[gdal-dev] Impact of tile size on artifacts in JPEG-compressed Tiffs

[Jacob Adams]

I'd like to check my understanding of the impact of the tile size on JPEG-compressed geotiffs. 

I've been getting dark bands 4 or 5 pixels wide on the bottom and right edges of the image when manually creating large image tiles from a collection of smaller tiles. I'm using the common aggressive compression settings for jpeg with a specified extent (in map units) as follows:
gdal_merge.py -ps .25 .25 -ul_lr 1528148 3773703 1529109 3772785 -co compress=jpeg -co interleave=pixel -co photometric=ycbcr -co tiled=yes -o <outfile> <infiles>

While subtle, they are distracting and are more visible when placed next to other imagery or when resampling. 

I've found two ways to get rid of them via trial and error: 
1) Don't use tiling.
2) Make sure your image size (in pixels) is a multiple of your tile size. 

I want to use tiling to speed up access time. If I adjust the extent to make the image size a multiple of some of the factors of the tile size, such as 128, 16, or 8 (for a default tile size of 256x256) the dark area is still there but shrinks to one or two pixels. This tells me it's not related to the MCU of the JPEG compression. 

Am I correct in saying that the image size should be a multiple of the tile size when using compress=jpeg and tiled=yes to avoid generating these darker bands on the bottom and right edges of the images?

Thanks,
Jake Adams
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