D-Stretch in Photoshop

D-Stretch is an image enhancement technique that I first learned about when I saw it applied to prehistoric pictographs. There is a plugin to the software ImageJ which will handle an image and produce a d-stretched version of it that can be found here. However, as someone who is comfortable working with digital images in Photoshop, I wondered if there was a way to replicate the effect in house. Here I present to you what I learned, and some thoughts on the process.

Let's start with a photograph of the Horned Dog, a particularly well preserved red ocre pictograph in Tennessee. The photograph was shot as raw, and processed for optimal display. White balance was adjusted based on the scale present in the image.

We will want to convert the image to the Lab color space to proceed (Image > Mode > Lab Color).

Check your channels tab, you should see these three channels present: Lightness, A, and B. We'll be individually adjusting these channels so keep this handy.

For each of the color channels you will want to do an auto-contrast (Image > Auto-contrast | Alt+Shift+Ctrl+l). This process will most dramatically affect the A and B color channels.

Above is the lightness channel.

Above is the A channel.

Above is the B channel.

Above is what the image looks like when all channels are active. This is the d-stretched image.

Photoshop is powerful software, is there anything more that we can do? Perhaps. Individually editing the A and B color channels in the raw dialog could be very powerful. Adjusting highs and lows through the whites, blacks, shadows, and lightlights sliders as well as contrast could be useful to elucidate meaningful data from the image. Texture is useful for bringing detail out in small areas, and clarity useful for bringing detail to larger areas. Dehaze as I have discovered is often a very powerful contrast, but has some preferences based on regions of application. In the next few steps I use dehaze and clarity to work on the A and B channels and provide an alternative final d-stretch image.

Channel A with clarity and dehaze maxed out.

Channel B with clarity and dehaze maxed out.

An alternative d-stretch image. Note that red is much more tightly aggregated on the subject. Obviously this won't work on all images equally, but perhaps provide some creative thoughts in dealing with d-stretch imaging situations.


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