Rectangle Detection Logic

[mahardhikapraja]

Describe the bug (mandatory)

I found incorrect logic in the rectangle detection.

To Reproduce (mandatory)

Case 1

line0 = [[2, 2], [6, 2]]
line1 = [[6, 2], [6, 4]]
line2 = [[6, 4], [10, 4]]

ll = line0[0]
lr = line0[1]
ur = line2[0]
ul = line2[1]

Passed the detection test

ll.y == lr.y
lr.x == ur.x
ur.y == ul.y

Case 2

line0 = [[2, 2], [6, 2]]
line1 = [[6, 2], [6, 4]]
line2 = [[6, 4], [4, 4]]

ll = line0[0]
lr = line0[1]
ur = line2[0]
ul = line2[1]

Passed the detection test

ll.y == lr.y
lr.x == ur.x
ur.y == ul.y

My Solution

I have 2 solutions to resolve this issue

Solution 1

Only works if the line0 is horizontal

if (ll.y != lr.y || ll.x != ul.x || ur.y != ul.y || ur.x != lr.x) {
	goto drop_out;
}

Solution 2

It can also detect rectangle if the line0 is vertical

if (ll.y == lr.y) {
	if (ll.x != ul.x || ur.y != ul.y || ur.x != lr.x) {
		goto drop_out;
	}
}
else if (ll.x == lr.x) {
	if (ll.y != ul.y || ur.x != ul.x || ur.y != lr.y) {
		goto drop_out;
	}
	// do swap lr => ul
	// It is required to keep orientation detection work properly
	fz_point tmp = lr;
	lr = ul;
	ul = tmp;
}
else {
	goto drop_out;
}

Your configuration (mandatory)

3.9.9 (tags/v3.9.9:ccb0e6a, Nov 15 2021, 18:08:50) [MSC v.1929 64 bit (AMD64)]
win32

PyMuPDF 1.21.0: Python bindings for the MuPDF 1.21.0 library.
Version date: 2022-11-08 00:00:01.
Built for Python 3.9 on win32 (64-bit).

[JorjMcKie]

Thanks for the report.
You are right: need to also check x-coordinate equality.

BTW: There is no intention to detect arbitrary rectangle-like things. Detection delivers a rectangle if and only if all of the following conditions are met:

1. Three consecutive lines
2. Lines 1 and 3 must be horizontal, line 2 must be vertical
3. The line commands must be followed by a "close path" command

While method .get_drawings() works for all supported document types, the reason of these conditions is how the PDF rectangle drawing command "re" is decomposed into single line drawings.

Depending on whether line 1 is "above" or "below" line 3, the rectangle orientation is determined: below means anti-clockwise, above is clockwise.

[julian-smith-artifex-com]

Fixed in PyMuPDF-1.21.1.