2018-soft-3d-renderer/src/geometry.cpp

#include "color.h"
#include "geometry.h"
#include "util.h"


// MACROS
#define DrawPixel(buffer, width, color, x, y)\
{\
    buffer[width * y + x] = color;\
}


// PRIVATE PROTOTYPES
static void FillTriangleFlatBottom(
        uint32_t *buffer, int width,
        float *zbuffer, uint32_t color,
        float x0, float y0, float z0,
        float x1, float y1, float z1,
        float x2, float y2, float z2);

static void FillTriangleFlatTop(
        uint32_t *buffer, int width,
        float *zbuffer, uint32_t color,
        float x0, float y0, float z0,
        float x1, float y1, float z1,
        float x2, float y2, float z2);


// PUBLIC FUNCTIONS
void FillTriangle(
        uint32_t *buffer, int width,
        float *zbuffer, uint32_t color,
        float x0, float y0, float z0,
        float x1, float y1, float z1,
        float x2, float y2, float z2)
{
    // Horizontal or vertical
    if ((FLOAT_EQUAL(x0,x1) && FLOAT_EQUAL(x1, x2))
        || (FLOAT_EQUAL(y0, y1) && FLOAT_EQUAL(y1, y2)))
    {
        return;
    }

    // Sort in ascending Y order
    float temp;
    if (y0 > y1)
    {
        SWAP(x0, x1, temp);
        SWAP(y0, y1, temp);
        SWAP(z0, z1, temp);
    }

    if (y0 > y2)
    {
        SWAP(x0, x2, temp);
        SWAP(y0, y2, temp);
        SWAP(z0, z2, temp);
    }

    if (y1 > y2)
    {
        SWAP(x1, x2, temp);
        SWAP(y1, y2, temp);
        SWAP(z1, z2, temp);
    }


    // Flat top triangle
    if (FLOAT_EQUAL(y0, y1))
    {
        FillTriangleFlatTop(buffer, width, zbuffer, color, x0, y0, z0, x1, y1, z1, x2, y2, z2);
    }

    // Flat bottom triangle
    else if (FLOAT_EQUAL(y1, y2))
    {
        FillTriangleFlatBottom(buffer, width, zbuffer, color, x0, y0, z0, x1, y1, z1, x2, y2, z2);
    }

    // Other - decompose into two triangles
    else
    {
        // Calculate x coordinate at point of decomposition using intercept theorem
        // New y is the same as y1
        float x3 = x0 + (((y1 - y0) / (y2 - y0)) * (x2 - x0));
        float y3 = y1;
        float z3 = z1;

        FillTriangleFlatBottom(buffer, width, zbuffer, color, x0, y0, z0, x1, y1, z1, x3, y3, z3);
        FillTriangleFlatTop(buffer, width, zbuffer, color, x1, y1, z1, x3, y3, z3, x2, y2, z2);
    }
}


// PRIVATE FUNCTIONS
static void FillTriangleFlatBottom(
        uint32_t *buffer, int width,
        float *zbuffer, uint32_t color,
        float x0, float y0, float z0,
        float x1, float y1, float z1,
        float x2, float y2, float z2)
{
    if (x2 < x1)
    {
        float temp;
        SWAP(x2, x1, temp);
        SWAP(y2, y1, temp);
        SWAP(z2, z1, temp);
    }

    // Top-left fill convention
    float dxdy_left = (x1 - x0) / (y1 - y0);
    float dxdy_right = (x2 - x0) / (y2 - y0);
    int yStart = (int)ceilf(y0);
    int yEnd = (int)ceilf(y2) - 1;
    float xStart = x0 + dxdy_left * (yStart - y0);
    float xEnd = x0 + dxdy_right * (yStart - y0);


    // Inverse z-buffering
    float z0Inv = 1.0f / z0;
    float z1Inv = 1.0f / z1;
    float z2Inv = 1.0f / z2;
    float dzdy_left = (z1Inv - z0Inv) / (y1 - y0);
    float dzdy_right = (z2Inv - z0Inv) / (y2 - y0);
    float zStart = z0Inv;
    float zEnd = z0Inv;


    // Fill
    for (int y = yStart; y <= yEnd; ++y)
    {
        float dzdx = (zEnd - zStart) / (xEnd - xStart);
        float z = zStart;

        int xStartInt = (int)xStart;
        int xEndInt = (int)xEnd;

        for (int x = xStartInt; x <= xEndInt; ++x)
        {
            int pixel = (y * width) + x;

            if (z > zbuffer[pixel])
            {
                DrawPixel(buffer, width, color, x, y);

                zbuffer[pixel] = z;
            }

            z += dzdx;
        }

        xStart += dxdy_left;
        xEnd += dxdy_right;

        zStart += dzdy_left;
        zEnd += dzdy_right;
    }
}

static void FillTriangleFlatTop(
        uint32_t *buffer, int width,
        float *zbuffer, uint32_t color,
        float x0, float y0, float z0,
        float x1, float y1, float z1,
        float x2, float y2, float z2)
{
    if (x1 < x0)
    {
        float temp;
        SWAP(x1, x0, temp);
        SWAP(y1, y0, temp);
        SWAP(z1, z0, temp);
    }

    // Top-left fill convention
    float dxdy_left = (x2 - x0) / (y2 - y0);
    float dxdy_right = (x2 - x1) / (y2 - y1);
    int yStart = (int)ceilf(y0);
    int yEnd = (int)ceilf(y2) - 1;
    float xStart = x0 + dxdy_left * (yStart - y0);
    float xEnd = x1 + dxdy_right * (yStart - y0);


    // Inverse z-buffering
    float z0Inv = 1.0f / z0;
    float z1Inv = 1.0f / z1;
    float z2Inv = 1.0f / z2;
    float dzdy_left = (z2Inv - z0Inv) / (y2 - y0);
    float dzdy_right = (z2Inv - z1Inv) / (y2 - y1);
    float zStart = z0Inv;
    float zEnd = z1Inv;

    // Fill
    for (int y = yStart; y <= yEnd; ++y)
    {
        float dzdx = (zEnd - zStart) / (xEnd - xStart);
        float z = zStart;

        int xStartInt = (int)xStart;
        int xEndInt = (int)xEnd;

        for (int x = xStartInt; x <= xEndInt; ++x)
        {
            int pixel = (y * width) + x;

            if (z > zbuffer[pixel])
            {
                DrawPixel(buffer, width, color, x, y);

                zbuffer[pixel] = z;
            }

            z += dzdx;
        }

        xStart += dxdy_left;
        xEnd += dxdy_right;

        zStart += dzdy_left;
        zEnd += dzdy_right;
    }
}