Christian Henning wrote:
I would be interested in a pointer to details of Partial Image Decoding. Depending on the amount of effort I'll either add it now or after the review. My time frame is kinda small right now. I think the situation was that you were decoding and discarding the stuff after the area of interest, which was obviously wasteful and fixable. The more difficult issue was whether it is possible to reduce the work done prior to the area of interest. I don't have a good enough understanding of libjpeg to know the answer to that. Presumably there are similar issues with the other formats.
For now, I consider this an optimization. I someone can show me how to do that I would be willing to add.
So for my problem of chopping up the TIFF into lots of tiles, I think I have two choices:
(1) Decode the whole TIFF into one gil image. For each tile, create a gil view and create and save a PNG.
Depending on the size of your TIFF you might need to use the BigTiff lib. You probably know that. If you are working in x64 why not load the whole thing? RAM is cheap. Unless you have your image is several TBs.
I assume Boost.GIL as general purpose library and not specific to particular environment, available memory, etc. configurations. Then, I think it's not an option. I often works with rasters of N GBs (not even TB) and swinging large monoliths of memory around is fragile. What if I have 4GB TIFF image with raster sample size of 1 BIT? Would Boost.GIL handle it? How? Would it it get 8 times larger in memory as each sample is loaded to basic unit of 1 byte, for instance? I think that the Boost.GIL I/O interface should be designed in a way that it doesn't force any limitations and at least allow highly optimised implementation of I/O specific to format. I'd be truly interested in discussing various aspects of handling large imagery in Boost.GIL.
Like this one: http://www.aperio.com/bigtiff/#Sample_Images
(2) For each tile, partially decode the TIFF into a gil image, and create and save a PNG.
Tile-based reading is being worked on right now. But as far as I know a tiff is either scanline or tile_based. You cannot read scanlines in a tile_based image and vice versa. I might be wrong, but that's my understanding right now.
Or srtip-based - a chunk of scanlines. Best regards, -- Mateusz Loskot, http://mateusz.loskot.net
