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General Interest Reducing PNG Scan Field Image Size
General Interest

Information in this article applies to:

  • uScope Navigator V4.5 and Later

Article ID: NGN2314 — Created: 24 Aug 2020 — Reviewed: 19 Oct 2020

Symptom

PNG has been selected as the image file format for uScope Navigator. PNG image files require significantly more disk space than JPEG image files. What can be done to reduce the size of the PNG images?

Cause

There are two file format options when scanning: JPEG and PNG.

  • JPEG images are compressed and lossy and are significantly smaller than PNG images.
  • PNG images are also compressed but are lossless—hence, the compression is not nearly as significant as JPEG.

PNG images saved by uScope Navigator are compressed but are not optimized and could be further reduced in size.

Resolution

When PNG output files are selected, uScope Navigator saves field images and deep zoom images as PNG files which are compressed. Third-party tools can optimize and re-compress PNG images even further and may provide an extra level of compression.

  • The open-source tool OptiPNG does a fine job optimizing PNG files and re-compressing them. It is a command-line utility that is available for a number of different platforms including Windows.
  • There are many other commercial utilities, like ImageOptim for MacOS, that incorporate several lossless (and lossy) PNG image compression algorithms including: PNGOUT, OxiPNG, AdvPNG, PNGCrush, and Zopfli. By using several different optimization/compression schemes, commercial utilities can minimize the final file size more than single-algorithm utilities.
Notes
  • Always make a backup copy of your job folder before making changes like those in this article. These are your source images and they cannot be recovered if they are destroyed or overwritten.
  • Lossy PNG image optimization results in significantly smaller images at the cost of image data. If lossy images are acceptable, JPEG is the better file format option.

More Information

For testing purposes, we used OptiPNG and ImageOptim to compress a 19.4 GigaByte folder of 4,731 PNG images with an average size of 4.1 MegaBytes. The results are shown in the following table.

ParameteruScope Navigator
Compression
OptiPNG
Compression
(Default Settings)
ImageOptim
Compression
(Default Settings)
Folder Size (bytes)19,444,071,66211,209,410,95811,191,569,632
Average File Size (bytes)4,109,9282,369,3532,365,582
Compression Ratio-1.73:11.74:1
Compression Savings-42.35%42.44%

Note
Optimizing PNG images is a slow process. It took several hours for both OptiPNG and ImageOptim to optimize the 4,000+ files in our test case.

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