How do I know the length of line?

I want to draw a line on the picture and know its length

Answer 1

[Image Analyst]

See examples in the help for imdistline

help imdistline

 IMDISTLINE Draggable Distance tool.
    H = IMDISTLINE creates a draggable distance tool on the current
    axes. The function returns H, a handle to an imdistline object.
 
    H = IMDISTLINE(HPARENT) creates a draggable distance tool on the object
    specified by HPARENT. HPARENT specifies the HG parent of the imdistline
    graphics, which is typically an axes but can also be any other object
    that can be the parent of an hggroup.
 
    H = IMDISTLINE(...,X,Y) creates a draggable distance tool with endpoints
    located at the locations specified by the vectors X and Y.  X and Y specify
    the initial endpoint positions of the draggable distance tool in the form
    X = [X1 X2], Y =[Y1 Y2].
 
    The draggable distance tool has a context menu associated with it that
    allows you to:
 
        Export endpoint and distance data to the workspace
        Toggle the distance label on/off
        Set the line color
        Specify horizontal and vertical drag constraints
        Delete the distance tool object
 
    Remarks 
    ------- 
    If you use IMDISTLINE with an axis that contains an image
    object, and do not specify a position constraint function, users can drag the
    line outside the extent of the image and lose the line.  When used with an
    axis created by the PLOT function, the axis limits automatically expand to
    accommodate the movement of the line.
 
    To understand how IMDISTLINE calculates the angle returned by
    getAngleToHorizontal, draw an imaginary horizontal vector from the bottom
    endpoint of the distance line, extending to the right.  The value returned
    by getAngleToHorizontal is the angle from this horizontal vector to the
    distance line, which can range from 0 to 180 degrees.
 
    Example 1 
    --------- 
    % Insert a distance tool into an image. Use makeConstrainToRectFcn to specify 
    % a position constraint function that prevents distance tool from being dragged 
    % outside the extent of the image.  Explore the context menu options of the 
    % distance tool by right clicking on the distance tool.
     
    figure, imshow('pout.tif');
    h = imdistline(gca);
    fcn = makeConstrainToRectFcn('imline',get(gca,'XLim'),get(gca,'YLim'));
    setPositionConstraintFcn(h,fcn);
         
    Example 2
    ---------
    % Use distance tool with XData and YData of associated image in non-pixel 
    % units. 
         
    start_row = 1;
    end_row = 1350;
    meters_per_pixel = 1;
    start_col = 731;
    end_col = 2955; 
    I  = imread('concordorthophoto.png');
    I1 = I(start_row:meters_per_pixel:end_row, start_col:meters_per_pixel:end_col);
    figure; 
    imshow(I1);
    title('1 meter per pixel');
     
    % Specify initial position of distance tool on image.
    h1 = imdistline(gca,[4 2218],[40 781]);
    setLabelTextFormatter(h1,'%02.0f meters');
  
    % Repeat process but work with a 2 meter per pixel sampled image. Verify
    % that the same distance is obtained.
    meters_per_pixel = 2;
    I  = imread('concordorthophoto.png');
    I1 = I(start_row:meters_per_pixel:end_row, start_col:meters_per_pixel:end_col);
    figure;    
    hImg = imshow(I1);
    title('2 meters per pixel');    
     
    % Convert XData and YData to meters using conversion factor.
    XDataInMeters = get(hImg,'XData')*meters_per_pixel; 
    YDataInMeters = get(hImg,'YData')*meters_per_pixel;
     
    % Set XData and YData of image to reflect desired units.    
    set(hImg,'XData',XDataInMeters,'YData',YDataInMeters);    
    set(gca,'XLim',XDataInMeters,'YLim',YDataInMeters);
     
    % Specify initial position of distance tool on image.
    h2 = imdistline(gca,[4 2218],[40 781]);
    setLabelTextFormatter(h2,'%02.0f meters');   
       
    See also makeConstrainToRectFcn.

    Documentation for imdistline
       doc imdistline

[Kantachai Chamnarnkit]

If I wanted to know the height and width of the figure without drawing a line, what are the ways?

[Image Analyst]

To get the pixel dimensions, see Steve's blog:

Too much information about the size function | Steve on Image Processing

to get real world distances you need to spatially calibrate to some object of known length in the image. See my attached demo and incorporate it into your code.

