demo: Add a test for simplification along attribute seam

This test is intended mostly to track future improvements to the error
behavior along the seam boundary. Right now, attribute error is not
correctly computed along the seam boundary which prevents some collapses
and can result in artificially inflated resulting error. We also seem to
have some issues with positional errors for collapses along the seam
which the test also demonstrates.

Author: zeux

demo/tests.cpp

@@ -1338,6 +1338,78 @@ static void simplifyErrorAbsolute()
 	assert(fabsf(error - 0.85f) < 0.01f);
 }
 
+static void simplifySeam()
+{
+	// xyz+attr
+	float vb[] = {
+	    0, 0, 0, 0,
+	    0, 1, 0, 0,
+	    0, 1, 0, 1,
+	    0, 2, 0, 1,
+	    1, 0, 0, 0,
+	    1, 1, 1, 0,
+	    1, 1, 1, 1,
+	    1, 2, 0, 1,
+	    2, 0, 0, 0,
+	    2, 1, 0, 0,
+	    2, 1, 0, 1,
+	    2, 2, 0, 1,
+	    3, 0, 0, 0,
+	    3, 1, 0, 0,
+	    3, 1, 0, 1,
+	    3, 2, 0, 1, // clang-format :-/
+	};
+
+	// 0   1-2   3
+	// 4   5-6   7
+	// 8   9-10 11
+	// 12 13-14 15
+
+	unsigned int ib[] = {
+	    0, 1, 4,
+	    4, 1, 5,
+	    2, 3, 6,
+	    6, 3, 7,
+	    4, 5, 8,
+	    8, 5, 9,
+	    6, 7, 10,
+	    10, 7, 11,
+	    8, 9, 12,
+	    12, 9, 13,
+	    10, 11, 14,
+	    14, 11, 15, // clang-format :-/
+	};
+
+	// note: vertices 1-2 and 13-14 are classified as locked, because they are on a seam & a border
+	// since seam->locked collapses are restriced, we only get to 3 triangles on each side as the seam is simplified to 3 vertices
+
+	// so we get this structure initially, and then one of the internal seam vertices is collapsed to the other one:
+	// 0   1-2   3
+	//     5-6
+	//     9-10
+	// 12 13-14 15
+	unsigned int expected[] = {
+	    0, 1, 5,
+	    2, 3, 6,
+	    0, 5, 12,
+	    12, 5, 13,
+	    6, 3, 14,
+	    14, 3, 15, // clang-format :-/
+	};
+
+	unsigned int res[36];
+	float error = 0.f;
+
+	assert(meshopt_simplify(res, ib, 36, vb, 16, 16, 18, 1.f, 0, &error) == 18);
+	assert(memcmp(res, expected, sizeof(expected)) == 0);
+	assert(fabsf(error - 0.22f) < 0.01f); // TODO: this is higher than normal due to border errors?
+
+	float aw = 1;
+	assert(meshopt_simplifyWithAttributes(res, ib, 36, vb, 16, 16, vb + 3, 16, &aw, 1, NULL, 18, 2.f, 0, &error) == 18);
+	assert(memcmp(res, expected, sizeof(expected)) == 0);
+	assert(fabsf(error - 0.49f) < 0.01f); // TODO: this is higher than normal due to merged attributes?
+}
+
 static void adjacency()
 {
 	// 0 1/4
@@ -1556,6 +1628,7 @@ void runTests()
 	simplifyLockFlags();
 	simplifySparse();
 	simplifyErrorAbsolute();
+	simplifySeam();
 
 	adjacency();
 	tessellation();