Assign dimensions for my image. I have an image and I need to give it a length that I know.

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The image attached is the one I have. In real life, the image has a specific length from top to bottom and it is 102. I just want to assign this value to the image length from top to bottom and thus the dimensions of the image would represent the real life dimensions.

Answers (2)

Image Analyst
Image Analyst on 3 Jan 2022
Run through the attached demo on spatial calibration. After that you'll see how you can draw a known distance in the image, give a known real world number for it, and have it calculate a calibration factor in millimeters (or whatever) per pixel. Then you can multiply all pixel distances by mmPerPixel to get distances in mm, and by mmPerPixel^2 to get areas in square mm from area in pixels.
Or if you know the full field of view and don't want to draw any distance interactively you can do
[rows, columns, numberOfColorChannels] = size(yourImage)
fovMm = 5; % Whatever width your FOV is in real world units.
mmPerPixel = fovMm / columns;
Normally people have square pixels, so you can use that factor for both directions. But if not, you can compute a calibration factor for the horizontal and vertical directions separately.
[rows, columns, numberOfColorChannels] = size(yourImage)
fovMmX = 5; % Whatever width your FOV is in real world units.
mmPerPixelX = fovMmX / columns;
fovMmY = 6; % Whatever height your FOV is in real world units.
mmPerPixelY = fovMmY / rows;

Turlough Hughes
Turlough Hughes on 3 Jan 2022
Edited: Turlough Hughes on 3 Jan 2022
The imref2d object is useful for exactly this. Let's have a look at an example:
% Example of a Binary image
BW = imbinarize(imread('coins.png'));
% For the example, let's assume we know the height/width in mm:
heightMM = 82;
widthMM = 100;
% Create an imref2d object, to do this we need to input the
% the size of the image, as well as the height and width of
% a pixel in millimeters.
R = imref2d(size(BW),heightMM/size(BW,1),widthMM/size(BW,2));
% Plot the results
xlabel('x - coordinate / [mm]')
ylabel('y - coordinate / [mm]')
Now, the dimensions of the image are represented in the world frame of reference. You can also analyse the image with datatips, and you'll see that X and Y are in terms of the real world coordinates. I suggest taking some time to understand the different coordinate systems here. You can also easily convert between intrinsic and world coordinates, let's take the following example:
% I'm using the coins' centroids as an example of data that we might want
% to convert
props = regionprops(BW,'centroid');
centroids = vertcat(props.Centroid);
xc = centroids(:,1); % these are intrinsic coordinates (pixel indices)
yc = centroids(:,2);
% Regular imshow
hf = figure; subplot(1,2,1)
hf.Position(3) = 2*hf.Position(3);
imshow(BW), title('Intrinsic Coordinates')
axis on
xlabel('x - coordinate / [px]')
ylabel('y - coordinate / [px]')
hold on, plot(xc, yc, 'r+', 'LineWidth', 2)
% imshow using a world frame of reference
imshow(BW, R), title('World Coordinates')
axis on
xlabel('x - coordinate / [mm]')
ylabel('y - coordinate / [mm]')
% The conversion is just the following:
[xc_mm, yc_mm] = intrinsicToWorld(R, xc, yc);
% And we can show the converted centroid coordinates on the image
hold on, plot(xc_mm, yc_mm, 'r+', 'LineWidth', 2)

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