// Adds sub-pixel resolution EdgeOffsets for the outline if the supplied
// pix is 8-bit. Does nothing otherwise.
// Operation: Consider the following near-horizontal line:
// _________
// |________
// |________
// At *every* position along this line, the gradient direction will be close
// to vertical. Extrapoaltion/interpolation of the position of the threshold
// that was used to binarize the image gives a more precise vertical position
// for each horizontal step, and the conflict in step direction and gradient
// direction can be used to ignore the vertical steps.
void C_OUTLINE::ComputeEdgeOffsets(int threshold, Pix* pix) {
if (pixGetDepth(pix) != 8) return;
const l_uint32* data = pixGetData(pix);
int wpl = pixGetWpl(pix);
int width = pixGetWidth(pix);
int height = pixGetHeight(pix);
bool negative = flag(COUT_INVERSE);
delete [] offsets;
offsets = new EdgeOffset[stepcount];
ICOORD pos = start;
ICOORD prev_gradient;
ComputeGradient(data, wpl, pos.x(), height - pos.y(), width, height,
&prev_gradient);
for (int s = 0; s < stepcount; ++s) {
ICOORD step_vec = step(s);
TPOINT pt1(pos);
pos += step_vec;
TPOINT pt2(pos);
ICOORD next_gradient;
ComputeGradient(data, wpl, pos.x(), height - pos.y(), width, height,
&next_gradient);
// Use the sum of the prev and next as the working gradient.
ICOORD gradient = prev_gradient + next_gradient;
// best_diff will be manipulated to be always positive.
int best_diff = 0;
// offset will be the extrapolation of the location of the greyscale
// threshold from the edge with the largest difference, relative to the
// location of the binary edge.
int offset = 0;
if (pt1.y == pt2.y && abs(gradient.y()) * 2 >= abs(gradient.x())) {
// Horizontal step. diff_sign == 1 indicates black above.
int diff_sign = (pt1.x > pt2.x) == negative ? 1 : -1;
int x = MIN(pt1.x, pt2.x);
int y = height - pt1.y;
int best_sum = 0;
int best_y = y;
EvaluateVerticalDiff(data, wpl, diff_sign, x, y, height,
&best_diff, &best_sum, &best_y);
// Find the strongest edge.
int test_y = y;
do {
++test_y;
} while (EvaluateVerticalDiff(data, wpl, diff_sign, x, test_y, height,
&best_diff, &best_sum, &best_y));
test_y = y;
do {
--test_y;
} while (EvaluateVerticalDiff(data, wpl, diff_sign, x, test_y, height,
&best_diff, &best_sum, &best_y));
offset = diff_sign * (best_sum / 2 - threshold) +
(y - best_y) * best_diff;
} else if (pt1.x == pt2.x && abs(gradient.x()) * 2 >= abs(gradient.y())) {
// Vertical step. diff_sign == 1 indicates black on the left.
int diff_sign = (pt1.y > pt2.y) == negative ? 1 : -1;
int x = pt1.x;
int y = height - MAX(pt1.y, pt2.y);
const l_uint32* line = pixGetData(pix) + y * wpl;
int best_sum = 0;
int best_x = x;
EvaluateHorizontalDiff(line, diff_sign, x, width,
&best_diff, &best_sum, &best_x);
// Find the strongest edge.
int test_x = x;
do {
++test_x;
} while (EvaluateHorizontalDiff(line, diff_sign, test_x, width,
&best_diff, &best_sum, &best_x));
test_x = x;
do {
--test_x;
} while (EvaluateHorizontalDiff(line, diff_sign, test_x, width,
&best_diff, &best_sum, &best_x));
offset = diff_sign * (threshold - best_sum / 2) +
(best_x - x) * best_diff;
}
offsets[s].offset_numerator =
static_cast<inT8>(ClipToRange(offset, -MAX_INT8, MAX_INT8));
offsets[s].pixel_diff = static_cast<uinT8>(ClipToRange(best_diff, 0 ,
MAX_UINT8));
if (negative) gradient = -gradient;
// Compute gradient angle quantized to 256 directions, rotated by 64 (pi/2)
// to convert from gradient direction to edge direction.
offsets[s].direction =
Modulo(FCOORD::binary_angle_plus_pi(gradient.angle()) + 64, 256);
prev_gradient = next_gradient;
}
}
