Compression

March 5th, 2009
Gwyn Headley

by Gwyn Headley

Managing Director

On January 21st we started (gradually) to convert all the TIFFs on fotoLibra to top quality (Photoshop Level 12) JPEGs.

This will save about a terabyte of space with our current number of images, and provide far more room for members to upload photographs. All buyers seem to prefer JPEGs nowadays. Long gone are the days when the 24 MB TIFF was the only path to salvation.

The big debate we had in-house was how much compression should we apply? We want to preserve of much of the original image as commercially possible. Rumour has it that one very major picture library saves its JPEGs at Level 8, which is 60%.

We are more perfectionist. I was against anything less than Level 12, which holds between 90% and 96% of the original data. This will reduce a 24 MB TIFF to an 8 MB JPEG, a substantial saving. It is impossible to detect a visual difference on a computer screen. We settled on 94%.

JPEGs work well where there are large areas of the same colour, such as the sky. To put it as simply as possible, a TIFF file of a photograph of blue sky would read as follows:

  • blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel, blue pixel,

whereas the JPEG file of the same image would read

  • blue pixel x 24

Which as you can see is much more concise. Hence it’s smaller.

Here’s an image shown as a TIFF, then at Level 12, Level 11 and Level 10 JPEG compression:

Compression

Frankly, I can’t really tell the difference. Only when compression gets down to Level 7 can a difference be detected.

Worries about the degradation of the JPEG image through repeated copying are unfounded because the original upload is always used as the source asset for image sales. The purchaser will always be getting a Level 12 JPEG.

Why camera makers never adopted the much more sophisticated JPEG2000 standard I’ll never know.

Of course, any Pro or Platinum members who have uploaded TIFFs to fotoLibra and who do not want them converted to JPEGs only have to tell us and their uploads will remain untouched. Only one so far.

Add your comment