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A string list of additional options controlling the warp operation in name=value format. A suitable string list can be prepared with CSLSetNameValue().
The following values are currently supported:
- INIT_DEST=[value] or INIT_DEST=NO_DATA: This option forces the destination image to be initialized to the indicated value (for all bands) or indicates that it should be initialized to the NO_DATA value in padfDstNoDataReal/padfDstNoDataImag. If this value isn't set the destination image will be read and overlayed.
- WRITE_FLUSH=YES/NO: This option forces a flush to disk of data after each chunk is processed. In some cases this helps ensure a serial writing of the output data otherwise a block of data may be written to disk each time a block of data is read for the input buffer resulting in alot of extra seeking around the disk, and reduced IO throughput. The default at this time is NO.
Normally when computing the source raster data to load to generate a particular output area, the warper samples transforms 21 points along each edge of the destination region back onto the source file, and uses this to compute a bounding window on the source image that is sufficient. Depending on the transformation in effect, the source window may be a bit too small, or even missing large areas. Problem situations are those where the transformation is very non-linear or "inside out". Examples are transforming from WGS84 to Polar Steregraphic for areas around the pole, or transformations where some of the image is untransformable. The following options provide some additional control to deal with errors in computing the source window:
- SAMPLE_GRID=YES/NO: Setting this option to YES will force the sampling to include internal points as well as edge points which can be important if the transformation is esoteric inside out, or if large sections of the destination image are not transformable into the source coordinate system.
- SAMPLE_STEPS: Modifies the density of the sampling grid. The default number of steps is 21. Increasing this can increase the computational cost, but improves the accuracy with which the source region is computed.
- SOURCE_EXTRA: This is a number of extra pixels added around the source window for a given request, and by default it is 1 to take care of rounding error. Setting this larger will incease the amount of data that needs to be read, but can avoid missing source data.
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