Edge effects for tiles viewed at higher magnification

[Keith Hughitt]

Currently, when zooming in far enough for certain images, the tile borders become readily visible due to some edge effects created during tile generation. For example, see streak in lower right corner of C3 image in:

http://helioviewer.nascom.nasa.gov/?date=2002-08-30T14:00:00.000Z&imageScale=10.52&imageLayers=[SOHO,LASCO,C2,white light,1,100],[SOHO,LASCO,C3,white light,1,100]

After some investigation, it appears that the artifacts are created as a result of the scaling that is done as a final step of the tile generation. Attached are images which reflect two tiles manually placed side-by-side from each step of the tile generation process. Notice that the edge artifacts only show up during the last step, and are not present when scaling is disabled.

There are two approaches that could be taken to solve this issue:

1. Use a different interpolation algorithm when resizing (see http://www.imagemagick.org/Usage/resize/#filter)
2. Generate a tile that is slightly larger than what is actually needed (e.g. 8px larger), and then crop to desired
size after scaling is done.

The second approach will probably provide better results in the end and perform faster, but will require a larger modification to the system. If the first could be used as an intermediate step, but should not be considered a permanent solution.