Intermediate result using edge(canny).

Question

mangat rai 2012-9-24

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https://ww2.mathworks.cn/matlabcentral/answers/48969-intermediate-result-using-edge-canny

评论： Engr. Omit Debnath 2020-6-18

Matlab Version : 7.8.0(R2009a)

I am getting edges from an image by using Canny edge detector using standard 'edge' function. But for my project I need to get intermediate Gradient Magnitude matrix. I.e. gradient magnitude values for each pixel.

I know we could do it manually, But I don't know the implementation used by Matlab for Canny. I need the exact values computed by edge function. Is there any way to do it or do I have to implement canny from scratch?

Background: I am basically changing Gray scale values for some pixels on the edge detected by canny. I need to know that after the change will they still come under Threshold values

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mangat rai 2012-9-27

在 MATLAB Online 中打开

By the way found the answer. We can access and edit Edge function by following command in work space.

edit edge

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Answer 1

Kapil Nagwanshi 2012-9-27

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在 MATLAB Online 中打开

if you tpe the help it will show following I think it is much better

EDGE Find edges in intensity image. EDGE takes an intensity or a binary image I as its input, and returns a binary image BW of the same size as I, with 1's where the function finds edges in I and 0's elsewhere.

    EDGE supports six different edge-finding methods:
       The Sobel method finds edges using the Sobel approximation to the
       derivative. It returns edges at those points where the gradient of
       I is maximum.
       The Prewitt method finds edges using the Prewitt approximation to
       the derivative. It returns edges at those points where the gradient
       of I is maximum.
       The Roberts method finds edges using the Roberts approximation to
       the derivative. It returns edges at those points where the gradient
       of I is maximum.
       The Laplacian of Gaussian method finds edges by looking for zero
       crossings after filtering I with a Laplacian of Gaussian filter.
       The zero-cross method finds edges by looking for zero crossings
       after filtering I with a filter you specify.
       The Canny method finds edges by looking for local maxima of the
       gradient of I. The gradient is calculated using the derivative of a
       Gaussian filter. The method uses two thresholds, to detect strong
       and weak edges, and includes the weak edges in the output only if
       they are connected to strong edges. This method is therefore less
       likely than the others to be "fooled" by noise, and more likely to
       detect true weak edges.
    The parameters you can supply differ depending on the method you
    specify. If you do not specify a method, EDGE uses the Sobel method.
    Sobel Method
    ------------
    BW = EDGE(I,'sobel') specifies the Sobel method.
    BW = EDGE(I,'sobel',THRESH) specifies the sensitivity threshold for 
    the Sobel method. EDGE ignores all edges that are not stronger than 
    THRESH.  If you do not specify THRESH, or if THRESH is empty ([]), 
    EDGE chooses the value automatically.
    BW = EDGE(I,'sobel',THRESH,DIRECTION) specifies directionality for the
    Sobel method. DIRECTION is a string specifying whether to look for
    'horizontal' or 'vertical' edges, or 'both' (the default).
    BW = EDGE(I,'sobel',...,OPTIONS) provides an optional string
    input. String 'nothinning' speeds up the operation of the algorithm by
    skipping the additional edge thinning stage. By default, or when 
    'thinning' string is specified, the algorithm applies edge thinning.
    [BW,thresh] = EDGE(I,'sobel',...) returns the threshold value.
    [BW,thresh,gv,gh] = EDGE(I,'sobel',...) returns vertical and
    horizontal edge responses to Sobel gradient operators. You can 
    also use these expressions to obtain gradient responses:
    if ~(isa(I,'double') || isa(I,'single')); I = im2single(I); end
    gh = imfilter(I,fspecial('sobel') /8,'replicate'); and
    gv = imfilter(I,fspecial('sobel')'/8,'replicate');
    Prewitt Method
    --------------
    BW = EDGE(I,'prewitt') specifies the Prewitt method.
    BW = EDGE(I,'prewitt',THRESH) specifies the sensitivity threshold for
    the Prewitt method. EDGE ignores all edges that are not stronger than
    THRESH. If you do not specify THRESH, or if THRESH is empty ([]),
    EDGE chooses the value automatically.
    BW = EDGE(I,'prewitt',THRESH,DIRECTION) specifies directionality for
