Configurable RGB

This space is a device Red Green and Blue colorspace, where the conversion from measured, device independent values to device dependent RGB values can be configured.

Most often this would be used to set the Reference Measurement value in terms of a desired device value, so that a device like a Video Display or TV can be measured and adjusted to minimize the difference (Delta) to the desired reference color. Combining this with Send RGB to TPG option is a convenient way of measuring and comparing to a specific reference color.

Note that the values displayed or entered are not constrained to the limited device range, although out of range values will be displayed in RED. Values sent to the TPG will always be clipped to the device range.

There are seven parts to the configuration of the RGB device model:

RGB configuration

1) Color Space - primary colorant values and white point

Three primary colors plus white define the fundamental colors that are mixed together by the display, and the balance between them when they are all 100%.
There is a pull down list to choose between standard RGB encoding colorspaces:

sRGB Standard for computer display and the Web

AdobeRGB1998 Photographic and graphic arts standard

ProPhoto RGB Wide gamut photographic space

PAL/EBU 3212 Standard definition European TV

Standard definition USA TV

ITU-R BT.709 High Definition TV


Digital Cinema

Plus two ways of specifying custom primaries:

Custom primaries & white point

Where the measurement values of the R,G,B & White Primary Color References are used.

Custom Where the R,G,B & White x & y Chromaticity coordinates can be specified manually.

2) EOTF - Electro-Optical Transfer Function

The EOTF determines how the light produced in each of the R,G & B channels responds to the device value. Typically it will be power like curve, often called a "Gamma" curve. Standard encoding colorspaces have a standard EOTF, and by default this corresponding EOTF will be chosen when you select a standard colorspace. This will be the first entry in the EOTF pull down list.

For a Measured or Custom set of primaries there is no standard EOTF, so one must be specified, and for some purposes (such as display calibration) it usually desirable to use a specific display or decoding EOTF rather than the one used by the encoding space standard.


There are three choices for this:

BT.1886 Reference EOTF with a technical gamma (power function) of 2.4 and an input black offset strategy
Effective Gamma  
Configurable gamma that is such that the 50% output value has the chosen power, irrespective of any black offse, and that then has the black offset strategy applied to it.
Technical Gamma  
Configurable gamma (power function) value that then has the black offset strategy applied to it

Real world display don't have a perfect zero black, so some strategy is needed to fit the EOTF so that it matches the display black point at zero. (Encoding spaces don't make any allowance for this, and assume a perfect zero black). The black level is assumed to be measured and stored in the Black Primary Color Reference.

There are two basic strategies: Input black offset or Output black offset.

Input black offset scales and offsets the gamma curve so that zero device value mapped through the curve gives the black point of the display. This has the benefit of preserving shadow detail just above black. It has the disadvantage of sometimes over emphasizing shadow detail at the expense of contrast.

Output black offset scales and offsets the gamma curve so that the zero input and output point of the curve maps to the black point of the display. This is sometimes referred to as a "pure gamma curve", even though it is not pure. It has the advantage of preserving maximum contrast ratio, but can result in loss of shadow detail.

The configuration offers a choice of input or output offset with a slider, allowing a hybrid of the two to be chosen.

Note that the Black Primary Color Reference can be manually set to zero by configuring a readout to show the Black Reference, and then setting the value (double tap).

3) 100% Brightness

While the primaries and white chosen determine the color and relative brightness of a device RGB value, the absolute brightness of white is determined separately. There are two choices:

100% Brightness

Where the measurement values White Primary Color Reference Y value is used.

Set Where the brightness can be set numerically.

4) Color Representation

The readout value can be set to show the native RGB values, or several alternate representations. These alternates are simple transformations of the RGB values, intended to correspond to the view of color being composed of a Hue angle, a Color intensity number called Chroma or Saturation, and some sort of number representing the distance from black to white along the neutral axis named Value, Lightness or Intensity, depending on the particular model.

While all these representations were intended to be more intuitively understandable than RGB, in practice their naive Luminance values and the distortions necessary to map all of the device dependent RGB colorspace to/from these spaces without any gaps, makes them highly visually non-uniform and hence hard to use. They are provided here for easy comparison or translation to other systems that use these representations.

If any Representation other than RGB is chosen, then the encoding is always 0..360° for H, and 0..100% for Saturation, Value, Lightness and Intensity.

RGB Representation

Red, Green Blue
Native Representation

Hue Saturation Value
V=100% for Saturated colors

Hue Saturation Lightness
L=50% for Saturated colors

HSI Hue Saturation Intensity
I=33.3% for Saturated colors

5) Number Encoding

The final part of the configuration is determining what scale and number base to display the RGB in.
(Number Encoding is not available if an HSV, HSL or HSI Representation is selected.)

Number Encoding



0.0 - 1.0

0.0 - 100 % 

8 bit 0 - 255 0 - ff

10 bit 
0-1203 0 - 3ff

12 bit 
0-4095 0 - fff

There is also an independent choice of whether the numbers are over their full range where 0 is black and maximum is white, or whether the restricted Video range is used, where 8 bit value 16 corresponds the Black, and value 235 corresponds to White.

6) Test Pattern Generator

You can use a ChromeCast as a Video Test Pattern Generator, and enabling the Send RGB to TPG option will automatically send the RGB value from this readout to ChromeCast selected in the Video Test Pattern Generator configuration. Sending the RGB Readout value to the TPG to set both the test value displayed on the TV or projector, as well as the target XYZ value that should result with the displayed value. Typically the Readout Source will be set to Reference Measurement for making use of this feature.

RGB Video Test Pattern Generator

A button to open the Test Pattern Generator dialog is also present, for convenience.

7) Tag

The last entry is a text Tag name, used to distinguish the Readout. By default it will be a short version of the primaries color space, but can be overridden to add any other important distinguisher. The default can be restored by deleting the tag text.


Special Behavior after setting a value

By default all Readouts reflect the current XYZ source value expressed in the particular Readout format, and reflect any changes in the source values they use or their Readout configuration, by a change in their displayed value.

The Configurable RGB Readout also behaves in this way until you manually set a value. It then responds differently to all other Readouts when a value used in the calculation (such as the White or Black values, or Red, Green or Blue values for a Measure colorspace) changes, or the model parameters (i.e. colorspace, EOTF or brightness) are changed, when it re-sets the XYZ value from the last entered RGB values.

This is to avoid having to manually re-enter an RGB reference value if the white point or black point of a display are re-measured, or if the model parameters are tweaked, while the same test patch value is being compared against it.

Another special behavior is that the manually set values will be displayed and sent to the TPG, even if the required configuration and measurements such as white and black point are not valid. This is to allow the white and black references to be measured. In this situation a warning message will be displayed after entering value, indicating that the XYZ value cannot be set, even though the RGB values sent to the TPG will be set. When this is happening, the RGB values will be shown in light blue.

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