hue preservation and color correction

HUE PRESERVATION AND COLOR CORRECTION

- 2 papers with a brief study and practical strategies

a. Color correction for tone mapping, R. Mantiuk, et, al. EuroGraphics 2009b. Hue preservation using enhanced integrated multi-scale retinex for improved color correction, Wand-Jun Kyung, et, al. Journal of Imaging Science and Technology, 2011.

13年7月31⽇日星期三

OUTLINE

• Background and Purpose

• [MMTH09] Color Correction for Tone Mapping

• [KLLH11] Hue Preservation using Enhanced Integrated Multi-scale Retinex for Improved Color Correction

• The Current Color Correction Issues We Deal with

BACKGROUND

COLOR REPRODUCTION / TONE MAPPING

• For the purposes of displaying in different devices and get perceptively matched color when applying effects.

• Tone Reproduction Curve (TRC) - a global tool which bases on global adaption to adjust the whole image. simple and efficient.

• Tone Reproduction Operator (TRO) - is a multi-resolution decomposition algorithm to find the best solution operated in local. precise and preserved local quality.

COLOR REPRODUCTION / TONE MAPPING (CONT.)

• TRO has better result, but it also produces artifact or halo. ex: HDR mapping.

• Although TRC has no halo or artifacts, it cannot preserve the local contrast or saturation.

COLOR CORRECTION

• Both reproduction mechanisms have issue on Color Shift. Or what we say the color becomes not related to human eyes’ perception.

[MMTH09] COLOR CORRECTION FOR TONE MAPPING

• A method which focused on Color Correction with different contrast compression after Tone Curve Mapping (TRC), and then extended to TRO.

• Modeled the color reproduction properties and demonstrated the relation between saturation and contrast in color matching is not Linear.

• Pointed out the color shift was due to the tone mapping curve itself.

RELATED WORKGamut mapping

• Need modify luminance and chrominance at the same time to preserve color appearance.

• P1: For tone mapping, the luminance is given by curve.

• P2: Gamut mapping corrects color in small dynamic range. But tone mapping is not.

COLOR APPEARANCE FACTORS RELATED WORK ( CONT. )

• Luminance, Hunt Effect [Hun52]

• Image Size [NB06]

• Color of the surround [BM97]

• Perceived contrast of complex images decreases with reduced chroma and the relation is sigmoidal. [CF03]

COMMON APPROACHRELATED WORK ( CONT. )

• Preserving color ratios of tone mapping [Sch94]

• An ad-hoc formula is to adjust the saturation level by power s. [TT99]

• How to find find correct s ?

ENHANCEMENT

• The drawback is it alters the resulting luminance for

THE CONTRAST

Contrast compression

where b is the brightness(exposure) adjustment that normalizes for maximum display brightness. Under huge contrast compression, nor equation (2) or equation (3) can fit good result.

CONTRAST COMPRESSION CAUSES BAD RESULTS

EXPERIMENT

To find what the s should be under different contrast c.

EXPERIMENT (CONT.)

For equation (2) For equation (3)

1. By Analysis of Variance( ANOVA ), LDR and HDR has no statistical difference.

2. The relation between s and c can be approximated with a power function below for c <= 1.6

3. For general function, a sigmoid function has been proposed:

RESULT

CORRECTION IN CIELAB

In CIELAB, it almost no need to adjust saturation in low contrast compression range, (0.6, 1.6)

CORRECTION IN CIELAB (CONT.)

CIELAB seems great. But it’s hard to be used in high dynamic range image due to it’s hard to estimate reference white color.

So it is only workable in LDR images.

APPLICATION

•Eq2 model get little un-natured color, but the goal is the best reference instead of best look.

•Eq3 often results in a hue shift for red colors.

EXTEND TO TRO•[DD02] proposed a bilateral filtering tone operator

function. This operator uniformly reduces the contrast of the base layer( low pass + edges) while preserving details.

