One of the options that you have when using the ImageMagick library to do your conversions is Sepia Threshold, a number that, to me at least, makes very little sense.  The ImageMagick documentation suggests 80 as a good starting point, but anything between 0 and 99 is valid.  Rather than try and explain what the number means we’re just going to show some examples so you can pick a number for yourself.

Original Image

0% Threshold

With a zero percent threshold you’ll notice that the image is almost pure yellow except for the white areas on the left.

25% Threshold

Setting the threshold to 25 starts to bring in a little detail into the girl’s face.

50% Threshold

At 50% you can start to see the shape of the girl’s face.  The image is still primarily yellow with just a little bit of black and white for detail.

70% Threshold

A threshold setting of 70 finally starts to show true detail rather than just yellow, black, and white. 

80%, 85%, 90%, 95%, & 99% Threshold

The next five images are have a sepia threshold setting of 80, 85, 90, 95, and 99 respectively. As you can see the detail becomes smoother and the image gets redder as the threshold goes higher.

What if I don’t like the setting?

The sepia and grayscale images are cached to keep the server from having to redo the conversion every time a visitor wants to look at a sepia or grayscale image.  After changing the threshold setting you’ll need to clear out the cache files.  Do this by going to the Admin -> Tools menu and selecting the button marked Delete cache files.  The next time a visitor selects a sepia image the file will be reconverted with your new settings.

What about the GD Library?

Unfortunately for users using the GD library for their image conversions sepia toning is not available.  In theory it is possible for the GD library to sepia tone an image but in our tests it was taking 5+ minutes for the conversion on a 600 pixel image and no server is going to allow a script to run for that long and no visitor will wait that long. 

Can I see more?

Sure, just click on the link below to download all 100 images with sepia thresholds set from 0 to 99.