3drenderer/src/triangle.c

#include "triangle.h"
#include "display.h"
#include "swap.h"

// Draw a filled triangle with a flat bottom
void fill_flat_bottom_triangle(int x0, int y0, int x1, int y1, int x2, int y2, uint32_t color) {
    // Find the two slopes (two triangle legs)
    float inv_slope_1 = (float)(x1 - x0) / (y1 - y0);
    float inv_slope_2 = (float)(x2 - x0) / (y2 - y0);

    // Start x_start and x_end from the top vertex (x0, y0)
    float x_start = x0;
    float x_end = x0;

    // Loop all the scanlines from top to bottom
    for (int y = y0; y <= y2; y++) {
        draw_line(x_start, y, x_end, y, color);
        x_start += inv_slope_1;
        x_end += inv_slope_2; 
    }
}

// Draw a filled triangle with a flat top
void fill_flat_top_triangle(int x0, int y0, int x1, int y1, int x2, int y2, uint32_t color) {
    // Find the two slopes (two triangle legs)
    float inv_slope_1 = (float)(x2 - x0) / (y2 - y0);
    float inv_slope_2 = (float)(x2 - x1) / (y2 - y1);

    // Start x_start and x_end from the bottom vertex (x0, y0)
    float x_start = x2;
    float x_end = x2;

    // Loop all the scanlines from bottom to top
    for (int y = y2; y >= y0; y--) {
        draw_line(x_start, y, x_end, y, color);
        x_start -= inv_slope_1;
        x_end -= inv_slope_2; 
    }

}


void draw_triangle(int x0, int y0, int x1, int y1, int x2, int y2, uint32_t color) {
    draw_line(x0, y0, x1, y1, color);
    draw_line(x1, y1, x2, y2, color);
    draw_line(x2, y2, x0, y0, color);
};

void draw_filled_triangle(int x0, int y0, int x1, int y1, int x2, int y2, uint32_t color) {
    // We need to sort the vertices by the y-coordinate ascending (y0 < y < y2)
    if (y0 > y1) {
        int_swap(&y0, &y1);
        int_swap(&x0, &x1);
    }

    if (y1 > y2) {
        int_swap(&y1, &y2);
        int_swap(&x1, &x2);
    }

    if (y0 > y1) {
        int_swap(&y0, &y1);
        int_swap(&x0, &x1);
    }

    if (y1 == y2) {
        fill_flat_bottom_triangle(x0, y0, x1, y1, x2, y2, color);
    } else if (y0 == y1) {
        fill_flat_top_triangle(x0, y0, x1, y1, x2, y2, color);
    } else {
        // Calculate the new vertex (Mx, My) using triangle similarity
        int My = y1;
        int Mx = ((float)((x2-x0) * (y1 - y0)) / (float)(y2 - y0)) + x0;

        fill_flat_bottom_triangle(x0, y0, x1, y1, Mx, My, color);
        fill_flat_top_triangle(x1, y1, Mx, My, x2, y2, color);
    }
}

////////////////////////////////////////////////////////////////////////////////
// Draw a textured triangle with the flat-top/flat-bottom method
// We split the original triangle in two, half flat-bottom and half flat-top
////////////////////////////////////////////////////////////////////////////////
//
//         v0
//         /\
//        /  \
//       /    \
//      /      \
//     v1-------v3
//     \_        \
//       \_       \
//         \_      \
//           \_     \
//             \_    \
//               \_   \
//                 \_  \
//                   \_ \
//                      \\
//                        \
//                        v2
////////////////////////////////////////////////////////////////////////////////
// TODO: use color_t type or uint32_t type?
void draw_textured_triangle(
    int x0, int y0, float u0, float v0,
    int x1, int y1, float u1, float v1,
    int x2, int y2, float u2, float v2,
    color_t* texture
) {
    // We need to sort the vertices by y-coordinate asecnding (y0 < y1 < y2)
    if (y0 > y1) {
        int_swap(&y0, &y1);
        int_swap(&x0, &x1);
        float_swap(&u0, &u1);
        float_swap(&v0, &v1);
    }

    if (y1 > y2) {
        int_swap(&y1, &y2);
        int_swap(&x1, &x2);
        float_swap(&u1, &u2);
        float_swap(&v1, &v2);
    }

    if (y0 > y1) {
        int_swap(&y0, &y1);
        int_swap(&x0, &x1);
        float_swap(&u0, &u1);
        float_swap(&v0, &v1);
    }

    //////////////////////////////////////////////////////
    // Render the upper part of the triangle (flat-bottom)
    //////////////////////////////////////////////////////
    float inv_slope_1 = 0;
    float inv_slope_2 = 0;

    // first and second slope of triangle legs
    if (y1 - y0 != 0) inv_slope_1 = (float)(x1 - x0) / abs(y1 - y0);
    if (y2 - y0 != 0) inv_slope_2 = (float)(x2 - x0) / abs(y2 - y0);

    if (y1 - y0 != 0) {
        for (int y = y0; y <= y1; y++) {
            int x_start = x1 + (y - y1) * inv_slope_1;
            int x_end = x0 + (y - y0) * inv_slope_2;

            if (x_end < x_start) {
                int_swap(&x_start, &x_end); // swap if x_start is to the right of x_end
            }

            for (int x = x_start; x < x_end; x++) {
                // Draw our pixel with the color that comes from the texture
                draw_pixel(x, y, 0xFFFF00FF);
            }
        }
    }


    //////////////////////////////////////////////////////
    // Render the upper part of the triangle (flat-bottom)
    //////////////////////////////////////////////////////
    inv_slope_1 = 0;
    inv_slope_2 = 0;

    // first and second slope of triangle legs
    if (y2 - y1 != 0) inv_slope_1 = (float)(x2 - x1) / abs(y2 - y1);
    if (y2 - y0 != 0) inv_slope_2 = (float)(x2 - x0) / abs(y2 - y0);

    if (y2 - y1 != 0) {
        for (int y = y1; y <= y2; y++) {
            int x_start = x1 + (y - y1) * inv_slope_1;
            int x_end = x0 + (y - y0) * inv_slope_2;

            if (x_end < x_start) {
                int_swap(&x_start, &x_end); // swap if x_start is to the right of x_end
            }

            for (int x = x_start; x < x_end; x++) {
                // Draw our pixel with the color that comes from the texture
                draw_pixel(x, y, 0xFFFF00FF);
            }
        }
    }     
}