The Subtractive RG Color Space
The RG or red-green color space is a color space that uses only two colors, red and green. An additive RG space creates images by combining red and green light. Such images appear to have a yellow overlay, because white light cannot be generated without a blue component. But an early color film process, two-color Technicolor, was a subtractive RG process that generated surprisingly-realistic images using only the colors red and green.
An RGB image is transformed to RG by removing the blue component. The initial result is an additive image with a yellow cast like the second image of the macaw below. However, a subtractive process may be simulated by using Hue blend mode for the transformation. The resulting image displays the hue of the RG space, while preserving the chroma and luma of the original image. The most obvious result of this transformation is that whites stay white, as in the third image below. It resembles an image created by printing with (idealized) red and green pigments on white paper.
(In reality, black ink, and probably an intermediate color such as yellow, would be required to produce a high-quality image. The point is that the gamut of an RG image includes only hues from 0 to 120 degrees, the red-yellow-green sector.)
In a photo editor, the most straightforward way of transforming an RGB image to RG is to add a Curves adjustment layer, set the blend mode to Hue, and flatten the blue curve. (Select the blue curve, click its upper right endpoint, and drag it down to the bottom.) In Affinity Photo the process is straightforward:
In PhotoLine, the blend mode is set in the Layers panel (just above center in the screenshot below), not the Curves dialog:
Images whose main subject is not a scarlet bird generally look too green when converted to RG; it usually helps to lower the green curve:
PhotoLine can record these operations as an action, Affinity Photo as a macro. More drastic effects are possible, such as lowering the endpoint of the green curve, or raising the origin of the red curve. Raising the latter all the way to the top results in a red-orange-yellow, autumn colors gamut. Every image reacts differently to RG conversion.
Here is the digital dog test image, converted to RG and then “RY”:
The RG Color .8bf Plugin
Here is a plugin that converts RGB images to RG, and facilitates a number of editing functions that may improve RG images, such as pre- and post-conversion saturation and hue adjustments, and two different image blends. In addition, it includes selective red and green vibrance controls, hue limits, and a hue histogram.
The plugin works in Photoshop, PhotoLine, and IrfanView. (It works in Affinity Photo per se but the preview is grayed out; they are working on it . . . . It can even work in the GIMP but again, the preview is impaired.) Three versions are available: 64-bit for most 64-bit applications, 32-bit for 32-bit applications (running in either Windows 32 or 64 bit), and a “retro” 32-bit version compatible with Windows XP. (Please note that both IrfanView and GIMP 64 bit use only 32-bit filters.) It can also be used in Linux and Mac OS X in a Windows application; please see the RC Filters page for more information.
Some information about the plugin controls:
When balancing the color, red skin tones and green foliage often compete. This may be overcome by increasing the pre-conversion red-green color contrast. Vivid greens may be muted by reducing hue max, limiting the hue range.
Saturated blues and cyans may cause garish effects. One way to deal with them is to partly or completely desaturate them. Here the red and green vibrances were increased as well:
Shifting the blue hues targets the color of the sky:
Small discontinuities in the red value are not so noticeable in a dark, saturated sky, but they pop out after conversion to RG. This can happen in jpeg noise as well. The artifact reduction controls attempt to correct this by boosting the red or green values in dark saturated areas; but they may introduce new artifacts elsewhere.
If the whites and neutrals look out of place after conversion, adding a little of the yellow-tinted Normal (rather than Hue) blend mode chroma will warm them:
The Red-yellow gamut control raises the red curve, converting the image to an “RY” color space:
Purchase the RG Color plugin
If you would like to use the RG Color plugin, please send me $5.00 and your e-mail address and I will send you a link to the download page, which will be good for at least a month. You may try the other plugins at RCFilters.htm to see if they work on your system.
Like the rest of the filters, RG Color is available in three versions: 64-bit for most 64-bit applications, 32-bit for 32-bit applications (running in either Windows 32 or 64 bit), and a “retro” 32-bit version compatible with Windows XP.
Thank you . . . .
© 2017 by Russell Cottrell; all rights reserved.