W3C Graphics PNG

PNG (Portable Network Graphics) and Gamma

Cross Platform Problems

This page is about cross 
platform image differences. If you cannot see images, or have downloading 
switched off, this page may be a little dull for you

Current situation

With existing formats, there are substantial cross-platform differences with image viewing. The images will be displayed, sure, but what people see will be very different.

Create a graphic on a Mac, say, and balance it to look just right. This picture of two children, for example.

Trouble is, it will look dark and contrasty on a PC, because the default gamma correction is different from that on a Mac. On an SGI workstation, however, it will look pale and washed-out because there the gamma is different again.

On a PC

On an SGI

The problem is that your browser has no idea where the image was created or how it was originally displayed, so it cannot compensate for these differences. What is needed is for the authoring tools to include this information, which is readilly available to them. But existing image formats have no way of storing this information.

How PNG helps

PNG stores the gamma value used by the source platform which created the image, in a standard place in the file which browsers, image viewers and authoring tools know how to read and adjust for. So the gross lightness and contrast differences we have just seen are compensated for automatically, without the image designer or the reader having to make any adjustments themselves.

On a PC

On a Mac

On an SGI

Not identical, but a lot better.

But that's not all...