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Tips & Tricks

Why use the PNG format?

The default print screen image for Windows is BMP which creates huge bitmaps. GIF would be a satisfactory alternative but PNG is better. Therefore, Solid Capture defaults to the Portable Network Graphics (PNG) format, which is pronounced “Ping." It was designed to replace the older and simpler GIF format and, to some extent, the much more complex TIFF format.

For the Web, PNG really has three main advantages over GIF: alpha channels (variable transparency), gamma correction (cross-platform control of image brightness), and two-dimensional interlacing (a method of progressive display). PNG also compresses better than GIF in almost every case, but the difference is generally only around 5% to 25%, not a large enough factor to encourage switching on that basis alone. One GIF feature that PNG does not try to reproduce is multiple-image support, especially animations; PNG was and is intended to be a single-image format only. (A very PNG-like extension format called MNG was finalized in mid-1999 and is beginning to be supported by various applications, but MNGs and PNGs have different file extensions and different purposes.)

For image editing, either professional or otherwise, PNG provides a useful format for the storage of intermediate stages of editing. Since PNG's compression is fully lossless - and since it supports up to 48-bit true color or 16-bit grayscale - saving, restoring and re-saving an image will not degrade its quality, unlike standard JPEG (even at its highest quality settings). And unlike TIFF, the PNG specification leaves no room for implementers to pick and choose which features they'll support; the result is that a PNG image saved in one application is readable in any other PNG-supporting application. (Note that for transmission of finished true color images,especially photographic ones--JPEG is almost always a better choice. Although JPEG's lossy compression can introduce visible artifacts, these can be minimized, and the savings in file size even at high quality levels is much better than is generally possible with a lossless format like PNG. And for black-and-white images, particularly of text or drawings, TIFF's Group 4 fax compression or the JBIG format are often far better than 1-bit grayscale PNG.)

Compression

For professional results with print screen, lossless compression is important. PNG's compression is among the best available without losing image information and without paying patent fees, but not all implementations take full advantage of the available power. Even those that do can be thwarted by unwise choices on the part of the user.

PNG supports three main image types: true color, grayscale and palette-based ("8-bit"). JPEG only supports the first two; GIF only the third (although it can emulate grayscale by using a gray palette). The impact on compression comes from the ability to mix up image types in PNG. Specifically, forcing an application to save an 8-bit palette image as a 24-bit true color (or "RGB") image is not going to result in a small file. This may be unavoidable if the original has been modified to include more than 256 colors (for example, if a continuous gradient background has been added), but many images intended for the Web have 256 or fewer colors.

(See http://www.libpng.org/pub/png/pngintro.html for more detailed information).

 


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