Book 1: Chapter 6.1: Is this algorithm correct?

[artogahr]

I'm trying to rewrite this book in Rust, and as far as I can tell, I've done everything 1:1 with the current published book (4.0.0-alpha.2, 2024-04-07)

As I can tell, within the older versions these lines:

raytracing.github.io/books/RayTracingInOneWeekend.html

Lines 680 to 683 in 891e5ac

	// Calculate the location of the upper left pixel.
	auto viewport_upper_left = camera_center
	- vec3(0, 0, focal_length) - viewport_u/2 - viewport_v/2;
	auto pixel00_loc = viewport_upper_left + 0.5 * (pixel_delta_u + pixel_delta_v);

Were represented as just a calculation for lower_left_corner.
Implementing the old version from reference implementations works. But the newer version, doesn't produce the colorful sphere.

Within my code, if I add an offset to the return value of the hit_sphere function, the sphere with the Vectors is reproduced with correct colors, i.e. instead of:

let t = hit_sphere(Point3::new(0.0, 0.0, -1.0), 0.5, r);
    if t > 0.0 {
        let n: Vec3 = (r.at(t) - Vec3::new(0.0, 0.0, -1.0)).normalized();
        return 0.5 * Color::new(n.x() + 1.0, n.y() + 1.0, n.z() + 1.0);
    }

doing

let t = hit_sphere(Point3::new(0.0, 0.0, -1.0), 0.5, r);
    if t > -0.5 {
        let n: Vec3 = (r.at(t) - Vec3::new(0.0, 0.0, -1.0)).normalized();
        return 0.5 * Color::new(n.x() + 1.0, n.y() + 1.0, n.z() + 1.0);
    }

fixes the issue and the exact image from the book is produced. Values between -0.5 and -0.27 produces different width circles. I'm not mathematically versed enough to understand why, but I thought this could help us diagnose this issue.

Any ideas?