A few scanning tips


PNG - Portable Network Graphics

(.PNG file extension, the pronunciation 'Ping' is specifically mentioned in the PNG Specification). PNG needs to be mentioned. PNG is not the number one file format, but you will want to know about it. PNG is not so popular yet, but it's appeal is growing as people discover what it can do.

PNG was designed recently, with the experience advantage of knowing all that went before. The original purpose of PNG was to be a royalty-free GIF and LZW replacement (see LZW next page). However PNG supports a large set of technical features, including superior lossless compression from LZ77. Compression in PNG is called the ZIP method, and is like the 'deflate" method in PKZIP (and is royalty free).

But the big deal is that PNG incorporates special preprocessing filters that can greatly improve the lossless compression efficiency, especially for typical gradient data found in 24 bit photographic images. This filter preprocessing causes PNG to be a little slower than other formats when reading or writing the file (but all types of compression require processing time).

Photoshop 7 and Elements 2.0 corrected this, but earlier Adobe versions did not store or read the ppi number to scale print size in PNG files (Adobe previously treated PNG like GIF in this respect, indicated 72 ppi regardless). The ppi number never matters on the video screen or web, but it was a serious usability flaw for printing purposes. Without that stored ppi number, we must scale the image again every time we print it. If we understand this, it should be no big deal, and at home, we probably automatically do that anyway (digital cameras do the same thing with their JPG files). But sending a potentially unsized image to a commercial printer is a mistake, and so TIF files should be used in that regard.

Most other programs do store and use the correct scaled resolution value in PNG files. PNG stores resolution internally as pixels per meter, so when calculating back to pixels per inch, some programs may show excessive decimal digits, perhaps 299.999 ppi instead of 300 ppi (no big deal).

PNG has additional unique features, like an Alpha channel for a variable transparency mask (any RGB or Grayscale pixel can be say 79% transparent and other pixels may individually have other transparency values). If indexed color, palette values may have similar variable transparency values. PNG files may also contain an embedded Gamma value so the image brightness can be viewed properly on both Windows and Macintosh screens. These should be wonderful features, but in many cases these extra features are not implemented properly (if at all) in many programs, and so these unique features must be ignored for web pages. However, this does not interfere with using the standard features, specifically for the effective and lossless compression.

Names you may see are PNG8 for Indexed color, PNG24 for RGB images, and PNG32 for RGB with transparency.

Netscape 4.04 and MS IE 4.0 browsers added support for PNG files on web pages, not to replace JPG, but to replace GIF for graphics. For non-web and non-graphic use, PNG would compete with TIF. Most image programs support PNG, so basic compatibility is not an issue. You may really like PNG.

PNG may be of great interest, because it's lossless compression is well suited for master copy data, and because PNG is a noticeably smaller file than LZW TIF. Perhaps about 25% smaller than TIF LZW for 24 bit files, and perhaps about 10% to 30% smaller than GIF files for indexed data.

Different images will have varying compression sizes, but PNG is an excellent replacement for GIF and 24 bit TIFF LZW files. PNG does define 48 bit files, and Photoshop does support it.

Here are some representative file sizes for a 9.9 megabyte 1943x1702   24-bit RGB color image:

File type File size  
TIFF 9.9 megs  
TIFF LZW 8.4 megs  
PNG 6.5 megs  
JPG 1.0 megs (1.0 / 9.9) is 10% file size  
BMP 9.9 megs  

Seems to me that PNG is an excellent replacement for TIFF too.

More PNG info at www.libpng.org/pub/png.

Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.

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