I have been investigating the possibility of implementing exact area box filtering in GEGL. Specifically, I would like to build a sampler that behaves like exact area box filtering when downsampling but also behaves reasonably when performing other transformations.

The basic idea is to average the piecewise constant "nearest neighbour" surface on the smallest rectangle (width and height at least 1) that contains the pull-back (inverse image, anti-image) of an "output" pixel area by the inverse Jacobian (approximated if needed).

The main issues are:

1) Averaging over a region too large for the usual 64x128 tile.

If we downsample by a significant amount, the region to average over may be larger than the size of a tile. This could be solved by either calling gegl_sampler_get_ptr () repeatedly with a large, constant footprint, or by devising a system to change the footprint on a pixel-by-pixel basis. The latter solution just involves modifying context_rect on the fly, which should be feasible.

2) Passing the four numbers which define (an approximation of) the inverse Jacobian to the sampler from the operation which calls it.

I see that there are create_matrix () methods that seem to construct a Jacobian for various affine transformations. The proposed method would require the inverse Jacobian instead, which may require some changes to the API. Since more complex warping may also make use of this feature in the future, would it be acceptable to add methods to support this?

3) Is there already code which does this?

I have noticed that resample_boxfilter_u8 seems to do something related. How similar is this code and Is there too much overlap for this project to be worthwhile?

I would appreciate any feedback or help that anyone can provide.

Adam Turcotte