worldToSubscript

Read a 2-D grayscale image of a knee into the workspace.

m = dicominfo('knee1.dcm');
A = dicomread(m);

Create an imref2d object, specifying the size and the resolution of the pixels. The DICOM file contains a metadata field PixelSpacing that specifies the image resolution in each dimension in millimeters per pixel.

RA = imref2d(size(A),m.PixelSpacing(2),m.PixelSpacing(1))

RA = 
  imref2d with properties:

           XWorldLimits: [0.1562 160.1562]
           YWorldLimits: [0.1562 160.1562]
              ImageSize: [512 512]
    PixelExtentInWorldX: 0.3125
    PixelExtentInWorldY: 0.3125
    ImageExtentInWorldX: 160
    ImageExtentInWorldY: 160
       XIntrinsicLimits: [0.5000 512.5000]
       YIntrinsicLimits: [0.5000 512.5000]

Display the image, including the spatial referencing object. The axes coordinates reflect the world coordinates. Notice that the coordinate (0,0) is in the upper left corner.

figure
imshow(A,RA,'DisplayRange',[0 512])

Select sample points, and store their world x- and y- coordinates in vectors. For example, the first point has world coordinates (38.44,68.75), the second point is 1 mm to the right of it, and the third point is 7 mm below it. The last point is outside the image boundary.

xW = [38.44 39.44 38.44 -0.2];
yW = [68.75 68.75 75.75 1];

Convert the world coordinates to row and column subscripts using worldToSubscript.

[rS, cS] = worldToSubscript(RA,xW,yW)

rS = 1×4

   220   220   242   NaN

cS = 1×4

   123   126   123   NaN

The resulting vectors contain the row and column indices that are closest to the point. Note that the indices are discrete, and that points outside the image boundary have NaN for both row and column indices.

Also, the order of the input and output coordinates is reversed. The world x-coordinate vector, xW, corresponds to the second output vector, cS. The world y-coordinate vector, yW, corresponds to the first output vector, rS.

Read a 3-D volume into the workspace. This image consists of 27 frames of 128-by-128 pixel images.

load mri;
D = squeeze(D);
D = ind2gray(D,map);

Create an imref3d spatial referencing object associated with the volume. For illustrative purposes, provide a pixel resolution in each dimension. The resolution is in millimeters per pixel.

R = imref3d(size(D),2,2,4)

R = 
  imref3d with properties:

           XWorldLimits: [1 257]
           YWorldLimits: [1 257]
           ZWorldLimits: [2 110]
              ImageSize: [128 128 27]
    PixelExtentInWorldX: 2
    PixelExtentInWorldY: 2
    PixelExtentInWorldZ: 4
    ImageExtentInWorldX: 256
    ImageExtentInWorldY: 256
    ImageExtentInWorldZ: 108
       XIntrinsicLimits: [0.5000 128.5000]
       YIntrinsicLimits: [0.5000 128.5000]
       ZIntrinsicLimits: [0.5000 27.5000]

Select sample points, and store their world x-, y-, and z-coordinates in vectors. For example, the first point has world coordinates (108,92,52), the second point is 3 mm above it in the +z-direction, and the third point is 5.2 mm to the right of it in the +x-direction. The last point is outside the image boundary.

xW = [108 108 113.2 2];
yW = [92 92 92 -1];
zW = [52 55 52 0.33];

Convert the world coordinates to row, column, and plane subscripts using worldToSubscript.

[rS, cS, pS] = worldToSubscript(R,xW,yW,zW)

rS = 1×4

    46    46    46   NaN

cS = 1×4

    54    54    57   NaN

pS = 1×4

    13    14    13   NaN

The resulting vectors contain the column, row, and plane indices that are closest to the point. Note that the indices are discrete, and that points outside the image boundary have index values of NaN.

Also, the order of the input and output coordinates is reversed. The world x-coordinate vector, xW, corresponds to the second output vector, cS. The world y-coordinate vector, yW, corresponds to the first output vector, rS.