'flavors' in TIF parlance. Not every application can import all types of TIF files, and PowerPoint often gets hiccups in importing TIF files for this very reason. For more TIF information, visit The Unofficial TIFF Homepage.
The Portable Network Graphic (PNG) format was introduced as an alternative to the GIF format. A patent held on GIF made its use prohibitive. Apart from being an open format, PNG includes a wider spectrum of colors and 256 transparency levels to impart the format a full alpha channel and effective anti-aliasing. PowerPoint supports PNG and all its features including alpha channels. You can learn more about the PNG format at the PNG Home Site, maintained by Greg Roelofs.Back
PNG files exist in a few variants. Like GIF, PNG has an 8-bit definition limited to 256 colors. There's also a 256-color level greyscale PNG definition. However, there are standards for higher color depths as well.
More importantly, PNG supports transparency in incremented levels from opaque to fully transparent. This means you don't have to put up with the blocky halos usually associated with GIF files. This factor alone opens up many avenues, allowing you to implement anti-aliased graphics, drop shadows, opaque to transparent gradients, and so much more. The best part is that, PowerPoint supports all these features!
Wait, there's more. Yes, this sounded too good to be true. PNGs need to be sourced from somewhere—this includes readymade collections or something you create yourself in a graphic application. Adobe Photoshop, Adobe Fireworks, Corel PhotoPaint and most other image editing and illustration software can export to the PNG format, and PowerPoint seems to accept them all. However, you may sometimes come across a PNG file that does not import properly into PowerPoint. In that case, import your PNG file into Adobe Fireworks and create a new PNG file using it's PNG Export feature.Back
To see for yourself the fabulous effects generated by alpha channels in PowerPoint, you could try out Crystal Graphics' PowerPlugs Headings product. This is a large collection of PNG clip media that you can insert them into PowerPoint using the Insert | Picture | From File option.
A full review of PowerPlugs Headings is available elsewhere on the Indezine site.Back
Creating Alpha Channels
After reading so much about creating alpha channel effects, you may have an urge to create a few pictures with alpha channels yourself. It goes without saying that you need a professional image editing application to create a picture with alpha channels. Adobe Photoshop is the de-facto standard, but Corel PhotoPaint and Adobe Fireworks are both viable alternatives. All these applications come with excellent help files and manuals which could teach you more about the creation of alpha channel pictures within that application.Back
So, how do you decide which format to use? Both formats work well, but we do have a bias towards PNG since the files are smaller in size than TIF. Also PowerPoint's PNG support is fantastic; it has yet to reject any PNG we've thrown at it.
At the other end of the spectrum, if you want to use TIF files which already have an alpha channel, it makes little sense converting them to PNG. A lot of stock photos are available in the TIF format with existing alpha channels (clipping paths) you could insert straightaway into PowerPoint.
The bottom line: if you are web leaned use PNG and if you're print leaned, you're going to use TIF! Anyway, don't use GIF unless you want animated GIFs.Back
Your alpha channel equipped PNG or TIF files may appear dithered in PowerPoint. This isn't a cause for worry, since they appear with their fidelities intact in show mode. For more information visit Microsoft Knowledge Base Article Q212420.
If PowerPoint cannot import your TIF files and displays jitters, it's time to take help from other applications. A simple solution would be to open such TIF files in Debabelizer or Paint Shop Pro and save to a generic TIF flavour.
In the same way, most problem PNG files can be rectified by importing them into Adobe Fireworks and using Fireworks' Export function to output 'pure' PNG files.Back