Lab → RGB 변환기
이 Lab → RGB 변환기는 회화 작품 촬영시 절대색 기준 기반 보정 (absolute color correction) 접근을 위한 Lab 소수점 반영 계산을 위해 만들 었습니다.
Lab → RGB 변환기 (ProPhoto / AdobeRGB / sRGB)
스펙트로포토미터 등으로 측정한 Lab 색상값을 RGB로 변환하는 계산기입니다.
포토샵, ACR 등에서 컬러체커 패치의 실제 측정값과 현상된 이미지의 RGB 일치 여부를 확인할 때 유용합니다.
사용 방법
L, a, b 값 입력 - 측정한 Lab 값을 그대로 입력하세요. (소수점 가능)
화이트포인트 선택 - 일반적으로 Lab은 D50 기준입니다. D65 측정이면 D65로 변경하세요.
- Custom (K+Tint) : 측정된 색온도와 틴트값을 입력해 더 정확한 계산을 할 수 있습니다.
Target 색공간 선택 - ProPhoto, AdobeRGB, sRGB 중 하나를 선택합니다.
Chromatic adapt - 일반적으로 Auto 선택. (D50 ↔ D65 간 크로매틱 어댑테이션(Bradford 변환)을 자동 적용할 수 있습니다.)
결과 확인
각 색공간의 R/G/B 값이 표시됩니다.
이 값을 기준으로 포토샵 또는 ACR에서 색을 교정하면 됩니다.
주의
ACR이 ProPhoto RGB일 경우 ProPhoto 결과값을 참고하세요.
8bit 작업 시 밴딩이 생길 수 있으므로 16bit로 현상하는 것을 권장합니다.
모니터가 색역을 충분히 표현하려면 하드웨어 캘리브레이션이 필요합니다.
sotheb
TheChef
1
Hey, awesome tool! I'm totally digging the custom K/Tint input—that's a game-changer for the real-world spectrophotometer crowd. But a couple of geeky questions on the color-science plumbing you've got running here
CAT's Out of the Bag, When I hit 'Auto' for Chromatic Adaptation, and I've got my wonky custom K/Tint light source firing away, what secret sauce CAT are you pouring on that white point conversion? Is it the classic Bradford, something newer like CIECAT02, or are we rocking the simpler von Kries? Just gotta know how my Custom K/Tint-D50 (for ProPhoto) conversion is flexing its muscles.
The 0.005 Tint Voodoo: That little 0.005 factor you're multiplying the Tint fraction by in tempToXY—that feels super empirical. Is that factor a neat approximation for a CC filter unit, or are we talking a calculated Mired or Duv shift that's just been highly simplified? Basically, how much can I trust that tiny number when my light meter yells +3/8 Mg at me? 🤔
CAT's Out of the Bag, When I hit 'Auto' for Chromatic Adaptation, and I've got my wonky custom K/Tint light source firing away, what secret sauce CAT are you pouring on that white point conversion? Is it the classic Bradford, something newer like CIECAT02, or are we rocking the simpler von Kries? Just gotta know how my Custom K/Tint-D50 (for ProPhoto) conversion is flexing its muscles.
The 0.005 Tint Voodoo: That little 0.005 factor you're multiplying the Tint fraction by in tempToXY—that feels super empirical. Is that factor a neat approximation for a CC filter unit, or are we talking a calculated Mired or Duv shift that's just been highly simplified? Basically, how much can I trust that tiny number when my light meter yells +3/8 Mg at me? 🤔
1일 전 14:55
sotheb 글쓴이
TheChef
Thank you for your interest in my writing and source code.
Here is the explanation regarding the 0.005 tint correction factor.
The 0.005 factor is an empirical approximation used to simulate the effect of the 1/8 tint unit (Green/Magenta shift) in the CIExyY color space, specifically by adjusting the y-coordinate (chromaticity).
What the Tint Input Represents.
The input value (ex: −1/8 Gr, +3/8 Mg) is based on the system used by many digital color meters (like Sekonic or Minolta color meters). This 1/8 unit closely correlates to a standard CC05 color compensating filter unit.
1/8 unit ≈ CC 05 filter value.
1 unit (ex : 1 Gr) ≈ CC 40 filter value.
for 0.005 Factor, The core challenge is translating the meter's tint unit, which is perceived linearly, into a shift on the CIE chromaticity diagram (xy or u’v’).
In the provided JavaScript code, the logic is.
Parse the input: The input string (-1/8 Gr) is converted into a fraction (−0.125).
