20

Sounds a bit iffy to me... A card, by itself, DOESN'T have a fixed L*a*b* colour - that's a product of the reflectivity of the card at different wavelengths and the intensity and wavelength distribution of the illumination. In the dark, L* will be 0. Light it with a coloured light and your a*b* will change. Light it with a "white" light with a different ...


12

This frequently comes up in photographic reproduction jobs where one is trying to closely approximate some other object such as a painting antique drawing. This cannot be done with typical photographs even when adjusting them to a specific, matching, LAB color. Regular photographs increase color saturation and tailor contrast, boosting the midranges and ...


3

Let's say I want to change this image, "Still, by Sara Dorweiler via Unsplash" to make the leaf blue. Let's start with an info point. So we have -12, 20. For blue it should be something fairly close to 0 and then a negative value. A simple curve can handle that. A Channel curve: B Channel curve: Which you can see brought the Info Point to 0, -60. You ...


3

Alright well AdobeRGB vs Lab in this case isn't really the benefit or concern so you're kind of asking the wrong question. There's really two questions: Do the channels available in Lab make up for the greater number of tools available in RGB? Is it better to do things like Dodging and Burning on an RGB composite or on a Luminosity Channel? The first ...


3

I don't think that it has anything to do with using the CIE-Lab colour space, and you should only use absolute colorimetric rendering intent if you are doing colour proofing. Relative (or perceptual) intent is nearly always a better choice, and I wonder why you want to use absolute colorimetric, or the CIE-Lab colour space, for that matter. Fraser, Murphy ...


3

No. CIELAB images are just as subject to different displays as RGB images. As you note CIELAB is converted to RGB in order to be displayed since displays do not use CIELAB. However, if displays are calibrated to the same white point and profiled then they will display images consistently so long as the images are within the display's gamut. And CIELAB is ...


2

Photoshop's histogram provides means(averages) for Lightness, a, and b. They are not normalized but shown in the LAB PCS space used by ICC profiles which go from 0 to 255 in unsigned, 8 bit values. To convert these to real Lab* values, divide the lightness reading by 2.55 to get the actual L*. Subtract 128 from the a and b values to get the true, CIE a* ...


2

It's not exactly using the a*/b* CIELAB channels/values; even though that is what it looks like. When you set the temp towards the blue side of the slider you are telling the program that the light was warmer (lower kelvin) and it needs to add blue light to the scene... and light is additive color (RGB). However, anything in the scene that is showing a color ...


1

Programs such as Photoshop allow you to change to a variety of color spaces like sRGB, Colormatch RGB, Adobe RGB, and ProPhoto RGB (listed in order from smaller to widest color gamut). Does the image editor you use have this functionality? You don't mention what software you're using. BTW, it's best to convert to sRGB when you package images for customers ...


1

No. It is not possible. The sRGB color gamut can’t be made wider. Making it wider makes it something else, Adobe RGB for example. The reason to use a Lab based workflow is to reduce the effects of cumulative rounding errors and facilitate tools tuned to make adjustments based on human perception. In typical implementation, cumulative rounding error is ...


1

Adobe actually describes extremely well their white balancing process in the Digital Negative (DNG) Specification, specifically in Chapter 6 Mapping Camera Color Space to CIE XYZ Space. The process is not super trivial but the core principle is to compute a matrix converting from Camera Space, i.e. space the captured image is upon capture, to CIE XYZ ...


1

This is a more tricky question that you might think. First, the RGB value that you see with your eyes on the screen are the result of an operation on the processed output from Lightroom, that is defined by the calibration of you screen. So beware what RGB values you measure. Second, when you are adjusting color temperature, what you are really doing is ...


1

If you look at my related answer here: https://photo.stackexchange.com/a/122262/20809, the White Balance process of Adobe Photography products, i.e. Adobe Lightroom and Camera Raw, is outlined. As for automatic White Balance, and given an image, you can, for example, compute the average chromaticity coordinates in a centre area of the image and use that as ...


1

What you describe, editing saturation curves in RGB mode, may not be possible. However, it is possible to enhance a few colors in an image with the curves tool. The way I think of saturation, which may be incomplete or even just plain wrong, is decreased saturation is more gray, while increased saturation is less gray. In RGB, the gray tones have an even ...


1

Lab* = (100,0,20) does not make sense, as L*=100 is maximum intensity, and thus it is white. If it were not white (has some color cast), you could additively add another complementary color, and then L* would be >100, which is not possible per definition. My guess is that the Lab reading cited is the result of a wrong calibration. Lab* is a calibrated ...


1

Assuming you properly calibrated and profiled the monitor to D65 with the computer's GPU set to output to a D65 monitor it should. Photoshop doesn't really take your monitor profile into account. But it doesn't have to because Photoshop isn't what displays your image: Your GPU and monitor do that. Photoshop outputs the image to your GPU in much the same way ...


1

Whenever an illuminant is not specified, assume it's D50 as that is the standard the ICC adapts its profiles to even if the device illuminant differs. For instance a Lab reading in a Photoshop image is relative to D50 whether the image is sRGB or ProPhoto. Scanner profile reference data sets in Lab use D50 even though the scanner's light is rarely close to ...


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