Originally Posted by fafrd
I understand that use of a white subpixel will introduce 'distortion' into a pure RGB mapping, but I'm not understanding why that distortion is inherently non-linear.
If we assume the RGB composition of each subpixel (R, G, B, W) is known and if we assume the color filters are pure (like QDCF) so R contains 100% red and 0% Blue and Green, etc... and W contains aR, bG, cB, then wouldn't the resulting mapping of any r,g,b triplet to RGBW be linear if the mapoing was properly implemented?
Is the 'inherent nonlinearity' you are referring to because the red filters pass a small amout of blue and green (and same for green and blue filters) or because the filters pass a 'bell-curve' of color spectra rather than a 'pure' spike, or because the white subpixel passes a complex curve of varibg intensities all WOLED spectra which is impossible to model (and shifts over age)? Or all three?
Said another way, with 'pue' color fiters for RGB (like QDCF) and proper modeling/translation algorithms could WRGB WOLED be linear or does the complex spectral output of the WOLED itself mean that even in that case linearity is impossible?
And if is is impossible even in that case, what about if the white subpixel istelf also has RGB QDCF (all 3, like for QDEF film used on current QLEDs)?
The output of the display is not linear and predictable when the input signal is changed in a linear and predictable way.
The causes display volumetric non-linearity.
In its simplest form, a displayed color is simply the sum of its components C= R+G+B.
If it is ‘linear’, then the combination is predictable in that changes in the input red signal has no effect on the light coming from the blue or green channel.
This is mostly true for old school CRT's, and for the most part LCD's and even LED's.
With a WOLED display, it has 4 emitters.
So, when the red signal is adjust, it will change the emission from the red LED, and may be also the white LED depending on the overall RGB signal that it is receiving.
This breaks the simple C= R+G+B rule.
The device now has a complex signal to light behavior.
This does not mean it is non-linear in its response.
If you have a non-linear system, you need a full 3d model to make it do what you want it to. The greater the non-linearity, the bigger the size of the 3D LUT required to manage it.
(Actually, if you had direct access to RGB and W, then you would be best generating a 4 channel cube! This is often done internally in printers that have more than 3 inks; same basic idea.)
Additionally, it seems that the in the WOLED display, the that the W channel is sometimes “off” completely and that there is a step change to minimum “on” luminance (again defendant on the RGB signal received). This step discontinuity may well be very small, but it compounds the problem, especially in the blacks/shadows.