bullet3/examples/TinyRenderer/our_gl.cpp

#include <cmath>
#include <limits>
#include <cstdlib>
#include "our_gl.h"

Matrix ModelView;
Matrix Viewport;
Matrix Projection;

IShader::~IShader() {}

void viewport(int x, int y, int w, int h) {
    Viewport = Matrix::identity();
    Viewport[0][3] = x+w/2.f;
    Viewport[1][3] = y+h/2.f;
    Viewport[2][3] = 1.f;
    Viewport[0][0] = w/2.f;
    Viewport[1][1] = h/2.f;
    Viewport[2][2] = 0;
}

void projection(float coeff) {
    Projection = Matrix::identity();
    Projection[3][2] = coeff;
}

void lookat(Vec3f eye, Vec3f center, Vec3f up) {
    Vec3f z = (eye-center).normalize();
    Vec3f x = cross(up,z).normalize();
    Vec3f y = cross(z,x).normalize();
    Matrix Minv = Matrix::identity();
    Matrix Tr   = Matrix::identity();
    for (int i=0; i<3; i++) {
        Minv[0][i] = x[i];
        Minv[1][i] = y[i];
        Minv[2][i] = z[i];
        Tr[i][3] = -center[i];
    }
    ModelView = Minv*Tr;
}

Vec3f barycentric(Vec2f A, Vec2f B, Vec2f C, Vec2f P) {
    Vec3f s[2];
    for (int i=2; i--; ) {
        s[i][0] = C[i]-A[i];
        s[i][1] = B[i]-A[i];
        s[i][2] = A[i]-P[i];
    }
    Vec3f u = cross(s[0], s[1]);
    if (std::abs(u[2])>1e-2) // dont forget that u[2] is integer. If it is zero then triangle ABC is degenerate
        return Vec3f(1.f-(u.x+u.y)/u.z, u.y/u.z, u.x/u.z);
    return Vec3f(-1,1,1); // in this case generate negative coordinates, it will be thrown away by the rasterizator
}

void triangle(mat<4,3,float> &clipc, IShader &shader, TGAImage &image, float *zbuffer) {
    mat<3,4,float> pts  = (Viewport*clipc).transpose(); // transposed to ease access to each of the points
    mat<3,2,float> pts2;
    for (int i=0; i<3; i++) pts2[i] = proj<2>(pts[i]/pts[i][3]);

    Vec2f bboxmin( std::numeric_limits<float>::max(),  std::numeric_limits<float>::max());
    Vec2f bboxmax(-std::numeric_limits<float>::max(), -std::numeric_limits<float>::max());
    Vec2f clamp(image.get_width()-1, image.get_height()-1);
    for (int i=0; i<3; i++) {
        for (int j=0; j<2; j++) {
            bboxmin[j] = std::max(0.f,      std::min(bboxmin[j], pts2[i][j]));
            bboxmax[j] = std::min(clamp[j], std::max(bboxmax[j], pts2[i][j]));
        }
    }
    Vec2i P;
    TGAColor color;
    for (P.x=bboxmin.x; P.x<=bboxmax.x; P.x++) {
        for (P.y=bboxmin.y; P.y<=bboxmax.y; P.y++) {
            Vec3f bc_screen  = barycentric(pts2[0], pts2[1], pts2[2], P);
            Vec3f bc_clip    = Vec3f(bc_screen.x/pts[0][3], bc_screen.y/pts[1][3], bc_screen.z/pts[2][3]);
            bc_clip = bc_clip/(bc_clip.x+bc_clip.y+bc_clip.z);
            float frag_depth = clipc[2]*bc_clip;
            if (bc_screen.x<0 || bc_screen.y<0 || bc_screen.z<0 || zbuffer[P.x+P.y*image.get_width()]>frag_depth) continue;
            bool discard = shader.fragment(bc_clip, color);
            if (!discard) {
                zbuffer[P.x+P.y*image.get_width()] = frag_depth;
                image.set(P.x, P.y, color);
            }
        }
    }
}