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imwarp

Apply geometric transformation to image

Syntax

  • B = imwarp(A,tform)
    example
  • B = imwarp(A,D)
  • [B,RB] = imwarp(A,RA,tform)
  • B = imwarp(___,Interp)
  • [B,RB] = imwarp(___,Name,Value)

Description

example

B = imwarp(A,tform) transforms the image A according to the geometric transformation defined by tform, which is a geometric transformation object. B is the transformed image.

B = imwarp(A,D) transforms the input image A according to the displacement field defined by D.

[B,RB] = imwarp(A,RA,tform) transforms the spatially referenced image, specified by the image data A and the associated spatial referencing object RA. The output is a spatially referenced image specified by the image data B and the associated spatial referencing object RB.

B = imwarp(___,Interp) specifies the form of interpolation to use

[B,RB] = imwarp(___,Name,Value) specifies parameters that control various aspects of the geometric transformation. Parameter names can be abbreviated, and case does not matter.

Code Generation support: Yes.

MATLAB Function Block support: Yes.

Examples

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Read grayscale image into workspace and display it.

I = imread('cameraman.tif');
imshow(I)

Create a 2-D geometric transformation object.

tform = affine2d([1 0 0; .5 1 0; 0 0 1])
tform = 

  affine2d with properties:

                 T: [3×3 double]
    Dimensionality: 2

Apply the transformation to the image.

J = imwarp(I,tform);
figure
imshow(J)

Read MRI data into the workspace and visualize it.

s = load('mri');
mriVolume = squeeze(s.D);
sizeIn = size(mriVolume);
hFigOriginal = figure;
hAxOriginal  = axes;
slice(double(mriVolume),sizeIn(2)/2,sizeIn(1)/2,sizeIn(3)/2);
grid on, shading interp, colormap gray

Create a 3-D geometric transformation object. First create a transformation matrix that rotates the image around the Y axis. Then pass the matrix to the affine3d function.

 theta = pi/8;
 t = [cos(theta)  0      -sin(theta)   0
     0             1              0     0
     sin(theta)    0       cos(theta)   0
     0             0              0     1]

tform = affine3d(t)
t =

    0.9239         0   -0.3827         0
         0    1.0000         0         0
    0.3827         0    0.9239         0
         0         0         0    1.0000


tform = 

  affine3d with properties:

                 T: [4×4 double]
    Dimensionality: 3

Apply the transformation to the image.

mriVolumeRotated = imwarp(mriVolume,tform);

Visualize three slice planes through the center of the transformed volumes.

sizeOut = size(mriVolumeRotated);
hFigRotated = figure;
hAxRotated  = axes;
slice(double(mriVolumeRotated),sizeOut(2)/2,sizeOut(1)/2,sizeOut(3)/2);
grid on, shading interp, colormap gray

Link views of both axes together.

linkprop([hAxOriginal,hAxRotated],'View');

Set view to see effect of rotation.

set(hAxRotated,'View',[-3.5 20.0])

Input Arguments

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Image to be transformed, specified as a nonsparse, real-valued array of any numeric class or logical.

2-D or 3-D geometric transformation to perform, specified as a geometric transformation object.

  • If tform is 2-D and ndims(A) > 2, such as for an RGB image, imwarp applies the same 2-D transformation to all 2-D planes along the higher dimensions.

  • If tform is 3-D, A must be a 3-D image volume.

Displacement field, specified as nonsparse numeric matrix.

When A is 2-D, D is an m-by-n-by-2 numeric array. The first plane of the displacement field, D(:,:,1) describes the X component of additive displacement that is added to column and row locations in D to produce remapped locations in A. Similarly, D(:,:,2) describes the Y component of additive displacement values.

When A is 3-D, D is an m-by-n-by-p-by-3 numeric array. In the 3-D case, D(:,:,:,1) contains displacements along the x-axis, D(:,:,:,2) contains displacements along the y-axis, and D(:,:,:,3) contains displacements along the z-axis. The unit of displacement values in D is pixels. When A is m-by-n-by-p and D is m-by-n-by-2, imwarp applies the displacement field to one plane at a time. imwarp assumes that D is referenced to the default intrinsic coordinate system.

Spatial referencing information associated with the image to be transformed, specified as a spatial referencing object.

  • If tform is a 2-D geometric transformation, RA must be a 2-D spatial referencing object (imref2d).

  • If tform is a 3-D geometric transformation, RA must be a 3-D spatial referencing object (imref3d).

Form of interpolation used, specified as one of the following values:

Interpolation MethodDescription
'linear'Linear interpolation
'nearest'Nearest-neighbor interpolation—the output pixel is assigned the value of the pixel that the point falls within. No other pixels are considered.
'cubic'Cubic interpolation

Data Types: char

Name-Value Pair Arguments

Specify optional comma-separated pairs of Name,Value arguments. Name is the argument name and Value is the corresponding value. Name must appear inside single quotes (' '). You can specify several name and value pair arguments in any order as Name1,Value1,...,NameN,ValueN.

Example: J = imwarp(I,tform,'FillValues',255) uses white pixels as fill values.

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Size and location of output image in world coordinate system, specified as the comma-separated pair consisting of 'OutputView' and an imref2d or imref3d spatial referencing object. The ImageSize, XWorldLimits, and YWorldLimits properties of the specified spatial referencing object define the size of the output image and the location of the output image in the world coordinate system. The use of 'OutputView' is not available when applying displacement fields.

Value used for output pixels outside the input image boundaries, specified as the comma-separated pair consisting of 'FillValues' and a numeric array. Fill values are used for output pixels when the corresponding inverse transformed location in the input image is completely outside the input image boundaries.

  • If the input image is 2-D, FillValues must be a scalar.

  • If the input image is 3-D and the geometric transformation is 3-D, FillValues must be a scalar.

  • If the input image is N-D and the geometric transformation is 2-D, FillValues may be either scalar or an array whose size matches dimensions 3 to N of the input image.

    For example, if the input image is a uint8 RGB image that is 200-by-200-by-3, FillValues can be a scalar or a 3-by-1 array. In this RGB image example, possibilities for FillValues include:

    FillValueEffect
    0Fill with black
    [0;0;0]Fill with black
    255Fill with white
    [255;255;255]Fill with white
    [0;0;255]Fill with blue
    [255;255;0]Fill with yellow

  • If the input image is 4-D with size 200-by-200-by-3-by-10, FillValues can be a scalar or a 3-by-10 array.

Pad image to create smooth edges, specified as the logical value true or false. When set to true, imwarp pads the input image (with values specified by FillValues) to create a smoother edge in the output image. When set to false, imwarp does not pad the image. Not padding the input image results in a sharper edge in the output image, which can be useful to minimize seam distortions when registering two images side by side.

Output Arguments

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Transformed image, returned as a nonsparse real-valued array of any numeric class or logical.

Spatial referencing information associated with the transformed image, returned as a spatial referencing object.

More About

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Code Generation

This function supports the generation of C code using MATLAB® Coder™. Note that if you choose the generic MATLAB Host Computer target platform, the function generates code that uses a precompiled, platform-specific shared library. Use of a shared library preserves performance optimizations but limits the target platforms for which code can be generated. For more information, see Understanding Code Generation with Image Processing Toolbox.

When generating code, note:

  • The geometric transformation object input, tform, must be either an affine2d or projective2d object.

  • The interpolation method and optional parameter names must be constants.

MATLAB Function Block

You can use this function in the MATLAB Function Block in Simulink.

The TFORM input argument must be a compile-time constant.

Introduced in R2013a

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