•To calculate the contrast factor c, can be approximated by the slope of the tone curve on the log-log plot. which given by the derivative of the tone-curve in the logarithmic space.

EXTEND TO TRO(CONT.)

[KLLH11] HUE PRESERVATION USING ENHANCED INTEGRATED

MULTI-SCALE RETINEX FOR IMPROVED COLOR CORRECTION

[KLLH11] HUE PRESERVATION USING ENHANCED INTEGRATED MULTI-SCALE RETINEX FOR IMPROVED

COLOR CORRECTION

• Aimed on fixing the hue shift issue when extreme lightening a picture.

• Apply the TRO on CIELAB Space.

• Enhance the downgraded highlight due to normalization.

• Use gamut mapping and gamut extension to solve out of gamut and desaturation.

BACKGROUND PROBLEM

• To reveal the information which comes from the dark image.

• TRO method can preserve the contrast than TRC.

An example of lightening but de-contrasting image by r correction (TRC).

RETINEX BASED METHODRELATED WORK

• Retinex Basic Operator

Output Image Pixel

Input Image Pixel

L of Input Image Pixel

L of surround pixels

RETINEX BASED METHODRELATED WORK (CONT.)

• Single Scale (SSR) v.s. Multi-Scale (MSR), the convolution methods.

SSRMSRThe distance scale c is scalable

RETINEX BASED METHODRELATED WORK (CONT.)

• The MSR Problem, Chroma Shift and Artifact

a. original image, b. background, c. standard MSR under linear space.

IMSR (INTEGRATED)RELATED WORK (CONT.)

• It calculates luminance images by different scale of convolution filters.

• Merge all of the luminance result can get a Integrated Surround Image, SR.

• The SR image can be used as weight mask to adjust the light.

• Better chroma preserving. No artifact.

user param13年7月31⽇日星期三

IMSR ISSUES

1. Lightness2. The dark region desaturated

PROPOSED METHOD

PROPOSED METHODSTEP 1. COLOR SPACE

• Transfer Color Space to CIELAB and replace function to Luminance.

(3’)

(2’)

(1’)

PROPOSED METHODSTEP 2. LIGHTNESS CORRECTION

• The original problem is due to Normalization. To fix it, use a cumulative distribution function, cdf to instead it.

Gamut mapping Cdf correction

PROPOSED METHODSTEP 2. LIGHTNESS CORRECTION - GAMUT (CONT.)

PROPOSED METHODSTEP 3. SIMPLE HUE EXTENSION FOR SATURATION COMPENSATION

EXP RESULT

RESULT

RESULT(CONT.)

q1. hue shiftq2. nature look

OUR ISSUES

TONE IN LIGHTNESS

• The key reason that the level of highlight tone cannot be further enhance might due to normalization or familiar operation.

• cdf or the curve slope is a possible solution.

ARTIFACT OF GRADIENT MASK• The mask now will skip hue

correction if the mask fits maximum.

• It’s kind of a TRO operation so that the artifact is almost inevitable.

• A basic solution is apply hue correction to whole image. Or should we separate the procedures with different effect ?

TONE CURVE HUE CORRECTION

TONE CURVE HUE CORRECTION• In extremely tone curve mapping, nature look is based on

human’s perception, It’s hard to provide a general solution.

• Consider the curve slope, find an equation model to

1. do tone correction if slope <= k

2. adjust luminance if slope > k, due to the bigger slope means user applied an extremely adjust, and it might can be adapted to nature light.

3. TRO with gaussian filter might be a solution to smooth img.

VIGNETTE PERFORMANCE TUNING

• In last version, Vignette Performance was stocked on SSE tuning due to color correction makes effect quality bad.

• could cdf help to solve the problem?

DOES LR USE SAME ALGORITHM BOTH IN TRC AND TRO ?

• In the other product, the color correction is a black box we need to investigate.

• The effect in LR not only includes TRC but also TRO. It’s hard to infer they are all adapted to the same color correction strategy.

• But how to know ?

THANKS FOR YOUR ATTENTIONAlfred

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