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mapproject does (bicubic) interpolation. When being used to generate low-res imagery, this of course results in aliasing. It would be nice, per Ben, to add to it the ability to create antialised images.
The same applies to dg_mosaic.
I would argue that this, if ever implemented, should be done carefully or as an option, to not increase the run-time when this is not necessary.
I am not sure how much of a priority to assign to this item. As Ben points out, one can use gdalwarp to do the job.