Sobel Edge Detection without a buffer - c++

Question

For an embedded design I am attempting to implement sobel's edge detection on a board without the use of a buffer. i.e. I am reading and writing directly from the screen. I can however, store about one or two imge width full of data to be referenced later. This is due to limitations set forth by the board. However I have fallen into some issue. All that I recieve is noise regardless if I attempt to do sobel or another edge detection algorithm. The code is below, does anyone have any suggestions
Version 1
void sobelEdgeDetection2() {
int GX[3][3];
int GY[3][3];
int sumX[3];
int sumY[3];
int SUM[3];
int piX = 0;
int piY = 0;
//uint8_t R, G, B = 0;
int I, J = 0;
//UnpackedColour pixVal;
uint16_t *buffer;
// allocate space for even scan lines and odd scan lines
buffer = new uint16_t[_gl->getWidth()];
//buffer for previous line
uint16_t *bufT;
// allocate space for even scan lines and odd scan lines
bufT = new uint16_t[_gl->getWidth()];
// Masks //////////////////////////////////////
//X//
GX[0][0] = -1;
GX[0][1] = 0;
GX[0][2] = 1;
GX[1][0] = -2;
GX[1][1] = 0;
GX[1][2] = 2;
GX[2][0] = -1;
GX[2][1] = 0;
GX[2][2] = 1;
//Y//
GY[0][0] = 1;
GY[0][1] = 2;
GY[0][2] = 1;
GY[1][0] = 0;
GY[1][1] = 0;
GY[1][2] = 0;
GY[2][0] = -1;
GY[2][1] = -2;
GY[2][2] = -1;
for (int Y = 0; Y < _gl->getHeight(); Y++) {
for (int X = 0; X < _gl->getWidth(); X++) {
sumX[0] = sumX[1] = sumX[2] = 0;
sumY[0] = sumY[1] = sumY[2] = 0;
if (Y == 0 || Y == _gl->getHeight() - 1) {
SUM[0] = SUM[1] = SUM[2] = 0;
} else if (X == 0 || X == _gl->getWidth() - 1) {
SUM[0] = SUM[1] = SUM[2] = 0;
} else {
for (I = -1; I <= 1; I++) {
for (J = -1; J <= 1; J++) {
piX = J + X;
piY = I + Y;
pixel16 pix = getPixel(piX, piY);
uint8_t Red = pix.Red;
uint8_t Green = pix.Green;
uint8_t Blue = pix.Blue;
sumX[0] += (Red) * GX[J + 1][I + 1];
sumX[1] += (Green) * GX[J + 1][I + 1];
sumX[2] += (Blue) * GX[J + 1][I + 1];
sumY[0] += (Red) * GY[J + 1][I + 1];
sumY[1] += (Green) * GY[J + 1][I + 1];
sumY[2] += (Blue) * GY[J + 1][I + 1];
}
}
SUM[0] = abs(sumX[0]) + abs(sumY[0]);
SUM[1] = abs(sumX[1]) + abs(sumY[1]);
SUM[2] = abs(sumX[2]) + abs(sumY[2]);
}
if (SUM[0] > 255)
SUM[0] = 255;
if (SUM[0] < 0)
SUM[0] = 0;
if (SUM[1] > 255)
SUM[1] = 255;
if (SUM[1] < 0)
SUM[1] = 0;
if (SUM[2] > 255)
SUM[2] = 255;
if (SUM[2] < 0)
SUM[2] = 0;
int newPixel[3];
newPixel[0] = (255 - ((unsigned char) (SUM[0])));
newPixel[1] = (255 - ((unsigned char) (SUM[1])));
newPixel[2] = (255 - ((unsigned char) (SUM[2])));
pixel16 pix(newPixel[0], newPixel[1], newPixel[2]);
buffer[X] = packColour(pix).packed565;
}
//Need to move cursor back
// draw it
this->paintRow(Point(0, Y), buffer, _gl->getWidth());
}
delete[] buffer;
}
Version2
/**
* https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/image-processing/edge_detection.html
* 1 Iterate over every pixel in the image
* 2 Apply the x gradient kernel
* 3 Apply the y gradient kernel
* 4 Find the length of the gradient using pythagoras' theorem
* 5 Normalise the gradient length to the range 0-255
* 6 Set the pixels to the new values
*/
void sobelEdgeDetection4() {
UnpackedColour colour;
for (int x = 1; x < _gl->getWidth() - 1; x++) {
for (int y = 1; y < _gl->getHeight() - 1; y++) {
// initialise Gx and Gy to 0
int Gx = 0;
int Gy = 0;
unsigned int intensity = 0;
// Left column
pixel16 pixel = this->getPixel(x - 1, y - 1);
intensity = pixel.Red + pixel.Green + pixel.Blue;
Gx += -intensity;
Gy += -intensity;
pixel = this->getPixel(x - 1, y);
intensity = pixel.Red + pixel.Green + pixel.Blue;
Gx += -2 * intensity;
