Mushroom Duotones

Posted a few duotone images of mushrooms to Flickr. Didn’t mean to spend this morning making these images, but when you get the right subject matter, there is an eerie presence to toned images: Maybe it’s the intersection of image qualities that recall old photography and emulsions hand-coated onto paper coupled with high tech display and printing—or maybe the mushrooms (just shitakes) have an effect on me.

This tutorial on duotones over at Luminous Landscape was useful.

Duotone Soft Proof

Can’t soft proof a duotone in Photoshop. Have to convert to RGB to soft proof; however, the color changes in subtle ways.

One way to compare the images is by checking their histograms.

The duotone image is just a little darker on average (mean 106.97 compared to mean 112.53), and it clearly has pixels in the darkest bins, including a spike of 0% black over on the left edge of its histogram. The RGB image has no black pixels until you reach level 11, about a 4% gray.

Comparison of two histograms

Comparison of RGB and duotone image histograms

You can use Curves adjustment layers to compare the dynamic ranges of the two versions of the image. Option-drag on the black point or the white point in the Curves dialog to see where in the image the darkest and lightest values are found. The duotone reveals solid black (0%) in a few places. The RGB version has no solid blacks. In the light tones, the two are practically the same.
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Bezier Circle

Approximate a circle with cubic Bezier curves: here’s a simple method, written in Processing. The key to approximating the circle is a constant, kappa, that will help us calculate the distance from a Bezier anchor point on the circle to its associated control point, off the circle. Kappa is the distance between the anchor point and the control point divided by the circle radius, when the circle is divided into 4 sectors of 90 degrees. See these notes by G. Adam Stanislav for the math.

Circle approximated by five Bezier curves. Click to view applet.

Notes: Kappa is scaled by the number of sectors into which the circle is divided (k = 4 * kappa / sectors). Four sectors is the minimum required to get a good approximation. With a little bit of work it should be possible to create a circle with unequal sectors, scaling kappa by the portion of the circle occupied by each sector. From there, one could make various sorts of blobs by scaling the the control points and anchor points around the center of the original circle.

Lab Enhancements

Using Photoshop’s Lab color mode, you can perform a number of simple image enhancements. For some of these enhancements there are similar RGB operations; however, the results are subtly (and sometimes not-so-subtly) different. The international standard L*a*b* color space from which the Lab mode in Photoshop was derived was constructed to capture the range of human vision. It was based on statistical evaluations of the range of color vision (the “a” and “b” channels)  and of just-perceptible differences in brightness (the Lightness channel).

Several techniques are illustrated by Photoshop actions that you can download. Explanations and a few tips on how to perform the actions manually follow.

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