photodesk : one good turn...

...or even eighteen. This article was originally going to be just a brief overview of the rotation facilities in Photodesk, but while putting it together I came across something rather surprising....

Setting the scene

Rotating an image is a fairly common operation. Photodesk has a basic image rotation feature, accessed from the image menu Image>Rotate... or by Ctrl-R. Here's the dialogue:

The image rotation dialogue

Here are some general points to note about rotation:

Rotation is always about the centre of the image; unlike ArtWorks, you can't specify an arbitrary axis point.
Masks have no effect on rotation; it always affects the entire image. But any mask channel will also be rotated.
You cannot rotate a selection. If you want to do this, you'll have to make the selection, copy it to the clipboard, rotate the copy, then paste it back.
You don't get a preview using this method of rotation. An alternative is to use the sprite import feature: just drop a sprite on to a blank image, and you'll be able to interactively set up a rotation. You don't have any control over the quality, though. Photodesk 3.06 won't rotate imported JPEGs (even in Draw files), but you can get round this if you have ArtWorks.

The dialogue offers various options:

If you just want to turn an image on its side or upside down, always use the Fixed section of the dialogue. Rotation by these amounts – 90° in either direction or 180° – is lossless; that is, however many times you do it, you won't degrade the image.
The Arbitrary section is for all other amounts. You can enter fractional or decimal numbers here for high accuracy. The rotation is anticlockwise (CCW), the usual convention for this operation (as in Draw and Paint).
The Resize to fit option will make the image bigger to accommodate the rotation, filling the blank areas with white.
Lose corners keeps the image the same size, with the effect that the corners will get chopped off.
The Interpolation section offers three different methods. According to the manual, Bi-cubic is the best, followed by Bi-linear then Nearest neightbour. But hold your horses; here's where it gets interesting.

Test spin

Note

This is not something you'd ever do in real life, of course. The point of the repeated rotations is to amplify any deficiences in the algorithms used by Photodesk's rotation functions. Note that all rotation methods are lossy to some extent (apart from 90° multiples) – there are better algorithms than the ones Photodesk uses, but they're extremely processor-intensive.

I've carried out a simple trial to test how good each interpolation method is. Using the image at the top of the page, rotate it in 10° steps until it's upside down (for a total of 18 consecutive rotations), then Image>X flip and Image>Y flip to bring it the right way up again (the flips are lossless, so make no difference to the result). This is a fairly exacting test of any rotation algorithm.

All tests were carried out using Photodesk 3.06 on an Iyonix running RISC OS 5.11 with 512MB RAM.

I added the black rectangles and single-pixel white lines to see how well these would survive rotation.

The RISC OS feature of Adjust-clicks keeping dialogues open makes doing this operation very quick – set up the window as below and click Adjust on Rotate 18 times. I've used the Lose corners option as otherwise the image keeps on getting bigger and bigger (try it and see). The successive corner-chopping gives the circular effect seen below.

Setting up for the test

Here are the results for each method:

This is Bi-cubic interpolation (allegedly the best):

[5991cub/jpg]

Bicubic interpolation

This is Bi-linear interpolation (medium quality):

[5991lin/jpg]

Bilinear interpolation

And this is Nearest neighbour interpolation (low quality):

[5991nr/jpg]

Nearest neighbour interpolation

Commentary

Here's what the Photodesk manual says about the interpolation methods:

As usual, Bi-cubic is the highest quality but takes the longest. Nearest neighbour is the quickest but yields the poorest quality result. Bi-linear is a good compromise between speed and quality.

It's clear that Nearest neighbour is by far the worst, as expected. It's difficult to think of any reason for using this method, other than as a fast preview of rotating a very large image (but see the timings below).

The Bi-cubic interpolation, allegedly the best, also suffers from pixelisation and staircasing artefacts.

The Bi-linear interpolation, while blurred, is – to my eyes at least – clearly the best. Here's a detail of each, zoomed in to double size:

[5991detail/png]

Detail at double size

Bi-linear also responds well to sharpening, unlike the Bi-cubic method. Here I've applied Image processing:Sharpen (coarse) at 75% Strength:

[5991shdetail/png]

Sharpened at double size

Timings

I've also done some rough timings for each of the rotation methods. The image was 3498x3581 pixels at 24bpp; together with one main and one alpha channel this gives a total size of 64MB when loaded into Photodesk. If you're interested, it's an image of Mars taken by the Spirit rover, and can be found here. The image was chosen so that it and the undo buffer would fit into the 128MB dynamic area I'm using, so virtual memory is not an issue. Rotation was by 10°, with Lose corners selected.

I've also included the timing for Paint, though because Paint doesn't have a 'lose corners' option it's not strictly comparable (it has to resize the image too). And just to show that RISC OS machines aren't quite as slow as some people make out, the last row is for Photoshop 5 on a Mac Mini with 512MB RAM running OS X 10.39 (more RAM would doubtless speed this up, as would a more recent Photoshop; and note that like Paint, Photoshop does not offer a 'lose corners' option so the image is also being resized).

Bi-cubic	13 sec
Bi-linear	12 sec
Nearest neighbour	11 sec
Paint	15 sec
Photoshop/Mac Mini	51 sec

There's not a huge difference in the Photodesk figures, perhaps suggesting that memory access is the critical bottleneck, rather than the algorithm.

Conclusions

Judging by this test, Bi-linear interpolation is the best. Given the almost identical timings, there's very little reason to use any other option, and Nearest neighbour should definitely be avoided.

According to theory (and practice in other graphics applications), Bi-cubic interpolation should be substantially better than it appears to be in Photodesk. It's possible that the implementation is flawed, or the dialogue could be triggering the wrong option (so selecting bilinear actually does bicubic, and vice versa).

Rob Davison's Variations also offers rotation, and has more options. There is no bicubic, but bilinear gives about the same result as the Bicubic option in Photodesk, supporting the theory that the latter program has got its options crossed.

I've reported these findings to the current maintainer, and await developments with interest.

Notes

Photoshop uses bicubic interpolation, and in addition automatically applies a sharpening on every rotation. This preserves more detail, but also introduces some staircasing and the outline artefacts typical of excessive sharpening. Here's the result of 18 10° rotations in Photoshop:

Rotated in Photoshop
For comparison, here's the same rotation process (18 consecutive 10° rotations) applied using Paint:

Rotated in Paint
This is slightly better than Photodesk's Nearest neighbour method, but there's not much in it. Paint doesn't offer a 'remove corners' option, so it's been cropped.
Since doing this test I've discovered a very similar comparison by Helmut Dersch on this page. It contains some theory and demonstration images from various PC and Mac programs.