Sandbox dimensions are too large for C2 and C3 Images

[Keith Hughitt]

The computed sandbox dimensions (which are a function of the maximum relative image size for a given zoom level and set of layers) are much too large for C2 and C3 images. The result is that the user is that at reasonably close-up zoom-levels, the user is able to drag the image around much further than there could possibly be data resulting in decreased usability and a number of unnecessary tile requests.

The is due to the fact that during the rescaling process, images that do not fall naturally on the chosen scale hierarchy have padding added to them. So for a C2 JP2 image that is 2048x2048, the actual data only spans about 1200x1200 of those pixels.

To fix the problem, Helioviewer needs to use the region where data may exist (e.g. 1200x1200) instead of the JP2 dimensions, which include the padding.