Posted by on 11月 28, 2023

DSTRETCH (https://www.dstretch.com/) is a powerful tool for enhancing faint images of Rock Art. However, the enhanced images have often strong false colors.  It is difficult to use for public presentation purposes. One solution to this problem is to use Hue masks of DSTRETCH to extract the painted areas and paste them onto the original image, but this is a complicated process and the result is unnatural.

The solution to this problem found by Mr. Hideharu Yamada, a professional designer and Photoshop expert (https://lithos-graphics.com/) , is to blend the ‘luminosity’ of the image obtained by DSTRETCH with the original image by using Photoshop. This avoids the false colors of DSTRETCH, and allows for a natural-looking result. This process is simple and can be done within 10 seconds. It is also possible to  achieve an even more natural result using various function of Photoshop. This process is possible not only with Photoshop CC  but also  with Photoshop Elements and some older versions of Photoshop such as CS.

Original image of rock art (Iheren, Tasdsili n’Ajjer, Central Sahara)

DSTRETCH (https://www.dstretch.com/) は見えにくくなった壁画を鮮明化する強力なツールであるが、鮮明化によって画像に偽色が含まれてしまうため一般的なプレゼンテーションには使いにくいという問題がある。この対応として、DSTRETCHのHue maskによって描画部分を抽出して元画像に貼り付ける方法があるが、作業が煩雑で仕上がりも不自然なものとなる。

デザイナーでPhotoshopに通じた山田英春さん (https://lithos-graphics.com/) がこの問題の解決策として見つけた方法は、DSTRETCHで得られた画像の輝度(濃淡)のみをPhotoshopで元の画像に合成するもの。これによりDSTRETCHの偽色を回避して自然な仕上がりが可能となる。この処理は非常に簡単で、DSTRETCH画像と元画像があればわずか10秒程度の時間でできる。また、Photoshopを使ってさらに自然な仕上がりにすることも可能。Photoshop ElementsやCSなどの古いバージョンのPhotoshopでも可能な方法を紹介したい。


元の不鮮明な壁画の画像 (イヘーレン、タッシリ・ナジェール、中央サハラ)


Tracing by the mission Henri Lhote in 1970



DStretch-YBK (scale 15)  Enhanced image with DStretch containing strong false colors.

DStretch-YBK (scale 15) 強い偽色を含むDSTRETCH処理された画像


Image after blending processes of Luminosity & Soft Light



Work procedure:

Open the original image and the DStretched image with Photoshop.  Copy DStretched image, Paste it on the original image. Change the blending mode of the pasted layer  from ‘Normal’ to ‘Luminosity’. If the blending looks unnatural, adjust the Opacity.





Adjustments after Luminosity blending:

Reduce contrast to reduce the harshness of the shading. No detail is lost when contrast is reduced.





Luminosity blending often results  a monotone appearance with low color saturation. In such cases, increase the saturation to make the color closer to that of the original rock surface.




Soft Light blending is another effective method for natural-looking results. After luminosity blending, duplicate  the two layers and merge the two duplicated layers. After making the DStretch image layer invisible, change the blending mode of the merged layer from ‘Normal’ to ‘Soft Light’.



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