PowerPoint And Alpha Channels
By: Geetesh Bajaj
Last Updated: March 2nd 2009
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 devour. Yes! 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 colour channels - which comprise RGB - red, green and blue. These are the colours 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 colour 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 shades 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 channel is basically a selection or mask represented in 256 colours of the greyscale 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 - professional file formats (with multi-layer capabilities) used by applications like Adobe Photoshop (PSD) and Corel PhotoPaint (CPT) can store multiple alpha channels.
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 - 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.
Before discussing the differences - let's discuss the 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 - it 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 format which worked across operating systems, colour spectrums and graphic applications. In addition, it allowed more control in print layouts. Resolution and colours are important factors in the print media and the TIF format excels at coping with these challenges. The TIF format offers LZW compression - this is the same compression used by the GIF 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 'flavours' in TIF parlance. Not every application can import all types of TIF files - PowerPoint often has hiccups in importing TIF files for this very reason. For more TIF information, visit The Unofficial TIFF Homepage. You can also download the specifications from the Adobe site in a PDF format.
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 colours and 256 transparency levels to impart the format a full alpha channel and effective anti-aliasing. Unfortunately, its use on the WWW - its intended objective has not been very successful. It is thus surprising that PowerPoint supports PNG and all its features including alpha channels. This is all the more amazing since even Microsoft's own Internet Explorer 5 doesn't support as many PNG features as PowerPoint does. And PNG is supposed to be a web graphics format! You can learn more about the PNG format at the PNG Home Site, maintained by Greg Roelofs.
PNG files exist in a few variants - like GIF, PNG has an 8-bit standard limited to 256 colours. There's also a 256 colour level greyscale PNG definition. However, there are standards for higher colour 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... And the best part is, PowerPoint supports all such features!
Wait, there's more. I know all 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, Macromedia Fireworks, Corel PhotoPaint and most other image editing and illustration software can export to the PNG format - and PowerPoint seems to accept them all - but sometimes you may come across a PNG file that does not import properly into PowerPoint. In that case, import your PNG file into Macromedia Fireworks and create a new PNG file using it's PNG Export feature.
To see for yourself the fabulous effects generated by alpha channels in PowerPoint, you could try out Crystal Graphics' PowerPlugs Headings. This is a large collection of PNG clip media - you can insert them into PowerPoint using Insert -> Picture -> From File.
A full review of PowerPlugs Headings is available elsewhere on the Indezine site:
After reading so much about creating alpha channel effects, you may have an urge to create a few pictures 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 Macromedia 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.
So, how do you decide which format to use. Personally, I use both. In the end, it depends on the usage. I 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 I'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 readymade stock photos are available in the TIF format with readymade 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 anyway going to use TIF! Anyway - don't use GIF unless you want animated GIFs - even those don't work with all versions of PowerPoint.
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 Macromedia Fireworks and using Fireworks' Export function to output 'pure' PNG files.