display normal on sphere

in_one_weekend/src/main.zig

@@ -16,21 +16,27 @@ const SDL_WINDOWPOS_UNDEFINED = @bitCast(c_int, sdl.c.SDL_WINDOWPOS_UNDEFINED_MA
 
 const fps = 60;
 
-fn hitSphere(center: Point3, radius: f32, ray: Ray) bool {
+fn hitSphere(center: Point3, radius: f32, ray: Ray) f32 {
     const ssr = ray.origin.sub(center); // Sphere-space ray, (A - C) in book
     const a = Vec3.dot(ray.direction, ray.direction);
     const b = 2.0 * Vec3.dot(ssr, ray.direction);
     const c = Vec3.dot(ssr, ssr) - (radius * radius);
     const discriminant = b * b - 4 * a * c;
-    return discriminant > 0;
+    if (discriminant < 0) {
+        return -1;
+    } else {
+        return (-b - std.math.sqrt(discriminant)) / (2 * a);
+    }
 }
 
 fn rayColor(ray: Ray) Color {
-    if (hitSphere(Point3{ .x = 0, .y = 0, .z = -1 }, 0.5, ray)) {
-        return Color{ .x = 1, .y = 0, .z = 0 };
+    var t = hitSphere(Point3{ .x = 0, .y = 0, .z = -1 }, 0.5, ray);
+    if (t > 0) {
+        const normal = vec3.unitVector(ray.at(t).sub(Vec3{ .z = -1 }));
+        return normal.add(Vec3{ .x = 1, .y = 1, .z = 1}).div(2);
     }
     const unitDirection = vec3.unitVector(ray.direction);
-    const t = 0.5 * (unitDirection.y + 1.0);
+    t = 0.5 * (unitDirection.y + 1.0);
     const white = Color{ .x = 1.0, .y = 1.0, .z = 1.0 };
     const blue = Color{ .x = 0.5, .y = 0.7, .z = 1.0 };
     return white.mul(1.0 - t).add(blue.mul(t));
@@ -45,7 +51,7 @@ pub fn main() anyerror!void {
 
     // Image
     const aspectRatio: f32 = 16.0 / 9.0;
-    const imageWidth = 400;
+    const imageWidth = 600;
     const imageHeight = @floatToInt(usize, imageWidth / aspectRatio);
 
     // Camera
@@ -87,7 +93,7 @@ pub fn main() anyerror!void {
                 const v = @intToFloat(f32, (imageHeight - 1) - j) / @intToFloat(f32, (imageHeight - 1));
                 const r = Ray{
                     .origin = origin,
-                    .direction = lowerLeftCorner.add(horizontal.mul(u).add(vertical.mul(v).sub(origin))),
+                    .direction = lowerLeftCorner.add(horizontal.mul(u)).add(vertical.mul(v)).sub(origin),
                 };
                 const pixelColor = rayColor(r);
                 sdl.setSurfacePixel(surface, i, j, pixelColor);

in_one_weekend/src/ray.zig

@@ -5,7 +5,7 @@ pub const Ray = struct {
     origin: Point3,
     direction: Vec3,
 
-    pub fn at(t: f32) Point3 {
-        return origin.add(direction.mul(t));
+    pub fn at(self: Ray, t: f32) Point3 {
+        return self.origin.add(self.direction.mul(t));
     }
 };