pixel = this->getPixel(x - 1, y + 1);
intensity = pixel.Red + pixel.Green + pixel.Blue;
Gx += -intensity;
Gy += +intensity;
// middle column
pixel = this->getPixel(x, y - 1);
intensity = pixel.Red + pixel.Green + pixel.Blue;
Gy += -2 * intensity;
pixel = this->getPixel(x, y + 1);
intensity = pixel.Red + pixel.Green + pixel.Blue;
Gy += +2 * intensity;
// right column
pixel = this->getPixel(x + 1, y - 1);
intensity = pixel.Red + pixel.Green + pixel.Blue;
Gx += +intensity;
Gy += -intensity;
pixel = this->getPixel(x + 1, y);
intensity = pixel.Red + pixel.Green + pixel.Blue;
Gx += +2 * intensity;
pixel = this->getPixel(x + 1, y + 1);
intensity = pixel.Red + pixel.Green + pixel.Blue;
Gx += +intensity;
Gy += +intensity;
// calculate the gradient length
unsigned int length = (unsigned int) sqrt(
(float) (Gx * Gx) + (float) (Gy * Gy));
// normalise the length to 0 to 255
length = length / 17;
// draw the pixel on the edge image
pixel16 pixel2(length,length,length);
this->setPixel(x, y, pixel2);
}
}
}
Version 3
// sobel map for the x axis
const double _SOBEL_Gx[3][3] = { { -1.0, +0.0, +1.0 }, { -2.0, +0.0, +2.0 },
{ -1.0, +0.0, +1.0 } };
// sobel map for the y axis
const double _SOBEL_Gy[3][3] = { { +1.0, +2.0, +1.0 }, { +0.0, +0.0, +0.0 },
{ -1.0, -2.0, -1.0 } };
double get_sobel_gradient(int width, int height, int x, int y) {
double sobel_gradient_x = 0, sobel_gradient_y = 0;
int mx = 0, my = 0, sx = 0, sy = 0;
for (mx = x; mx < x + 3; mx++) {
sy = 0;
for (my = y; my < y + 3; my++) {
if (mx < width && my < height) {
//int r, g, b, idx;
int idx = (mx + width * my) * 3;
pixel16 pixVal = this->getPixel(idx);
//r = pixVal.Red;
//g = pixVal.Green;
//b = pixVal.Blue;
UnpackedColour col = this->packColour(pixVal);
sobel_gradient_x += col.packed565 * _SOBEL_Gx[sx][sy];
sobel_gradient_y += col.packed565 * _SOBEL_Gy[sx][sy];
}
sy++;
}
sx++;
}
return abs(sobel_gradient_x) + abs(sobel_gradient_y);
}
void sobelEdgeDetection3() {
double threshold = 50000.0;
UnpackedColour colour;
for (int y = 0; y < _gl->getHeight(); y++) {
for (int x = 0; x < _gl->getWidth(); x++) {
if (get_sobel_gradient(_gl->getWidth(), _gl->getHeight(), x, y)
>= threshold) {
colour.packed565 = 0x0000; //set white
} else {
colour.packed565 = 0xFFFF; //set black
}
this->setPixel(x, y, colour);
}
}
}

Answer 1

For Version 1, after you allocate 2 buffers (just use buffer and bufT), create 2 pointers to point to the current and previous rows, like this:
uint16_t *currentRow = buffer;
uint16_t *prevRow = bufT;
Inside the row loop, write to currentRow instead of buffer:
pixel16 pix(newPixel[0], newPixel[1], newPixel[2]);
currentRow[X] = packColour(pix).packed565;
Because the Sobel filter reads from the previous row, you can't overwrite a row until after you have finished calculating the filtered values for the row after it. So at the end of the loop, where you are currently calling paintRow(), draw the previous row (if one exists), and then swap the buffers so that the current becomes the previous, and the previous becomes the new current row (to be overwritten on the next pass through the loop). On the last row the current row is also drawn, because otherwise it won't be since the outer loop is about to terminate.
if(Y > 0) // draw the previous row if this is not the first row:
this->paintRow(Point(0, Y-1), prevRow, _gl->getWidth());
if(Y == _gl->getHeight()-1) // draw the current row if it is the last:
this->paintRow(Point(0, Y), currentRow, _gl->getWidth());
// swap row pointers:
uint16_t *temp = prevRow;
prevRow = currentRow;
currentRow = temp;
The same strategy should work for the other versions.