sobel_dir

Herror ::sobel_dir ( Hobject Image, Hobject *EdgeAmplitude, Hobject *EdgeDirection, const HTuple &FilterType, const HTuple &Size ) HImage HImage::SobelDir ( HImage *EdgeDirection, const HTuple &FilterType, const HTuple &Size ) const HImageArray HImageArray::SobelDir ( HImageArray *EdgeDirection, const HTuple &FilterType, const HTuple &Size ) const

Detect edges (amplitude and direction) using the Sobel operator.

::sobel_dir 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 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_dir(I:Amp,Dir:FilterType,S)

for Size > 3 is equivalent to

     gauss_image(I:G:S-2) >
     sobel_dir(G:Amp,Dir:FilterType,3).

The edge directions are returned in EdgeDirection, and are stored in 2-degree steps, i.e., an edge direction of x degrees with respect to the horizontal axis is stored as x / 2 in the edge direction image. Furthermore, the direction of the change of intensity is taken into account. Let [Ex,Ey] denote the image gradient. Then the following edge directions are returned as r/2:

intensity increase                     Ex / Ey        edge direction r

from bottom to top                      0 / +         0
from lower right to upper left          + / -         ]0,90[
from right to left                      + / 0         90 
from upper right to lower left          + / +         ]90,180[
from top to bottom                      0 / +         180 
from upper left to lower right          - / +         ]180,270[
from left to right                      + / 0         270
from lower left to upper right          - / -         ]270,360[.

Points with edge amplitude 0 are assigned the edge direction 255 (undefined direction).

Parameters

`Image` (input_object)	(multichannel-)image(-array) `->` Hobject: HImage(Array) ( byte / int2 )
Input image.

`EdgeAmplitude` (output_object)	(multichannel-)image(-array) `->` Hobject : HImage(Array)* ( byte / int2 )
Edge amplitude (gradient magnitude) image.

`EdgeDirection` (output_object)	multichannel-image(-array) `->` Hobject : HImage(Array)* ( direction )
Edge direction image.

`FilterType` (input_control)	string `->` HTuple.char *
Filter type.
Default value: 'sum_abs'
List of values: 'sum_abs', 'sum_sqrt'

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

Example

read_image(&Image,"fabrik");
sobel_dir(Image,&Amp,&Dir,"sum_abs",3);
threshold(Amp,&Edg,128.0,255.0);

Result

::sobel_dir returns 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.

Possible Predecessors

::gauss_image, ::mean_image, ::anisotrope_diff, ::sigma_image

Possible Successors

::nonmax_suppression_dir, ::hysteresis_threshold, ::threshold

Alternatives

::edges_image, ::frei_dir, ::kirsch_dir, ::prewitt_dir, ::robinson_dir

Module

Image filters