ISSUES WITH RGB to LAB . . .
2022-03-10-180708
All --
I am working with a customer that needs to convert from the RGB COLOR SPACE to the LAB COLOR SPACE.
There are two tools within IP10 that I am looking at to assist.
One is are the REGION MEASUREMENTS of LAB COLOR L, LAB COLOR a, and LAB COLOR b.
Another is
MEASURE RIBBON +
COLOR SECTION +
COLOR MANAGEMENT +
COLOR MEASUREMENT
When I use a TEST IMAGE like the one shown here
where the CIRCLES are PURE RED, PURE GREEN, and PURE BLUE (PURE being 255) I get different values for L, a, and b when I use the REGION MEASUREMENTS and when I extract images for L, a, and b shown here.
The REGION MEASUREMENTS numbers are pretty close to the Lab numbers generated by two websites that I found but the extracted images are way different. Here are the REGIONS.
This is shown in the EXCEL SPREADSHEET shown below.
The REGION MEASUREMENTS are pretty close to the WEBSITE CONVERSIONS from the RGB VALUES but the IMAGES extracted using the TOOL are much different.
The IP10 HELP FILE regarding the EXTRACT for the L, a, and b does not suggest a reason for this difference.
Will someone here explain this difference please?
A copy of my TEST IMAGE with the IP10 REGIONS stored in it is attached in the ZIP FILE.
Thanks.
-- Matt
0
Best Answer
-
Hi Matt,
If you convert your color image to "Color FPoint" type and then extract L*a*b* channels, the values will be unscaled.
Yuri0
Answers
Extracted L*a*b* images are converted to the bit depth of the original image, so if your original image is RGB24, the result is converted to Gray8 scaling the intensity values. Every L*a*b* component has different range (https://en.wikipedia.org/wiki/CIELAB_color_space), which is then scaled to 0..255. So, if you are interested in non-scaled L*a*b* values, it's better to use L*a*b* measurements.
Note that the L*a*b* values also depend on illuminant, so if you place measurement regions and check L*a*b* values in the measurement table, the values will change selecting different illuminant (e.g. D50, D65). So, to get proper L*a*b* measurements you must know which illuminant was used when images were acquired.
Yuri
Thanks.
Click the Dara Range button on the Adjust tab, display group, and then Reset Range. It will set image range from the data, histogram will also be adjusted.
Yuri