Sigmoidal Contrast Compared by Flo

An Image Magick Tute by Flo

Look under the Enhance tab to find Sigmoidal Contrast.

Sigmoidal Contrast works very well with some images, but not all. So this is just like the other tools, in that respect - it all depends on the image type.

You need middle tones, dark tones and light tones for this to work well. Art Deco wouldn't work well, for instance, nor would poster-type images.

To quote Mr. Cristy: "Sigmoidal Contrast: sigmoidal non-lineraity contrast control. Give two values, contrast and midpoint. Increase the contrast of the image using a sigmoidal transfer function without saturating highlights or shadows. Contrast indicates how much to increase the contrast (0 is none; 3 is typical; 20 is a lot); mid-point indicates where midtones fall in the resultant image (0 is white; 50% is middle-gray; 100% is black). A good starting point is 3x50%."

Unlike Contrast-Stretch, Sigmoidal Contrast allows control over two factors: the contrast range from light to dark, and where the middle value of the mid-tones falls.

You will need to enter 2 numbers: The first number (1-20) increases the contrast. So if you have a "soft" low contrast image, try a 3 or a 5 or 10 and see if you like the results.

The 2nd number relates to how light or dark the mid-tones are to be. 0-100%. 50% is an average scene.

To make the scene look lighter, go below 50%; for a darker feel, go higher than 50%. The higher you go, the less highlights there'll be, so the scene will look darker.

IM suggests starting with 3x50%.


Original	Sig Con @ 3x75%, twice

Original	Sig Con @ 10x30%

For the image in the top row above, to get the near pond bank looking brighter than the far bank, I lost lots of the subtle details in the shadowed areas at the left and top of the image. So you have to make a judgement call here as to what you want to emphasize and what you're willing to give up.

Sigmoidal Contrast Compared
with Composite & Enhance/Saturate

Original	Sig Con @ 15x80%

Composites: 1: saturate; 2: bumpmap; 3: darken; 4: multiply	Saturation @ 500

Sigmoidal contrast was not used for the images in the lower row.

Another Example:

Original ~ ~ ~ Sig Cont @ 10x63%

Compare the above examples. Five ways to use to get more contrast and more color saturation in an image:

Enhance/Sigmoidal Contrast (this tute);

Use several Composites, one after the other as above, or several of the same Composite;

Enhance/Saturate;

Enhance/Contrast-Stretch;

Enhance/Level (Flo's tute coming)
- SEE:

Stranjer's Level tute
Tiger Lily's Level tute
Flame's Level tute

Each way produces different effects, so choose the way that gives you the most pleasing results.