what do I get by calling 'PixelIdxList'?

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Ziming Zhong 2017-11-4

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评论： DGM 2022-12-4

Hi! I recently using 'PixelIdxList' in the 'regionprop' function to calculate the color of the particle I labeled. But I don't really understand what color that 'PixelIdxList' gives?

I mean, I extract the 'PixelIdxList' from a RGB image, and calculate the mean value of pixels. But as we know, RGB image has red, green and blue in its array. When I calculate the mean(PixelIdxList) in an RGB image, I only get one value. So what colour does this mean value present?

My goal is to generate the mean grey value from the RGB image. Can I use this mean value from PixelIdxList?

Thank you!

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Walter Roberson 2018-5-28

Approximate translation:

Good evening please how I can convert the pixels of stats (i) .area to image in matlab for example I have the result as follows cc # 1 - area 202965 nuemoro of pixel and size I like the poster in a binary image

Image Analyst 2018-5-28

marwa, you can't convert an area (a single scalar), or a list of pixel values in an irregularly-shaped blob, into an image - you need more than that. See my Image Segmentation Tutorial to see how you can crop out the regions from the image into sub-images using the bounding box from regionprops(), and imcrop(). Start your own question with your own image if you need more help.

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Walter Roberson 2017-11-4

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PixelIDxList does not give any image value. PixelIDxList gives array locations.

R = original_image(:,:,1);
G = original_image(:,:,2);
B = original_image(:,:,3);
blobinfo = regionprops(BWorLabeledImage, 'PixelIdxList');
one_idxlist = blobinfo(1).PixelIdxList;
R_in_region = R(one_idxlist);
G_in_region = G(one_idxlist);
B_in_region = B(one_idxlist);
mean_R_in_region = mean(R_in_region);
mean_G_in_region = mean(G_in_region);
mean_B_in_region = mean(B_in_region);

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Walter Roberson 2017-11-9

The color image that you are reading in does not contain measures of the amount of melanin.

Cameras take time-integrated electron charge readings of sets of sensors that are tuned to be most efficient at different wavelengths, and process those readings through an internal lighting model to come up with red, green, and blue readings for each pixel. The lighting model that is used has the property that the luminance (brightness) can be calculated by a linear combination of the R, G, and B values. rgb2gray does that calculation, calculating luminance. It is not measuring grey in the original image: it is calculating brightness of the image.

You are taking some color measure as a proxy for the amount of melanin. The validity of using color as a proxy depends a lot on the spectra qualities of the illumination of the sample, and upon how those frequencies are absorbed differently by the melanin and non-melanin portions of the sample. For example if the illumination contains significant 785 nm then the melanin is going to fluoresce, giving it a heightened brightness -- but areas can also be bright without containing melanin, and the lack of brightness for a spot could mean the lack of 785 nm illumination rather than a lack of melanin.