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Remove spikes when drawing thin pieslices #5460

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merged 2 commits into from May 14, 2021
Merged

Remove spikes when drawing thin pieslices #5460

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0c9ffcf
Fixed #5432
xtsm May 2, 2021
ca67a0c
Added test for #5432
xtsm May 2, 2021
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30 changes: 30 additions & 0 deletions Tests/test_imagedraw.py
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Expand Up @@ -538,6 +538,36 @@ def test_pieslice_wide():
assert_image_equal_tofile(im, "Tests/images/imagedraw_pieslice_wide.png")


def test_pieslice_no_spikes():
im = Image.new("RGB", (161, 161), "white")
draw = ImageDraw.Draw(im)
cxs = (
[140] * 3
+ list(range(140, 19, -20))
+ [20] * 5
+ list(range(20, 141, 20))
+ [140] * 2
)
cys = (
list(range(80, 141, 20))
+ [140] * 5
+ list(range(140, 19, -20))
+ [20] * 5
+ list(range(20, 80, 20))
)

for cx, cy, angle in zip(cxs, cys, range(0, 360, 15)):
draw.pieslice(
[cx - 100, cy - 100, cx + 100, cy + 100], angle, angle + 1, fill="black"
)
draw.point([cx, cy], fill="red")

im_pre_erase = im.copy()
draw.rectangle([21, 21, 139, 139], fill="white")

assert_image_equal(im, im_pre_erase)


def helper_point(points):
# Arrange
im = Image.new("RGB", (W, H))
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16 changes: 16 additions & 0 deletions src/libImaging/Draw.c
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Expand Up @@ -1347,6 +1347,22 @@ pie_init(clip_ellipse_state *s, int32_t a, int32_t b, int32_t w, float al, float
s->root->l = lc;
s->root->r = rc;
s->root->type = ar - al < 180 ? CT_AND : CT_OR;

// add one more semiplane to avoid spikes
if (ar - al < 90) {
clip_node *old_root = s->root;
clip_node *spike_clipper = s->nodes + s->node_count++;
s->root = s->nodes + s->node_count++;
s->root->l = old_root;
s->root->r = spike_clipper;
s->root->type = CT_AND;

spike_clipper->l = spike_clipper->r = NULL;
spike_clipper->type = CT_CLIP;
spike_clipper->a = (xl + xr) / 2.0;
spike_clipper->b = (yl + yr) / 2.0;
Comment on lines +1362 to +1363
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Here's my explanation of what is happening here.
This is taking the average of the two points on the outside of the circle. Going through the center of the circle as well, that gives us a line down the middle of the pieslice.
Then by swapping x and y (normally y would be with a), and multiplying one of them by -1, it gives us a perpendicular line. Because the line also goes through the center of the circle, that cuts off the spike.

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yeah, that's one way to put it
line equation Ax+By+C=0 can be also written as rn + C = 0, r(x, y), n(A, B) - i.e. n(A, B) is a normal vector for line

spike_clipper->c = 0;
}
}

void
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