# The tessellate polygon drawing algorithm

The algorithm is based on libtess2 here and we optimizated some implementation and fixed some bugs.

The differents between our algorithm and libtess2’s algorithm:

• We change the coordinate system and the sweep direction (sweep line by horizontal here).
• We need not project the vertices because our graphic engine is 2d, so our algorithm will be faster.
• We processed more cases of the intersection with some numerical errors, so our algorithm will be more stable.
• We change the algorithm of comparing the active edge and make it more stable for numerical errors.
• We optimizate the algorithm of merging into the convex polygon from the triangulated mesh.
• We have not counted the vertices for each region, so it will be faster than libtess2.

(you can see libtess2/alg_outline.md if want to known more details of algorithm.)

There are four stages to the algorithm:

1. Build a mesh (DCEL, be similar to quad-edge) from polygon.
2. Tessellate the mesh into the monotone regions if the polygon is concave.
3. Triangulate the monotone regions.
4. Merge the triangulated regions into the convex regions.

There are seven stages to the tessellation algorithm:

1. Simplify the mesh and process some degenerate cases.
2. Build a vertex event queue and sort it (uses the priority queue with min-heap).
3. Build an active edge region list and sort it (uses the partial insertion sort).
4. Sweep all events from the event queue using the Bentley-Ottman line-sweep algorithm and calculate the intersection and winding number.
5. We need fix it if the intersection with numerical errors violate the mesh topology or active edge list ordering.
6. Process some degenerate cases for the mesh faces which were generated when we fixed some cases.
7. Get the monotone regions with the left face marked “inside”

The triangulation test result:   The drawing result with opengl(60 fps on my mac pro): Please refer to the source code monotone.c if you want to know more verbose monotone algorithm implementation.

And the tessellator interfaces usage from gbox/gl/render.c:

``````    static tb_void_t gb_gl_render_fill_convex(gb_point_ref_t points, tb_uint16_t count, tb_cpointer_t priv)
{
// check
tb_assert(priv && points && count);

// apply it
gb_gl_render_apply_vertices((gb_gl_device_ref_t)priv, points);

#ifndef GB_GL_TESSELLATOR_TEST_ENABLE
// draw it
gb_glDrawArrays(GB_GL_TRIANGLE_FAN, 0, (gb_GLint_t)count);
#else
// the device
gb_gl_device_ref_t device = (gb_gl_device_ref_t)priv;

// make crc32
tb_uint32_t crc32 = 0xffffffff ^ tb_crc_encode(TB_CRC_MODE_32_IEEE_LE, 0xffffffff, (tb_byte_t const*)points, count * sizeof(gb_point_t));

// make color
gb_color_t color;
color.r = (tb_byte_t)crc32;
color.g = (tb_byte_t)(crc32 >> 8);
color.b = (tb_byte_t)(crc32 >> 16);
color.a = 128;

// enable blend
gb_glEnable(GB_GL_BLEND);
gb_glBlendFunc(GB_GL_SRC_ALPHA, GB_GL_ONE_MINUS_SRC_ALPHA);

// apply color
if (device->version >= 0x20) gb_glVertexAttrib4f(gb_gl_program_location(device->program, GB_GL_PROGRAM_LOCATION_COLORS), (gb_GLfloat_t)color.r / 0xff, (gb_GLfloat_t)color.g / 0xff, (gb_GLfloat_t)color.b / 0xff, (gb_GLfloat_t)color.a / 0xff);
else gb_glColor4f((gb_GLfloat_t)color.r / 0xff, (gb_GLfloat_t)color.g / 0xff, (gb_GLfloat_t)color.b / 0xff, (gb_GLfloat_t)color.a / 0xff);

// draw the edges of the filled contour
gb_glDrawArrays(GB_GL_TRIANGLE_FAN, 0, (gb_GLint_t)count);

// disable blend
gb_glEnable(GB_GL_BLEND);
#endif
}
static tb_void_t gb_gl_render_fill_polygon(gb_gl_device_ref_t device, gb_polygon_ref_t polygon, gb_rect_ref_t bounds, tb_size_t rule)
{
// check
tb_assert(device && device->tessellator);

#ifdef GB_GL_TESSELLATOR_TEST_ENABLE
// set mode
gb_tessellator_mode_set(device->tessellator, GB_TESSELLATOR_MODE_TRIANGULATION);
//      gb_tessellator_mode_set(device->tessellator, GB_TESSELLATOR_MODE_MONOTONE);
#endif

// set rule
gb_tessellator_rule_set(device->tessellator, rule);

// set func
gb_tessellator_func_set(device->tessellator, gb_gl_render_fill_convex, device);

// done tessellator
gb_tessellator_done(device->tessellator, polygon, bounds);
}
``````