Tint Factor=Fractional Tint Value×0.005
(−0.125)×0.005=−0.000625
Apply the shift: The resulting value is added directly to the calculated y chromaticity coordinate.
New y=Initial y+Tint Factor
This 0.005 value serves as a coarse conversion constant derived from correlating typical CC 05 filter effects to observed y coordinate shifts on the CIE diagram for general photographic color temperatures.
Comparison to Duv. This method is not based on a strict scientific Duv calculation, which measures the deviation from the Planckian Locus (Blackbody Locus) in the CIE u’v’ uniform color space. A true Duv calculation would be computationally intensive and require solving a minimization problem to find the nearest point on the Locus.
so my code uses the y shift as a simplified, practical workaround to push the calculated white point (from Kelvin) off the Blackbody Locus along the Green/Magenta axis, achieving an effect similar to what a Duv or CC filter value represents.
In summary, the 0.005 factor is an empirically determined constant to approximately map one 1/8 CC filter unit (the meter's Tint) to an equivalent shift in the y chromaticity coordinate. For precise, scientific color management, a dedicated Duv-based calculation is required, but this approximation offers reasonable accuracy for a web-based conversion tool.
Here is the explanation regarding the 0.005 tint correction factor.
The 0.005 factor is an empirical approximation used to simulate the effect of the 1/8 tint unit (Green/Magenta shift) in the CIExyY color space, specifically by adjusting the y-coordinate (chromaticity).
What the Tint Input Represents.
The input value (ex: −1/8 Gr, +3/8 Mg) is based on the system used by many digital color meters (like Sekonic or Minolta color meters). This 1/8 unit closely correlates to a standard CC05 color compensating filter unit.
1/8 unit ≈ CC 05 filter value.
1 unit (ex : 1 Gr) ≈ CC 40 filter value.
for 0.005 Factor, The core challenge is translating the meter's tint unit, which is perceived linearly, into a shift on the CIE chromaticity diagram (xy or u’v’).
In the provided JavaScript code, the logic is.
Parse the input: The input string (-1/8 Gr) is converted into a fraction (−0.125).
Tint Factor=Fractional Tint Value×0.005
(−0.125)×0.005=−0.000625
Apply the shift: The resulting value is added directly to the calculated y chromaticity coordinate.
New y=Initial y+Tint Factor
This 0.005 value serves as a coarse conversion constant derived from correlating typical CC 05 filter effects to observed y coordinate shifts on the CIE diagram for general photographic color temperatures.
Comparison to Duv. This method is not based on a strict scientific Duv calculation, which measures the deviation from the Planckian Locus (Blackbody Locus) in the CIE u’v’ uniform color space. A true Duv calculation would be computationally intensive and require solving a minimization problem to find the nearest point on the Locus.
so my code uses the y shift as a simplified, practical workaround to push the calculated white point (from Kelvin) off the Blackbody Locus along the Green/Magenta axis, achieving an effect similar to what a Duv or CC filter value represents.
In summary, the 0.005 factor is an empirically determined constant to approximately map one 1/8 CC filter unit (the meter's Tint) to an equivalent shift in the y chromaticity coordinate. For precise, scientific color management, a dedicated Duv-based calculation is required, but this approximation offers reasonable accuracy for a web-based conversion tool.
20시간 전 19:59
Kyle Davel
1
Look, I've got my Custom K/Tint WP locked in, and the calculator spits out those ProPhoto RGB values 241.13, 242.28, 233.07, awesome. I get the whole Lab to ProPhoto step, since Lab is device-independent and ProPhoto is the go-to large working space for my Canon RAW files.
But what, exactly, am I supposed to do with the converted RGB values next?
Am I supposed to take those numbers and manually punch them into the RGB fields of a file already tagged with the ProPhoto profile? Or is the idea that I should be using those values to build a custom ICC profile that acts as the source for my color-managed workflow? I'm trying to make sure I bridge the gap from my initial spectrophotometer measurement to the actual image file correctly, especially when I eventually down-convert to srgb for web viewing. Help me connect the dots here s this a validation tool or a creation tool?
But what, exactly, am I supposed to do with the converted RGB values next?
Am I supposed to take those numbers and manually punch them into the RGB fields of a file already tagged with the ProPhoto profile? Or is the idea that I should be using those values to build a custom ICC profile that acts as the source for my color-managed workflow? I'm trying to make sure I bridge the gap from my initial spectrophotometer measurement to the actual image file correctly, especially when I eventually down-convert to srgb for web viewing. Help me connect the dots here s this a validation tool or a creation tool?
