LibTiff Mailing List
TIFF and LibTiff Mailing List Archive
April 2008
Previous Thread
Next Thread
Previous by Thread
Next by Thread
Previous by Date
Next by Date
Contact
The TIFF Mailing List Homepage
This list is run by Frank Warmerdam
Archive maintained by AWare Systems
Thread
2008.04.16 16:05 "Re: efficient thumbnailing", by Phillip Crews
Seth Price wrote:
> I would like to say that I've just been having the exact same problem,
> except I've been using ImageMagick and GDAL. The images I'm working
> with are 14400x14400px.
>
> Any solutions out there?
> ~Seth
You can skip most scan lines in untiled, non-YCbCr images if they are
uncompressed, or compressed with rowsperstrip = 1, and only load enough
scan lines to get a good thumbnail. (I use 3 * the thumbnail height,
then reduce). This doesn't produce as nice of a thumbnail as a fully
resampled image, but it does a good job.
Here's some pseudo-code with no error checking. Variable names should be
obvious. All scan lines retain their full width, so your 14400 x 14400
images would load as (for example) 14400 x 300, eliminating 14100 scan
line reads and decompressions, requiring 2% of the memory required for a
full image load, and much less CPU for resampling to 100x100.
- For planar-contiguous images, I only use this code if height >
thumbnail_height * 8.
- For planar-separate images, because of the disk seeks between the
planes, I only use this code if height > thumbnail_height * planes * 8.
- There may be some improvement with rowsperstrip > 1, especially with
memory usage.
- For compressed images with rowsperstrip = height, the library will
decompress the whole file into memory anyway
loadheight = thumbnail_height * 3;
if (planar_config == PLANARCONFIG_CONTIG) {
PFNCTGOUT put;
BYTE *pbuf = malloc(TIFFScanlineSize(tif));
put = GetContigOutputRoutine();
for (row=0; row < loadheight; row++) {
fromrow = (row * height) / loadheight;
TIFFReadScanline(tif, pbuf, fromrow, 0);
(*put)(my_image, row, 0, pbuf, width, 1, 1, 0);
}
} else {
PFNSEPOUT put;
BYTE *r, *g, *b, *a;
r = pbuf = malloc(4*TIFFScanlineSize(tif));
g = r + TIFFScanlineSize(tif);
b = g + TIFFScanlineSize(tif);
a = b + TIFFScanlineSize(tif);
put = GetSeparateOutputRoutine();
for (row=0; row < loadheight; row++) {
fromrow = (row * height) / loadheight;
TIFFReadScanline(tif, r, fromrow, 0);
TIFFReadScanline(tif, g, fromrow, 1);
TIFFReadScanline(tif, b, fromrow, 2);
if (sampperpix == 4)
TIFFReadScanline(tif, a, fromrow, 3);
(*put)(my_image, row, 0, r, g, b, a, width, 1, 1, 0);
}
}
This significantly speeds thumbnailing of very large images as long as
they meet the criteria.