![]() To find tutorials using the "decompose, drag, and drop" procedure, enter the following terms into a search engine box: GIMP convert to black and white lab.Ĭomparing Figures 2 and 3, it's obvious that the commonly recommended procedure for using GIMP 2.8 to convert to black and white using the LAB Lightness channel doesn't actually produce an RGB rendition of the LAB Lightness channel. Drag the "L" channel back to the original color image layer stack.Do "Colors/Components/Decompose" and choose Color model "LAB", making sure "Decompose to layers" is checked and "Foreground as registration color" is not checked.So decomposing images in other RGB working spaces (eg AdobeRGB1998) to LAB will always give wrong results, regardless of what procedure you follow. GIMP 2.8's code for converting from RGB to LAB uses hard-coded sRGB red, green, and blue primaries. Open the image and assign or convert the image to GIMP's built-in sRGB working space.The commonly recommended "decompose, drag, and drop" produces incorrect results This is what you get when using GIMP 2.8, using the commonly recommended "decompose, drag, and drop" procedure: If you want to follow along and verify results shown below (highly recommended - you'll learn a lot more by doing than by just reading), here's a downloadable 16-bit png version of the color photograph in Figure 1, licenced as CC-BY-SA 3.0 unported. Now let's see what you actually get when following various tutorials on the internet for converting to black and white using the LAB "L" channel. But when contemplating the interactions between math and aesthetics, it helps to start with the right math! The complex relationship between "mathematically correct" and "aesthetically pleasing" is far outside the scope of this article. This is what you should get when you convert the color image in Figure 1 to black and white by extracting the LAB "Lightness" ("L") channel.īy "should" get I mean this is a mathematically correct RGB rendition of the "Lightness" channel of a correctly done conversion from RGB to LAB. The scene was set up to capture some nicely saturated red, green, and blue color gradients: This is a photograph of diffuse light shining through variously colored glass objects.įigure 2 shows what you should get when you convert the color image in Figure 1 to black and white by extracting the LAB "Lightness" ("L") channel: It does mean that GIMP and PhotoShop users are being misled (however unintentionally) about the actual nature of the LAB Lightness channel.įigure 1 below shows a color photograph of variously colored glass objects. ![]() This doesn't mean the resulting black and white image might not be useful or pleasing. But the ones I've read - whether written for PhotoShop or GIMP - are based on mathematical and "color management" mistakes. Somewhere on the internet there might be an article or tutorial that shows how to make a mathematically correct conversion from the LAB "L" channel to black and white. This article explains mathematically correct and incorrect ways to use GIMP 2.8 to convert to black and white using GIMP's "decompose to LAB". But it would be nice if tutorials didn't mislead users about the nature of the LAB Lightness channel. Summary and conclusion: "Mathematically incorrect" is a statement about the underlying math, not an aesthetic judgement about the result.When using GIMP 2.8, the mathematically correct procedure produces severe shadow posterization.The mathematically correct procedure requires linearized RGB.The commonly recommended "decompose, drag, and drop" produces incorrect results.Introduction: A sample color image and a mathematically correct conversion to black and white using the LAB Lightness channel.
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