22시간 전 18:10
sotheb 글쓴이
Kyle Davel
The RGB values obtained from the calculator are not intended for manual data entry, but rather serve as a "target value" to verify and correct the color accuracy of the ACR development result.
ACR Development Goal - White Balance Setting
The primary goal of the Lab→ProPhoto RGB conversion is to adjust ACR's White Balance so that the brightest white/gray areas under the measured light source appear color-neutral within the ProPhoto RGB space.
Custom White Balance Entry (Key Steps)
The measured K and Tint values become the direct reference for setting White Balance in ACR.
Prepare Light Source Information - Prepare the Color Temp (K) value (3250) and the Tint value (−1/8 Gr) that you entered into the Lab calculator.
Set ACR White Balance - Manually change the White Balance setting in ACR.
Input Kelvin - Enter the measured Kelvin value (3250) into the Temperature slider.
Input Tint - Input the measured Tint value into the Tint slider.
The ACR Tint slider is Duv based and typically uses integer units between +20 and −20. It does not directly correspond to the light meter's 1/8 Gr/Mg unit. The most accurate method is using ACR's eyedropper tool on a neutral gray patch.
RGB Value Verification and Correction (Validation)
The ProPhoto RGB values obtained from the calculator (241.13,242.28,233.07) are used as the target figures for color correction in ACR.
Select Sample Point - Select the white or gray point in the image (a neutral patch on a ColorChecker) that was the basis for your Lab measurement.
Check RGB Values - Check the RGB values of that specific point using the ACR histogram or information panel.
Compare Target Values - Compare the RGB values displayed in ACR with the ProPhoto RGB output values from your calculator. The goal is for the R, G, and B channel values in ACR to be very close to the calculator's output (241.13,242.28,233.07).
Manual Correction - If ACR RGB values deviate from the calculator figures, subtly adjust ACR Tint slider until the G and B values are as close as possible to the calculator's output.
Context on Workflow
While programs exist to measure the full ColorChecker patch set under the measured Kelvin+Tint conditions to create a corrective DCP profile, that level of full workflow automation is not the goal for my immediate needs. The purpose of this calculator, for my current environment, is to obtain accurate Lab data for color correction (chromaticity) and to manually adjust White Balance in ACR, using the DCP profile primarily for the overall camera color correction matrix.
I hope my explanation was conveyed well.
ACR Development Goal - White Balance Setting
The primary goal of the Lab→ProPhoto RGB conversion is to adjust ACR's White Balance so that the brightest white/gray areas under the measured light source appear color-neutral within the ProPhoto RGB space.
Custom White Balance Entry (Key Steps)
The measured K and Tint values become the direct reference for setting White Balance in ACR.
Prepare Light Source Information - Prepare the Color Temp (K) value (3250) and the Tint value (−1/8 Gr) that you entered into the Lab calculator.
Set ACR White Balance - Manually change the White Balance setting in ACR.
Input Kelvin - Enter the measured Kelvin value (3250) into the Temperature slider.
Input Tint - Input the measured Tint value into the Tint slider.
The ACR Tint slider is Duv based and typically uses integer units between +20 and −20. It does not directly correspond to the light meter's 1/8 Gr/Mg unit. The most accurate method is using ACR's eyedropper tool on a neutral gray patch.
RGB Value Verification and Correction (Validation)
The ProPhoto RGB values obtained from the calculator (241.13,242.28,233.07) are used as the target figures for color correction in ACR.
Select Sample Point - Select the white or gray point in the image (a neutral patch on a ColorChecker) that was the basis for your Lab measurement.
Check RGB Values - Check the RGB values of that specific point using the ACR histogram or information panel.
Compare Target Values - Compare the RGB values displayed in ACR with the ProPhoto RGB output values from your calculator. The goal is for the R, G, and B channel values in ACR to be very close to the calculator's output (241.13,242.28,233.07).
Manual Correction - If ACR RGB values deviate from the calculator figures, subtly adjust ACR Tint slider until the G and B values are as close as possible to the calculator's output.
Context on Workflow
While programs exist to measure the full ColorChecker patch set under the measured Kelvin+Tint conditions to create a corrective DCP profile, that level of full workflow automation is not the goal for my immediate needs. The purpose of this calculator, for my current environment, is to obtain accurate Lab data for color correction (chromaticity) and to manually adjust White Balance in ACR, using the DCP profile primarily for the overall camera color correction matrix.
I hope my explanation was conveyed well.
19시간 전 20:34
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