Re^2: Resizing large TIF files

I'm not sure to have understood what you did and why it didn't work. These times and sizes are not what I got here.

Actually, your results are entirely consistent with mine. The problem is that your code uses 'preview' quality rather than the default 'normal' or high quality scaling. If I use 'preview' quality I also can resize an image in about 0.3 seconds.

The images I'm working with are scans of paper forms. Some of the resulting image is the text and lines printed on the form and the rest (the more interesting bit) is the handwritten text that someone wrote. In the original files, lines are 1-2 pixels thick - say 1.5 pixels thick on average. If I resize the images in 'preview' mode, the resulting files use 1bit depth so the file size is good but the image quality is poor - sections of lines and letters disappear altogether and legibility is significantly reduced. However if I use 'normal' quality, there is barely any degradation in legibility. The resulting files essentially end up anti-aliased which necessarily uses more shades of gray so the file size increases. Obviously the extra quality also comes at a significant CPU cost (about a factor of 10 over preview quality).

So the bit that I obviously failed to make clear was that I'm not trying to produce thumbnails or previews. I want to reduce the images so that they display on screen at about the same size as the physical form and retain sufficient legibility that users never need to refer to the full size originals. The originals appear too big and 'zoomed' for practical purposes. All the resizing techniques that I've tried do meet these legibility requirements (with the exception of Imager's preview mode). I've also determined that reducing from 24bit RGB to 8 colour (3 bit) indexed colour barely affects legibility at all. 2 bit colour is border line (might be OK if I could preselect the palette) and 1 bit colour is not good enough. It's the processing overhead and file bloat that I'm keen to reduce if possible.

Thanks for taking the time to try it out and reply.

Comment on Re^2: Resizing large TIF files

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Re^3: Resizing large TIF files by dk (Chaplain) on Nov 23, 2007 at 08:32 UTC
So the bit that I obviously failed to make clear was that I'm not trying to produce thumbnails or previews Yes you didn't - and that changes the problem entirely. There are a number of pixel-interpolating algorithms, and the fastest is "nearest neighbor", or Imager::preview in your terms. A good visual balance for image downscaling is usually produced by the bicubic algorithm, but Image::Magick has also `qw(Quadratic Triangle Hermite Hanning Hamming Blackman Gaussian Catrom Mitchell Lanczos Bessel Sinc)` (at least those I know), and you might want to try which works best for you.	[reply] [d/l]

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Re^3: Resizing large TIF files
by dk (Chaplain) on Nov 23, 2007 at 08:32 UTC

So the bit that I obviously failed to make clear was that I'm not trying to produce thumbnails or previews

Yes you didn't - and that changes the problem entirely. There are a number of pixel-interpolating algorithms, and the fastest is "nearest neighbor", or Imager::preview in your terms. A good visual balance for image downscaling is usually produced by the bicubic algorithm, but Image::Magick has also qw(Quadratic Triangle Hermite Hanning Hamming Blackman Gaussian Catrom Mitchell Lanczos Bessel Sinc) (at least those I know), and you might want to try which works best for you.

[reply]
[d/l]