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Jens Bodal
Jens Bodal

Reputation: 1757

Why am I losing image quality with imagemagick when reducing a very large image to a much smaller image?

I have a bunch of images that I want to convert into a single PDF, the images are primarily images of text (similar to scanned images of a textbook). The image files are extremely large, I have no need for the amount of resolution that they offer.

So first, as a base file, I did a simple conversion of 26 of these "pages" to a single pdf, and the total filesize was 46MB for 26 pages. Viewing in page width mode resulted in a scale of 16% of the original image. 

convert *.png kapittel1.pdf

The quality of the PDF pages was perfect, they were just too large. So I figure since 16% of the image is more than adequate for viewing the entire width of the page on my screen, I could reduce the image sizes to 20% of their original values and still maintain the same image quality. The quality of the images is visibly less than before reducing the size.

convert -resize 20% -quality 100% *.png 20percent.pdf

I believe I'm going to need to start looking into filters, but before I potentially waste my time converting using all of the filters then comparing to find the one I want to use, is there a better way to just reduce the size, maintain quality, then convert to PDF? I don't see why I would be losing pixels here.

Edit

I tried with -scale instead of -resize but am really not seeing a difference in the output. It pretty much seems that once I go below 40% I start losing pixel data.

Upvotes: 6

Views: 3925

Answers (2)

Jens Bodal
Jens Bodal

Reputation: 1757

Ok well I discovered that the size of the PDF once following Shawn Patrick Rice's suggestion for Optimizing Scanned PDFs and OCR+ClearText was fairly negligible between a -resize setting of 30-50%. The primary goal here is to reduce the size of the resulting PDF to under 45" in height as this is the threshold for Adobe's OCR. I found no benefit from converting each image individually to a PDF then resizing, or playing with the plethora of other settings in Adobe. The below process kept (as far as I can tell) all of the image quality and reduces the images to the smallest size PDF (at full quality).

My process was as follows:

convert *.png -resize 50% name.pdf 
// resize amount dependent on original file dimensions, goal is document height < 45"
Adobe Acrobat => Document Processing => Optimize Scanned PDF (Edit => ClearScan output style) => OK

The size of the resulting PDF document is still quite large, however the size after reducing in Adobe goes down considerably (90MB => 4MB). If I first resized at 30% there would be noticeable image quality loss, however the amount of size I would save after optimizing would be around 800KB for the above file.

Upvotes: 2

nwellnhof
nwellnhof

Reputation: 33658

The excellent ImageMagick Examples state that by default, no image compression is used when creating PDFs and suggest to use Zip (Deflate Compression):

convert *.png -compress Zip -quality 100 kapittel1.pdf

If your images are only black and white, you can try the -monochrome option and optionally Group4 (Fax) compression using -compress Group4.

Upvotes: 3

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