CSS Color HDR Module Level 1

1. Introduction

This section is not normative.

CSS Color 4 adds Wide Color Gamut (WCG) color spaces to the Open Web Platform. By design, these are all Standard Dynamic Range (SDR) color spaces. This specification explores what additions might be suitable to also add High Dynamic Range (HDR).

While WCG specifications have been broadly stable for some decades, HDR standards are less mature and have undergone frequent revision in the last decade.

This specification is a collection of ideas and has no status whatsoever. It is not even an Editors Draft and has not been reviewed or adopted by the CSS Working Group.

1.1. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

2. Predefined color spaces for HDR:

In addition to the SDR color spaces defined by CSS Color 4, the following HDR color spaces are defined for use in the color function CSS Color 4 § 10.1 Specifying Predefined Colors: the color() function.

Serialization of values in these new color spaces is identical to the description in CSS Color 4 § 15 Serializing <color> Values.

2.1. rec2100-pq

The rec2100-pq [Rec_BT.2100] color space accepts three numeric parameters, representing the red, green, and blue channels of the color, with each having a valid range of [0, 1] regardless of the actual bit depth (10 or 12 bits per component).

This is to avoid confusion if [0, 1024] and [0, 4096] encoded values are mixed up. Add examples showing conversion from binary values to the [0, 1] range.

should this specification recommend that the CSS Object Model store these values in the 16-bit half-floating point representation (IEEE standard 754-2008.) from BT.2100 pp. 9-10? (or any wider representation, like float or double)?

should the values accept % as well as number? so 19% as well as 0.19. Probably yes.

The Perceptual Quantizer (PQ) electro-optical transfer function is used [SMPTE-ST-2084],[Rec_BT.2100]. PQ assumes a reference viewing condition where the screen surround is at 5 cd/m².

ITU Reference 2100 is used for HDR 4k and 8k television.

In contrast to SDR color spaces and some HDR color spaces, the PQ values are absolute rather than relative.

add a diagram showing SDR and HDR dynamic ranges on a log scale

It has the following characteristics: (The display primaries are the same as [Rec.2020]):

from BT.2100 p.3 "Note 3d – For PQ in a non-reference viewing environment, or for HLG (in any viewing environment), the black level should be adjusted using the PLUGE test signal and procedure specified in Recommendation ITU-R BT.814."

 color(rec2100-pq 1.0 1.0 1.0);

 color(rec2100-pq 0.58 0.58 0.58);

 color(rec2100-pq 0.34 0.34 0.34);

add other examples, including encoding of sRGB red, green, blue and P3 red, green, blue.

Linear-light RGB signals are converted to PQ encoded as follows. An absolute luminance scale is used, so the luminance of diffuse (media) white is required to scale relative luminances. The maximum encodable value (peak, small-area white) in PQ is 10,000 cd/m². Media white is 203 cd/m² [Rpt_BT.2408].

var Er;      // the red, green or blue component, [0, 1] for SDR, [0, 70ish] for HDR
var Yw = 203;  // the absolute luminance of diffuse white, cd/m²
var x = Er * Yw / 10000;   // absolute luminance of peak white is 10,000 cd/m².
const n = 2610 / (2 ** 14);
const m = 2523 / (2 ** 5);
const c1 = 3424 / (2 ** 12);
const c2 = 2413 / (2 ** 7);
const c3 = 2392 / (2 ** 7);
xPQ = (((c1 + (c2 * (x ** n))) / (1 + (c3 * (x ** n)))) ** m);

xPQ is the "gamma-corrected" (OETF-adjusted) signal [0, 1].

PQ encoded values are converted to linear-light as follows:

var xPQ;      // the red, green or blue PQ-encoded component, [0, 1]
const ninv = (2 ** 14) / 2610;
const minv = (2 ** 5) / 2523;
const c1 = 3424 / (2 ** 12);
const c2 = 2413 / (2 ** 7);
const c3 = 2392 / (2 ** 7);
var x = (((Math.max(((xPQ ** minv) - c1), 0) / (c2 - (c3 * (xPQ ** minv)))) ** ninv);
var Yw = 203;      // the absolute luminance of diffuse white, cd/m²
var Ea = x * 10000;   // absolute luminance, [0, 10,000].
var Er = x * 10000 / Yw;   // luminance relative to diffuse white, [0, 70 or so].

2.2. rec2100-hlg

The rec2100-hlg [Rec_BT.2100] color space accepts three numeric parameters, representing the red, green, and blue channels of the color, with each having a valid range of [0, 1] regardless of the actual bit depth (10 or 12 bits per component).

The Hybrid Log-Gamma (HLG) electro-optical transfer function is used [ARIB_STD-B67],[Rec_BT.2100]. HLG can be used with displays of different brightness in a wide range of viewing environments, with user control of the overall brightness level. A value of 0.75 represents "diffuse" or "media" white while an "18% reflectance grey" card has a value of 0.38. [Rec_BT.2390].

It has the following characteristics: (The display primaries are the same as [Rec.2020]):

add other examples

 color(rec2100-hlg 0.75 0.75 0.75);

 color(rec2100-hlg 0.38 0.38 0.38);

Linear-light RGB signals are converted to HLG encoded as follows [Rec_BT.2390]:

var E;      // the red, green or blue component, [0, 1]
const a = 0.17883277;
const b = 0.28466892;   // 1 - (4 * a)
const c = 0.55991073;   // 0.5 - a * Math.log(4 *a)
if (E <= 1/12) {
  Edash = Math.sqrt( 3 * E);
}
else {
  Edash = a * Math.log(12 * E - b) + c;
}

Edash is the "gamma-corrected" (OETF-adjusted) signal.

