townengine/src/rendering/twn_circles.c

#include "twn_util.h"
#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,
                            Color color,
                            float radius,
                            size_t num_vertices,
                            SDL_Vertex vertices[],
                            int indices[])
{
    /* the angle (in radians) to rotate by on each iteration */
    float seg_rotation_angle = (360.0f / (float)num_vertices) * ((float)M_PI / 180);

    vertices[0].position.x = (float)position.x;
    vertices[0].position.y = (float)position.y;
    vertices[0].color.r = color.r;
    vertices[0].color.g = color.g;
    vertices[0].color.b = color.b;
    vertices[0].color.a = color.a;
    vertices[0].tex_coord = (SDL_FPoint){ 0, 0 };

    /* 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;

        vertices[i].position.x =
            cosf(final_seg_rotation_angle) * start_x -
            sinf(final_seg_rotation_angle) * start_y;
        vertices[i].position.y =
            cosf(final_seg_rotation_angle) * start_y +
            sinf(final_seg_rotation_angle) * start_x;

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

        vertices[i].color.r = color.r;
        vertices[i].color.g = color.g;
        vertices[i].color.b = color.b;
        vertices[i].color.a = color.a;

        vertices[i].tex_coord = (SDL_FPoint){ 0, 0 };


        size_t triangle_offset = 3 * (i - 1);

        /* center point index */
        indices[triangle_offset] = 0;
        /* generated point index */
        indices[triangle_offset + 1] = (int)i;

        size_t index = (i + 1) % num_vertices;
        if (index == 0)
            index = num_vertices;
        indices[triangle_offset + 2] = (int)index;
    }
}