To automatically get the caliper dimensions, you can use bwferet

help bwferet

 BWFERET Measure Feret diameters and angles of image regions.
 
   OUT = BWFERET(I) measures the maximum Feret Properties of each
   component (object) in the image. I can be a binary image, connected 
   component or labeled matrix.
 
   OUT = BWFERET(BW, PROPERTIES) measures a set of Feret properties of each
   connected component (object) in the binary image BW, which is a logical
   array.
 
   OUT = BWFERET(CC, PROPERTIES) measures a set of Feret properties of each
   connected component (object) in CC, which is a struct returned by
   BWCONNCOMP.
 
   OUT = BWFERET(L, PROPERTIES) measures a set of Feret properties of
   each labeled component (object) in the label matrix L. Positive integer
   elements of L correspond to different regions. For example, the set of
   elements of L equal to 1 corresponds to region 1; the set of elements of
   L equal to 2 corresponds to region 2; and so on.
 
   [OUT, L] = BWFERET(...) measures a set of Feret properties of each
   component (object) in the binary image, connected component or labeled
   matrix.  This also returns the corresponding label matrix L such that
   the first value in the table OUT corresponds to the labeled region
   1, the second value with the labeled region 2 and so on.
 
   PROPERTIES can be an array of strings, a single character vector, a cell
   array of character vectors, or 'all'. If PROPERTIES is set to 'all' it
   returns all the Feret properties mentioned below. If no argument is
   provided, all maximum Feret properties are given as outputs. The set of
   valid measurement strings or character vectors includes
   'MaxFeretProperties', 'MinFeretProperties' and 'all'.
 
   'MaxFeretProperties' - Outputs all the properties related to maximum 
                          Feret Diameter.
       These properties are:
       MaxDiameter    - Maximum Feret diameter length.
       MaxAngle       - Angle of maximum Feret diameter with respect to X
                        axis in degrees. The value lies between 180 to -180
                        degrees.
       MaxCoordinates - Endpoint coordinates of maximum Feret diameter.
 
   'MinFeretProperties' - Outputs all the properties related to minimum 
                          Feret Diameter.
       These properties are:
       MinDiameter    - Minimum Feret diameter length.
       MinAngle       - Angle of minimum Feret diameter with respect to X
                        axis in degrees. The value lies between 180 to -180
                        degrees.
       MinCoordinates - Endpoint coordinates of minimum Feret diameter.
 
   Class Support
   -------------
   If the first input is BW, BW must be a logical array and it should be 2D.
   If the first input is CC, CC must be a structure returned by BWCONNCOMP.
   If the first input is L, L must be real, nonsparse and 2D. L can have 
   any numeric class. The output OUT is returned as a table.
 
   Example 1
   ---------
   % Calculate the minimum Feret diameter for objects in image
   
    I = imread('toyobjects.png');
    bw = imbinarize(I, 'adaptive');
    % Retain the two biggest objects in the image
    bw = bwareafilt(bw, 2);
    bw = imfill(bw, 'holes');
    % Calculate the Feret properties of the objects in the image along with
    % the label matrix
    [out, L] = bwferet(bw, 'MinFeretProperties');
 
   Example 2
   ---------
   % Plot the maximum Feret diameter for objects in the image
   
    % Read an image
    I = imread('toyobjects.png');
    % Binarize the image
    B = imbinarize(I, 'adaptive');
    % Fill in the holes in the binary image
    B = imfill(B, 'holes');
    % Show the image
    h = imshow(B)
    ax = h.Parent;
    % Convert to connected component struct using bwconncomp
    C = bwconncomp(B);
    % Calculate Feret Properties
    F = bwferet(C, 'MaxFeretProperties');
    hold on 
    % Display maximum Feret Diameters with their values for each object
    imdistline(ax, F.MaxCoordinates{1}(:,1), F.MaxCoordinates{1}(:,2)); 
    imdistline(ax, F.MaxCoordinates{2}(:,1), F.MaxCoordinates{2}(:,2)); 
    imdistline(ax, F.MaxCoordinates{3}(:,1), F.MaxCoordinates{3}(:,2)); 
    imdistline(ax, F.MaxCoordinates{4}(:,1), F.MaxCoordinates{4}(:,2)); 
   
   See also BWCONNCOMP, BWLABEL, BWLABELN, LABELMATRIX, REGIONPROPS.

    Documentation for bwferet
       doc bwferet