    the Prewitt method. DIRECTION is a string specifying whether to look
    for 'horizontal' or 'vertical' edges, or 'both' (the default).
    BW = EDGE(I,'prewitt',...,OPTIONS) provides an optional string
    input. String 'nothinning' speeds up the operation of the algorithm by
    skipping the additional edge thinning stage. By default, or when 
    'thinning' string is specified, the algorithm applies edge thinning.
    [BW,thresh] = EDGE(I,'prewitt',...) returns the threshold value.
    [BW,thresh,gv,gh] = EDGE(I,'prewitt',...) returns vertical and
    horizontal edge responses to Prewitt gradient operators. You can 
    also use these expressions to obtain gradient responses:
    if ~(isa(I,'double') || isa(I,'single')); I = im2single(I); end
    gh = imfilter(I,fspecial('prewitt') /6,'replicate'); and
    gv = imfilter(I,fspecial('prewitt')'/6,'replicate');
    Roberts Method
    --------------
    BW = EDGE(I,'roberts') specifies the Roberts method.
    BW = EDGE(I,'roberts',THRESH) specifies the sensitivity threshold for
    the Roberts method. EDGE ignores all edges that are not stronger than
    THRESH. If you do not specify THRESH, or if THRESH is empty ([]),
    EDGE chooses the value automatically.
    BW = EDGE(I,'roberts',...,OPTIONS) provides an optional string
    input. String 'nothinning' speeds up the operation of the algorithm by
    skipping the additional edge thinning stage. By default, or when 
    'thinning' string is specified, the algorithm applies edge thinning.
    [BW,thresh] = EDGE(I,'roberts',...) returns the threshold value.
    [BW,thresh,g45,g135] = EDGE(I,'roberts',...) returns 45 degree and
    135 degree edge responses to Roberts gradient operators. You can 
    also use these expressions to obtain gradient responses:
    if ~(isa(I,'double') || isa(I,'single')); I = im2single(I); end
    g45  = imfilter(I,[1 0; 0 -1]/2,'replicate'); and
    g135 = imfilter(I,[0 1;-1  0]/2,'replicate');
    Laplacian of Gaussian Method
    ----------------------------
    BW = EDGE(I,'log') specifies the Laplacian of Gaussian method.
    BW = EDGE(I,'log',THRESH) specifies the sensitivity threshold for the
    Laplacian of Gaussian method. EDGE ignores all edges that are not
    stronger than THRESH. If you do not specify THRESH, or if THRESH is 
    empty ([]), EDGE chooses the value automatically.
    BW = EDGE(I,'log',THRESH,SIGMA) specifies the Laplacian of Gaussian
    method, using SIGMA as the standard deviation of the LoG filter. The
    default SIGMA is 2; the size of the filter is N-by-N, where
    N=CEIL(SIGMA*3)*2+1. 
    [BW,thresh] = EDGE(I,'log',...) returns the threshold value.
    Zero-cross Method
    -----------------
    BW = EDGE(I,'zerocross',THRESH,H) specifies the zero-cross method,
    using the specified filter H. If THRESH is empty ([]), EDGE chooses 
    the sensitivity threshold automatically.
    [BW,THRESH] = EDGE(I,'zerocross',...) returns the threshold value.
    Canny Method
    ----------------------------
    BW = EDGE(I,'canny') specifies the Canny method.
    BW = EDGE(I,'canny',THRESH) specifies sensitivity thresholds for the
    Canny method. THRESH is a two-element vector in which the first element
    is the low threshold, and the second element is the high threshold. If
    you specify a scalar for THRESH, this value is used for the high
    threshold and 0.4*THRESH is used for the low threshold. If you do not
    specify THRESH, or if THRESH is empty ([]), EDGE chooses low and high 
    values automatically.
    BW = EDGE(I,'canny',THRESH,SIGMA) specifies the Canny method, using
    SIGMA as the standard deviation of the Gaussian filter. The default
    SIGMA is 1; the size of the filter is chosen automatically, based
    on SIGMA. 
    [BW,thresh] = EDGE(I,'canny',...) returns the threshold values as a
    two-element vector.
    Class Support
    -------------
    I is a nonsparse numeric array. BW is of class logical.
    Remarks
    -------
    For the 'log' and 'zerocross' methods, if you specify a
    threshold of 0, the output image has closed contours, because
    it includes all of the zero crossings in the input image.

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Engr. Omit Debnath 2020-6-18

Good Job

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Answer 2

Kapil Nagwanshi 2012-9-27

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/48969-intermediate-result-using-edge-canny#answer_60142

在 MATLAB Online 中打开

and finally you can use the following example

    Find the edges of the circuit.tif image using the Prewitt and Canny
    methods:
        I = imread('circuit.tif');
        BW1 = edge(I,'prewitt');
        BW2 = edge(I,'canny');
        figure, imshow(BW1)
        figure, imshow(BW2)