Which Bitmap Format Is Best?

Many screen capture or save from web commands are defaulting to saving image files in PNG format (portable network graphic). However, the PNG is not always the best format to use when populating a web page or sending an image file.

As an example, the image at left started out at 640 x 304 pixels at 100 dots per inch resolution. Presented as a JPEG ( Joint Photographic Experts Group ) image, the file size is 34KB. The same file, with no discernible difference in quality, stacks up at a whopping 250KB when saved as a PNG. This means that the JPEG loads seven times faster in a web browser … or can be sent seven times faster as an e-mail attachment.

The PNG was developed to supplant the GIF (graphics interchange format). The GIF format has been around since the earliest days of the web, and was often used to represent vector artwork, such as logos. The GIF also offered transparency … where the background of the image dropped out, allowing the page background to “butt-up” to the image, eliminating the square images often found in photographs. One main limitation of the GIF was that it was limited to 256 colors. The PNG went much further, in that it could accurately represent a full color spectrum.

Let’s compare two pieces of artwork. The image on the left shows gradient artwork saved as a GIF. Because of the 256-color limitation, the image shows severe aliasing and speckling … where the color gamut of the original could not be achieved.

The second image shown is a PNG. Color reproduction is accurate, and there is no evidence of speckling. The improvement in quality of the PNG over the GIF comes at a surprising size reduction. The GIF file shown here is 36KB in size, while the higher quality PNG is only 32KB!

When file size is important, whether it is to load a web page or quickly transmit an e-mail attachment, consider using a JPEG file rather than a PNG when the image is a photograph. If the image is artwork, the PNG is the hands-down winner over the GIF when it comes to quality and reduced size.


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