RISC World

The Education Column

Andrew Harmsworth with the latest Education news

Many of you must have wondered what had happened to the Education Column for the last few issues (Aaron certainly has!). Well, I've been busy. Getting married apparently takes huge amounts of organisation. That and the fact that seems to be generating lots of advertising revenue, little things like this get left to one side.

Anyway, it's back. This month I take a look at some simple digital image manipulation - that anyone can do - yet most either ignore or fail to realise it's possible. Oh, and a moan about PAT testing.

PAT Testing

I've been proud of the fact that I managed to prevent a large number of RiscPCs and A7000/+ machines from being skipped in my school. They've been incredibly useful in my department - for analysing experimental results (GraphDraw), for datalogging (Insight 2 with DCP Datameters or LogITs), for quickly throwing out a Textease document, for TechWriter, old BASIC teaching programs, etc.

Every year, as I suspect is standard practice, portable appliance testing is carried out (PAT testing). This year they brought with them more sophisticated equipment. Previously the only test carried out on computers was a cursory glance at the power lead (and checking of the case for holes). This year, they failed 4 RiscPCs, 5 monitors (AKF60) and 3 A7000s. I don't know what is wrong with them. They just produce an exciting "fail" on the display, despite working perfectly. Ugly red stickers with "DO NOT USE" have therefore been plastered all over the machines' power supply sockets. Great. Thanks. Very helpful. I wonder if anyone ever tests the PAT-testing equipment? I'd love to fail it...

It would seem, therefore, that the future of RISC OS computers in schools or businesses where PAT testing is "really sophisticated" is on PCs running Virtual Acorn. I think there's something deeply ironic about this, but I can't quite put my finger on it.

Gamma Correction (& more!) for Beginners

At time of writing, the much-hyped (and probably rightly so) Wakefield Show is about to happen. As with all RISC OS shows, websites will soon be filled with photographs taken on digital cameras (or camera phones: see Unfortunately, as most RISC OS shows seem to happen indoors, without sufficient lighting, most of these images tend to be a little disappointing. There is something that can be done about this!

In fact, this article arose after an email discussion with Aaron. I was quietly complaining that the images he'd taken for the RISC OS South-West Show report could have done with some gamma correction. Apparently, they had been corrected! Another question for Aaron then: why were the photos GIFs not JPEGs?.(Answer - because I can't remember why, but I am sure there was a good reason at the time - ED).


I'm sure this has been covered before, but the GIF file format is generally considered applicable to simple graphics, such as those you might produce using Draw or Paint. It is also limited to 256 colours and it can have a mask (to allow transparency).

On the other hand, the JPEG file format (which stands for Joint Photographic Expert Group) is - surprisingly enough - one of the best formats for producing photographic images that have millions of colours and extremely high levels of complexity.

Both the GIF and JPEG formats are compressive, meaning the file size you end up with is much smaller than if you had the same image as a SPRITE (for example). The subtle difference here is that the GIF format is lossless (unless you do something really silly) - meaning the final image has the same quality as the original - whereas the JPEG format is lossy - meaning you can choose to lose a certain amount of quality whilst saving huge amounts of file space.

Let's take a look at Aaron's first photo from the last issue:

RComp - Aaron's original image from Vol. 4, Issue 6: 62K

Let's see what happens if we load this using !ChangeFSI (which everyone has and can use on all modern RISC OS kit).

Firstly, ChangeFSI will turn it by default into a Sprite file. You can get it to produce a JPEG by clicking MENU on its iconbar icon:

ChangeFSI's iconbar menu, and JPEG options

As standard, the 'quality' is set to 75%, but this can easily be altered. 75% gives an excellent improvement to file sizes, without too much loss of quality. Let's see Aaron's image as a 75% quality JPEG:

RComp - Aaron's updated image as a JPEG: 30K

The file size is less than half that of the GIF, yet the image itself is almost indistinguishable from the first. Making the file size as small as possible is good practice for use on web pages.

Let's get back down to business regarding Gamma Correction. This is used to correct the quality of images but is more sophisticated than just altering the brightness or contrast. To a first approximation, it does both whilst retaining the overall "feel" of the photograph. ChangeFSI can apply gamma correction at the same time as many other effects. To do so, choose Processing from the iconbar menu:

ChangeFSI's Processing options

Low values of gamma correction (from 0 to 1) make colours darker; high values (above 1) make them lighter. 2.2 has a quite considerable effect, too much. Let's start from 1.1 and work through to 2.1. For ease of comparison, I've also asked ChangeFSI to make the images smaller:

Original: gamma=1.0












As is quite clear, the more gamma correction one applies, the clearer the final image can become. The colours are still not right, however. To improve the balance (that wall is white, presumably?) better software is really needed. Photodesk does a good job with white balance, and has a highly intricate approach to Gamma Correction:

RComp - Aaron's image passed through Photodesk

Finally, if you find messing around with ChangeFSI too time consuming, I strongly recommend the use of Jochen Lueg's excellent !Batch_FSI utility. This is just an additional front-end for ChangeFSI, but it allows you to dump an entire folder of images to undergo the same processing (of your choice) in one single go. So, for all of you wishing you could be bothered to slightly improve your digital images: fire it up and see if you like it!

If you have any questions or comments on the use of RISC OS computers in education, please either email or better still join the RISC OS Education Discussion List, and air them there.

Andrew Harmsworth