PLY/ RGB to 2D Grayscale Image

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Wookie on 7 Dec 2020
Answered: Image Analyst on 29 Dec 2020
I have a PLY from a stereo vision sensor (1024 x 1280). The PLY gives:
- Location (X,Y,Z) [1310442 x 3]
- Color (R, G, B) [1310442 x 3] and
- XLimits, YLimits, ZLimits.
I am trying to convert the Color into a grayscale image/matrix. My attempt has been the following:
Image = ptCloud.Color(:,:,1);
R = double(Image(:,1));
G = double(Image(:,2));
B = double(Image(:,3));
I = 0.2989 * R + 0.5870 * G + 0.1140 * B; % For grayscale
% Since the matrix size does not equal 1024 x 1280, I add zeros to the end of I to compensate
Image_2d = reshape (I, 1024, 1280)
The image on the left is by plotting the PLY with MatLab's pcshow feature. This is what I want to obtain as an image/matrix but instead by following the above code... I get the image on the right which does not look correct!

Answers (2)

Rohit Pappu
Rohit Pappu on 29 Dec 2020
Based on the above code,
  • Image stores the Color Matrix of just the Red channel
  • R, G, B refer to the values of first, second and third column of Image matrix respectively
A plausible workaround could be
Image = ptCloud.Color; %% Store the entire m-by-n-by-3 color matrix in Image
%% Extract the Red , Green and Blue Channels
R = double(Image(:,:,1));
G = double(Image(:,:,2));
B = double(Image(:,:,3));
%% the rest of the code is same
  1 Comment
Image Analyst
Image Analyst on 29 Dec 2020
[R, G, B] = imsplit(Image);
grayImage = rgb2gray(Image);

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Image Analyst
Image Analyst on 29 Dec 2020
What does this show?
whos Image
Assuming Image is an RGb image and you want grayscale, you can do
grayImage = rgb2gray(Image);
I'd recommend agains using Image as the name of your image since image is the name of a built-in function.
From the pseudocolored image on the right, it looks like you're using imagesc() which applies a colormap by default that is usually not wanted or helpful. I never use that. I use imshow(). So just use imshow() with no colormap.
If you need more help, attach ptCloud in a .mat file:
save('answers.mat', 'ptCloud');




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