RM0020 datasheet(38/69 Pages) STMICROELECTRONICS

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Function API

RM0020

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6.18

sobel3x3 - sobel filter

Function call:

void sobel3x3(unsigned char *a, unsigned char *b, unsigned short M,

unsigned short N);

Arguments:

Description:

Computes the sobel filter on MxN input image. The function adds together absolute

values of horizontal and vertical sobel filters and then limits output to 0..255. The

output image is of the same size as input image and has one pixel border that is not

computed. The first and the last row of output image is not changed, the first column

is zeroed and the last column will contain not meaningful results. The last column can

be zeroed when ZERO_LASTCOL is defined to 1 for the penalty of one more

instruction inside the inner loop. The ZERO_LASTCOL is a symbol defined in function

source file.

Algorithm:

Compute horizontal sobel filter, i.e. compute 2-D convolution with the following kernel:

Compute vertical sobel filter, i.e. compute 2-D convolution with the following kernel:

Add absolute values of horizontal and vertical sobel filters

Range limit to 0..255

Note: The input/output images must be properly aligned in memory as described in

Table 18.

Performance: See Section 7 and Table 49.

Example 18. sobel3x3

#include "libdsp2.h"

/* #pragma and ALIGN macros used to align to 4 bytes */

#ifdef __ghs__

#pragma alignvar (4)

Table 18.

sobel3x3 arguments

a(1)

a and b must be word aligned.

input image of size MxN

b(1)

out

output image of size MxN

number of rows of a and b, must be >= 3

number of columns of a and b, must be multiple of 4, must be >= 8

–

000

120

– 01

– 02

– 01

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