townengine/src/rendering/twn_circles.c

#include "twn_engine_context_c.h"
#include "twn_draw_c.h"
#include "twn_draw.h"

#include <SDL2/SDL.h>
#include <stb_ds.h>


void draw_circle(Vec2 position, float radius, Color color) {
    CirclePrimitive circle = {
        .radius = radius,
        .color = color,
        .position = position,
    };

    Primitive2D primitive = {
        .type = PRIMITIVE_2D_CIRCLE,
        .circle = circle,
    };

    arrput(ctx.render_queue_2d, primitive);
}


void create_circle_geometry(Vec2 position,
                            float radius,
                            size_t num_vertices,
                            Vec2 vertices[])
{
    SDL_assert(num_vertices <= CIRCLE_VERTICES_MAX);

    /* the angle (in radians) to rotate by on each iteration */
    float seg_rotation_angle = (360.0f / (float)(num_vertices - 2)) * ((float)M_PI / 180);

    vertices[0].x = (float)position.x;
    vertices[0].y = (float)position.y;

    /* this point will rotate around the center */
    float start_x = 0.0f - radius;
    float start_y = 0.0f;

    for (size_t i = 1; i < num_vertices - 1; ++i) {
        float final_seg_rotation_angle = (float)i * seg_rotation_angle;

        float c, s;
        sincosf(final_seg_rotation_angle, &s, &c);

        vertices[i].x = c * start_x - s * start_y;
        vertices[i].y = c * start_y + s * start_x;

        vertices[i].x += position.x;
        vertices[i].y += position.y;
    }

    // place a redundant vertex to make proper circling over shared element buffer
    {
        float final_seg_rotation_angle = (float)1 * seg_rotation_angle;

        float c, s;
        sincosf(final_seg_rotation_angle, &s, &c);

        vertices[num_vertices - 1].x = c * start_x - s * start_y;
        vertices[num_vertices - 1].y = c * start_y + s * start_x;

        vertices[num_vertices - 1].x += position.x;
        vertices[num_vertices - 1].y += position.y;
    }
}