Calibrating the background picture

The background picture feature comes with two main settings: main nodes location and gamma correction. Before starting, a general side note concerning background images: Java is not really fast (an elegant way to say it is creeping) at handling images. Try to keep your image sizes reasonable, and use paletted images (prefer GIFs to JPEGs) for background, which will really speed-up the gamma correction.

Gamma correction

Gamma correction is quite straightforward: just adjust the image "gamma" using the slider to make it lighter or clearer, so that the drawn circles and segments are easier to distinguish from the image.

Nodes calibration

The main nodes calibration seems very strange at start, but essentially needs some explanations, given around an example. Let's start with this picture, which is a part of a photo I took in Sintra (Portugal) a few years ago.

The pattern here clearly belongs to the p4* family. Digging a bit, you will agree that it is a p4g. The primary and secondary centres are located within neighbouring 8-branches stars, making it useful to rotate the image 45 degrees in some directions.

Now, the idea behind calibration is to place the image in such a position that the primary and secondary centres appear at the "right" place: primary in the middle, secondary on the upper right, at the right distance. Arabeske brings you two pairs of co-ordinates, which are the locations, in pixels, of our centres. The (0, 0) pixel is the upper left one, and co-ordinates given out of the picture will be accepted. The default values are the centre of the image for the primary centre, top-right for the secondary one. In our example, the first point is around (80, 91), and the second one is (146, 89), as we could see in an image editing programme.

Now, why should we use something external when Arabeske can do it internally? The idea here is that you can drag the reference primary or secondary centre from its "real" location (i.e. Just in the middle for the primary one) to the pixel of its location on the picture. This simple, two-step process is fairly easy once you've tried it.

Let's do it: first, drag from "somewhere near the middle" to the suitable place on the picture.
The primary centre moved to the right place, but the disturbing effect is that, as the secondary centre didn't move, the image gets scaled and rotated a lot. Now, the second step: drag from "somewhere in the top-right" to the suitable location for the secondary centre.
And that's all! Having a look at the pattern in "help" mode gives a better idea of where the nodes are.

If needed, you can adjust, never forgetting that you move the nodes, not the image. This is why you will sometimes feel that the picture moved the opposite way you wanted it to move.