RM0020 datasheet(36/69 Pages) STMICROELECTRONICS

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Function API

RM0020

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6.17

conv3x3 - 2-D convolution with 3x3 kernel

Function call:

void conv3x3(unsigned char *a, unsigned char *b, signed char *c,

unsigned short M, unsigned short N, unsigned short

shift);

Arguments:

Description:

Computes the 2-D convolution on MxN input image using a 3x3 convolution kernel.

The computed convolution value is shifted by shift bits to the right and then range

limited 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:

a - input matrix, c - 3x3 convolution kernel, b - output matrix

Compute 2-D convolution, i.e. for each i = 1,2 ... M-2, j = 1,2 ... N-2 compute bij:

Equation 34

Table 17.

conv3x3 arguments

a(1)

a and b must be word aligned.

input image of size MxN

b(1)

out

output image of size MxN

c(2)

c must be half word aligned and there must be 1 byte of readable memory behind c.

3x3 convolution kernel (mask)

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

shift

number of bits result is shifted down



–



–



–





–

0 b M1

–

1 b M1

–

2  bM1N

–



–



–



–





–

0 a M1

–

1 a M1

–

2  aM1N

–

 jp 1

–

 c 2k

–

 2p

–







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