sobel_amp

sobel_amp ( Image : EdgeAmplitude : FilterType, Size : )

Detect edges (amplitude) using the Sobel operator.

sobel_amp calculates first derivative of an image and is used as an edge detector. The filter is based on the following filter masks:

  A =
           1       2       1
           0       0       0
          -1      -2      -1

  B =
           1       0      -1
           2       0      -2
           1       0      -1

These masks are used differently, according to the selected filter type. (In the following, a und b denote the results of convolving an image with A und B for one particular pixel.)

     'sum_sqrt'              sqrt(a^2 + b^2)
     'sum_abs'               (|a| + |b|) / 2
     'thin_sum_abs'          (thin(|a|) + thin(|b|)) / 2
     'thin_max_abs'          max(thin(|a|),thin(|b|))
     'x'                     b
     'y'                     a

Here, thin(x) is equal to x for a vertical maximum (mask A) and a horizontal maximum (mask B), respectively, and 0 otherwise. Thus, for 'thin_sum_abs' and 'thin_max_abs' the gradient image is thinned. For the filter types 'x' and 'y' if the input image is of type byte the output image is of type int1, of type int2 otherwise. For a Sobel operator with size 3x3, the corresponding filters A and B are applied directly, while for larger filter sizes (Size = 5,7,9 and 11) the input image is first smoothed using a Gaussian filter of size Size-2. Therefore,

     sobel_amp(I:E:FilterType,S)

for Size > 3 is equivalent to

     gauss_image(I:G:S-2) >
     sobel_amp(G:E:FilterType,3).

Parameters

`Image` (input_object)	(multichannel-)image(-array) `->` object : byte / int2 / uint2
Input image.

`EdgeAmplitude` (output_object)	(multichannel-)image(-array) `->` object : int1 / int2 / uint2
Edge amplitude (gradient magnitude) image.

`FilterType` (input_control)	string `->` string
Filter type.
Default value: 'sum_abs'
List of values: 'sum_abs', 'thin_sum_abs', 'thin_max_abs', 'sum_sqrt', 'x', 'y'

`Size` (input_control)	integer `->` integer
Size of filter mask.
Default value: 3
List of values: 3, 5, 7, 9, 11, 13

Example

read_image(Image,'fabrik') 
sobel_amp(Image,Amp,'sum_abs',3) 
threshold(Amp,Edg,128,255).

Result

sobel_amp returns 2 (H_MSG_TRUE) if all parameters are correct. If the input is empty the behaviour can be set via set_system('no_object_result',<Result>). If necessary, an exception handling is raised.

Parallelization Information

sobel_amp is reentrant and automatically parallelized (on tuple level, channel level, domain level).

Possible Predecessors

gauss_image, mean_image, anisotrope_diff, sigma_image

Possible Successors

threshold, nonmax_suppression_amp, gray_skeleton

Alternatives

frei_amp, roberts, kirsch_amp, prewitt_amp, robinson_amp

Module

Image filters