The reverse conversion (HLG encoded to linear light) is as follows [Rec_BT.2390]::

var Edash;      // the red, green or blue encoded component, [0, 1]
const a = 0.17883277;
const b = 0.28466892;   // 1 - (4 * a)
const c = 0.55991073;   // 0.5 - a * Math.log(4 *a)
if (Edash <= 0.5) {
  E = (Edash ** 2) / 3;
}
else {
  E = Math.exp(((Edash - c) / a) + b) / 12;
}

add black level lift Β as defined on same page?

2.3. Jzazbz

The Jzazbz color space [Safdar-PUCS] accepts three numeric parameters, with J_z representing the Lightness (similar to L in Lab) while a_z and b_z represent the redness-greenness and yellowness-blueness opponent-color axes (similar to a and b in Lab).

CIE Lab uses a lightness scale with a [0%, 100%] range, relative to media white, and has been experimentally extended to L=400%. In contrast, the J_z axis in Jzazbz, which has a [0, 1.0] range, uses Perceptual Quantizer encoding, and can represent a dynamic range of more than 13 stops.

It has the following characteristics:

Note that unlike Lab, a D65 whitepoint is used. Thus, for most RGB spaces (which also use a D65 whitepoint) no chromatic adaptation step need be performed.

2.4. JzCzHz

Similar to LCH, which is the polar representation of Lab, JzCzHz is the polar form of Jzazbz. J_z is identical to the value in JzCzHz and represents the Lightness, while C_z represents the Chroma or colorfulness and H_z represents a hue angle, measured from the positive a_z axis, towards the positive b_z axis.

 color(jzazbz 0.17542 -0.1179 0.1092);

 color(jzczhz 0.17542 0.1614 132.50);

2.4.1. Converting Jzazbz colors to JzCzHz colors

Conversion to JzCzHz is trivial:

1. Hz = atan2(bz, az) // but convert to degrees!
2. Cz = sqrt(az^2 + bz^2)
3. Jz is the same

2.4.2. Converting JzCzHz colors to Jzazbz colors

Conversion to Jzazbz is trivial:

1. az = Cz cos(H) // convert to radians first!
2. bz = Cz sin(H) // convert to radians first!
3. Jz is the same

2.5. ICtCp

The ICtCp color space is defined as Constant Intensity IC_TC_P signal format in [Rec_BT.2100] and accepts three numeric parameters, with I representing the Intensity (similar to L in Lab, but covering a luminance range up to 10,000 cd/m²) while C_T and C_P represent the yellowness-blueness (tritanope) and redness-greenness (protanope) opponent-color axes (similar to b and a, respectively, in Lab).

It is based on human visual system LMS cone primaries; the Hunt-Pointer-Estevez (HPE) XYZ to LMS transform is used, normalized to a D65 white point. A crosstalk matrix is then applied, to reduce the gamut hull concavities of BT.2020 RGB, thus reducing interpolation errors. The crosstalk also provides improved lines of constant hue and improved uniformity of Just Noticeable Difference (JND) MacAdam ellipses.

It has the following characteristics:

Although [Rec_BT.2100] defines IC_TC_P by a conversion from linear-light BT.2100 RGB [Rec_BT.2100], this conversion proceeds via LMS and thus, any other color space can also be represented by applying an XYZ to LMS transform.

Note that unlike Lab, a D65 whitepoint is used.

Also unlike Lab, which has primarily been tested with lower-intensity reflective colors, IC_TC_P has been tested with high-chroma, self-luminous, and high intensity (HDR) colors. This makes it suitable for color difference measurement (deltaE ITP) and for gamut mapping of SDR and HDR colors.

 color(ictcp 0.5393 -0.2643 -0.0625);

3. Compositing SDR and HDR content

Compositing should take place in CIE XYZ, as it is a linear-light space without gamut limitations. Implementations may chose to composite in a linear-light RGB space instead, which will give the same result provided out-of-gamut values (negative, or greater than 100%) are correctly handled and not clipped or gamut mapped until the final transfer to the device color space.

Relative HDR, using the HLG transfer function, must map SDR media white to the same luminance as is used to display the 75% HLG value. [SMPTE-ST-2084]

For further details, see also tables 3 and 4 in ITU Rpt_BT.2408-0 [Rpt_BT.2408]

Absolute HDR, using the PQ transfer function, should map SDR media white to 203 cd/m², the same luminance as is used to display the 58% PQ value. [SMPTE-ST-2084] However, implementations may chose to incorporate a color re-rendering step (OOTF) to account for non-reference viewing conditions.

Do the PQ-based color spaces require tone-mapping (OOTF) for display in different viewing environments?

Privacy and Security Considerations

Make some considerations about privacy and security.

Accessibility Considerations

Some individuals may have a sensitivity to very bright colors, so user agents should provide a mechanism to limit the maximum luminance at user option. The toe and knee procedure in section 5.4.1 Mapping to display with limited brightness range of [Rec_BT.2390] is suggested as suitable.