Subsampling in ICTCP vs Y’C’BC’R

Summary

Y’C’BC’R introduces distortions when chroma subsampling (4:2:0 or 4:2:2) saturated BT.2100 colours due to Y’C’BC’R’s nonconstant luminance attributes. ICTCP , as a nearly constant luminance representation, does not producethese distortions.

Details

Chroma subsampling introduces chroma error into a system. Humans are substantially more sensitive to errors in luminance than colour; therefore, any chroma errors that propagate from chroma into luminance information will perceptually degrade the image.

The examples below show a BT.2100 image of flowers that has been spatially subsampled at 10 bits in ICTCP and Y’C’BC’R. The results have been mapped for a standard-dynamic-range (SDR) display, so some loss of detail is occurring; however, the appearance is similar.

The middle close-up view in the second row shows how subsampling this image with Y’C’BC’R introduces substantial distortion, creating the appearance of sparkling. There is some inherent noise in the original image that is below visible threshold. When subsampled in Y’C’BC’R, chroma noise that was not visible in the original propagates into luminance and becomes visible. ICTCP as seen on the bottom right, maintains a visually high-quality image despite the chroma subsampling.

Original Y'C'BC'R 4:2:0 ICTCP 4:2:0

The following owl images illustrate why minimizing error propagation from chroma into luminance is important and specifically why it is important to avoid having noise introduced into a system when there is crosstalk between chroma and luminance. The owl image at top left is the original. This is compared to two percent Gaussian noise added to the I channel (bottom left), to the CTCP channels (bottom right), and the C’BC’R channels (top right). Despite having only added noise to one out of three channels, the two percent noise introduced in luminance (I channel) is substantially more visible than the two percent noise added to both chroma channels (CTCP or C’BC’R channels). Therefore, the closer a colour representation is to a constant luminance system, the more advantageous it is for chroma spatial subsampling because subsampling errors (simulated in this example by added noise) are constrained to the chroma channels where they are less visible. The two percent noise added to C’BC’R illustrates why a nonconstant luminance system is problematic. The same degree of noise added to the C’BC’R channels as was added to the CTCP channels is more visible due to the chroma/luminance crosstalk.

The following two graphs show the relationship between luma and luminance of Y’C’BC’R and ICTCP. A perfectly constant luminance colour representation has a 1:1 relationship between electro-optical transfer function (EOTF) encoded luminance (Y of XYZ, cd/m2) and luma (Y’ or I).

The deviation from that 1:1 relationship in Y’C’BC’R (see graph at left) is the luminance/chroma crosstalk that Y’C’BC’R exhibits. This crosstalk causes the chroma subsampling errors to propagate into luminance and causes the distortions shown earlier. ICTCP (see graph at right) has a near 1:1 relationship and is therefore essentially constant luminance. ICTCP constrains the subsampling error to chroma only, where it is less visible.

© 2016 Dolby Laboratories, Inc. All rights reserved.

Original

2% Noise Added to I

2% Noise Added to C'BC'R

2% Noise Added to CTCP

Luma Y' of Y'C'BC'R Intensity I of ICTCP

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