 # Help with Image Convolution Convolution

Does anyone know how to do a convolution of a image using a simple 3x3 matrix? Basically all I want to do is filter my input image with a 3x3 mask:

M = 1/9 *
[indent] [ 1 1 1 ]
| 1 1 1 |
[ 1 1 1 ] [/indent]

I tried to follow the separable convolution example in the SDK but its a bit over my head. Also tried hacking it to make it work for my image which a 640*480 unsigned char array but with no luck (all i got was a black image).

If anyone can help or has a naive convolution implementation example please let me know.

Thanks.

you need a basic compute that looks like this:

``````for (int x=1; x < width-1; x++) {

for (int y=1, y < height-1; y++) {

int acc = 0;

for (int cx=0; cx < 2; cx++) {

for (int cy=0;cy < 2; cy++) {

acc += inpic[x][y]*matrix[cx][cy]

}

}

outpic[x][y]=acc/normalise;

}

}
``````

This is very inefficient though as it computes the array offsets for each pixel, much better to manipulate the pointers and unroll

the inner loop. Also watch out for overflow blowing up the maths, make sure that intermediate computations use more bits than the input and output data.

This is probably why you’re getting black picture.

Not too hard, here’s a 5 X 5 sample set up with a cuda friendly interface, but not yet set up to run on the GPU:

it takes RGB 8 bit pic and generates a 1 byte or RGB grayscale transformed image. The inner loop is already unrolled

for speed, so the code is tedious, but fast.

``````void convolve5x(const guint8 *inpic, guint8 *outpic, const gint16 *mat,

const int width, const int height, const bool sout) {

const guint8 *r1, *r2, *r3, *r4, *r5;

const int loopwidth = width-4;

guint8 *outr;

int norm = 0;

int acc, acc1, acc2, acc3, acc4, acc5;

int outwidth = 3;

if (sout) outwidth = 1;

printf("outwidth is %i in convolver\n",outwidth);

for (int i = 0; i <25; i++) {

norm += mat[i]*3;

}

if (norm == 0) norm = 1;

int maxval = -32000, minval = 32000;

for (int y = 0; y < height-4; y++) {

r1 = &inpic[y*width*3];

r2 = r1+width*3;

r3 = r2+width*3;

r4 = r3+width*3;

r5 = r4+width*3;

outr = &outpic[y*(width-4)*outwidth];

for (int x = 0; x < loopwidth; x++) {

acc1 =  ((int)r1[ 0]+(int)r1[ 1]+(int)r1[ 2])*(int)mat[ 0] + ((int)r1[ 3]+(int)r1[ 4]+(int)r1[ 5])*(int)mat[ 1]  + ((int)r1[ 6]+(int)r1[ 7]+(int)r1[ 8])*(int)mat[ 2]

+ ((int)r1[ 9]+(int)r1+(int)r1)*(int)mat[ 3] + ((int)r1+(int)r1+(int)r1)*(int)mat[ 4];

acc2 =  ((int)r2[ 0]+(int)r2[ 1]+(int)r2[ 2])*(int)mat[ 5] + ((int)r2[ 3]+(int)r2[ 4]+(int)r2[ 5])*(int)mat[ 6] + ((int)r2[ 6]+(int)r2[ 7]+(int)r2[ 8])*(int)mat[ 7]

+ ((int)r2[ 9]+(int)r2+(int)r2)*(int)mat[ 8] + ((int)r2+(int)r2+(int)r2)*(int)mat[ 9];

acc3  = ((int)r3[ 0]+(int)r3[ 1]+(int)r3[ 2])*(int)mat + ((int)r3[ 3]+(int)r3[ 4]+(int)r3[ 5])*(int)mat + ((int)r3[ 6]+(int)r3[ 7]+(int)r3[ 8])*(int)mat

+ ((int)r3[ 9]+(int)r3+(int)r3)*(int)mat + ((int)r3+(int)r3+(int)r3)*(int)mat;

acc4  = ((int)r4[ 0]+(int)r4[ 1]+(int)r4[ 2])*(int)mat + ((int)r4[ 3]+(int)r4[ 4]+(int)r4[ 5])*(int)mat + ((int)r4[ 6]+(int)r4[ 7]+(int)r4[ 8])*(int)mat

+ ((int)r4[ 9]+(int)r4+(int)r4)*(int)mat + ((int)r4+(int)r4+(int)r4)*(int)mat;

acc5  = ((int)r5[ 0]+(int)r5[ 1]+(int)r5[ 2])*(int)mat + ((int)r5[ 3]+(int)r5[ 4]+(int)r5[ 5])*(int)mat + ((int)r5[ 6]+(int)r5[ 7]+(int)r5[ 8])*(int)mat

+ ((int)r5[ 9]+(int)r5+(int)r5)*(int)mat + ((int)r5+(int)r5+(int)r5)*(int)mat;

acc = (acc1+acc2+acc3+acc4+acc5)/norm;

if (sout) {

outr = acc;

outr++;

} else {

outr = outr = outr = acc / 3;

outr += 3;

}

if (acc < minval) minval = acc;

if (acc > maxval) maxval = acc;

r1 += 3;

r2 += 3;

r3 += 3;

r4 += 3;

r5 += 3;

}

}

printf("min found: %i, max found: %i, norm is %i\n",minval,maxval,norm);

}
``````