hgkjshegfskef

hgkjshegfskef

The Ray Tracer Challenge: chapter 7 final image is flipped (camera issues) (pp. 105-107)

@jamis

Unfortunately, my final image of chapter 7 is flipped:

All the tests from the chapter pass.

My camera and ray generation are implemented as:

camera::camera(unsigned hsize, unsigned vsize, float fov) noexcept
    : hsize{hsize}, vsize{vsize}, fov{fov}, tform{} {
    float half_view = std::tan(fov / 2.f);
    float aspect = float(hsize) / float(vsize);
    if (aspect >= 1.f) {
        half_width = half_view;
        half_height = half_view / aspect;
    } else {
        half_width = half_view * aspect;
        half_height = half_view;
    }
    pixel_size = (half_width * 2.f) / hsize;
}

tform4 view(pnt3 const& from, pnt3 const& to, vec3 const& up) noexcept {
    vec3 const forward = normalize(to - from);
    vec3 const left = normalize(cross(forward, normalize(up)));
    vec3 const true_up = normalize(cross(left, forward));
    //    vec3 const true_up = -normalize(cross(left, forward));
    tform4 orientation{left.x,     left.y,     left.z,     0, //
                       true_up.x,  true_up.y,  true_up.z,  0, //
                       -forward.x, -forward.y, -forward.z, 0};
    return orientation * tform4::translate({-from.x, -from.y, -from.z});
}

ray ray_for_pixel(camera const& cam, float px, float py) noexcept {
    // offset from edge of canvas to pixel center
    float const xoffset = (px + 0.5f) * cam.pixel_size;
    float const yoffset = (py + 0.5f) * cam.pixel_size;

    // cam looks towards -z, x is to the left
    float const world_x = cam.half_width - xoffset;
    float const world_y = cam.half_height - yoffset;
    assert(-1.f <= world_x && world_x <= 1.f);
    assert(-1.f <= world_y && world_y <= 1.f);

    tform4 const inv_cam_tform = inverse(cam.tform);
    // canvas is at z=-1
    pnt3 const pixel = inv_cam_tform * pnt3{world_x, world_y, -1};
    pnt3 const origin = inv_cam_tform * pnt3{0, 0, 0};
    vec3 const direction = normalize(pixel - origin);
    return ray{origin, direction};
}

If I negate the true_up vector, as commented out, then the image is correct:

Obviously, if I negate it, some tests fail. This is a workaround, or one bug undoing the problem caused by another bug.

If I understand correctly, forward designates Z axis, which grows into the screen (assuming left hand coordinate system, X growing right and Y growing up). Then left vector designates X, since it is the result of the left hand coordinate cross product of Z and Y. Then true_up designates X and Z cross product. Flipping the sign on true_up cross is the same as making the cross of Z and X, which would point the Y axis downwards. But this is obviously wrong.

My render() is:

canvas render(camera const& cam, world const& w) noexcept {
    canvas image{cam.hsize, cam.vsize};
    image.fill({0, 0, 0});
    for (int y = cam.vsize - 1; y >= 0; --y) {
        for (unsigned x = 0; x < cam.hsize - 1; ++x) {
            image(x, y) = color_at(w, ray_for_pixel(cam, x, y));
        }
    }
    return image;
}

My canvas’s (x,y) addressing differs from the book. In the book, the canvas’s X grows right, Y grows down, like in 2D array. Since this is difficult to reason about, my canvas’s abstraction is that X grows right, and Y grows up, same as the left hand coordinate system. But it is also implemented as 2D array (addressed as 1D array), where Y grows down, so I transform the coordinates inside the canvas implementation:

canvas::canvas(unsigned int w, unsigned int h) noexcept
    : canvas_{std::make_unique<color[]>(w * h)}, w_{w}, h_{h} {}

color& canvas::operator()(unsigned int x, unsigned int y) noexcept {
    // Internally, X grows right, but Y grows down, hence the conversion
    return canvas_[(h_ - y - 1) * w_ + x];
}

This could be the reason, but I don’t think so.

Any ideas what could be going wrong? The full code is here.

First Post!

gautierdag

gautierdag

@jamis

Curious, what was the bug with this in the end?

Where Next?

Popular Pragmatic Bookshelf topics Top

jon
Some minor things in the paper edition that says “3 2020” on the title page verso, not mentioned in the book’s errata online: p. 186 But...
New
sdmoralesma
Title: Web Development with Clojure, Third Edition - migrations/create not working: p159 When I execute the command: user=&gt; (create-...
New
edruder
I thought that there might be interest in using the book with Rails 6.1 and Ruby 2.7.2. I’ll note what I needed to do differently here. ...
New
herminiotorres
Hi @Margaret , On page VII the book tells us the example and snippets will be all using Elixir version 1.11 But on page 3 almost the en...
New
Mmm
Hi, build fails on: bracket-lib = “~0.8.1” when running on Mac Mini M1 Rust version 1.5.0: Compiling winit v0.22.2 error[E0308]: mi...
New
hazardco
On page 78 the following code appears: &lt;%= link_to ‘Destroy’, product, class: ‘hover:underline’, method: :delete, data: { confirm...
New
jwandekoken
Book: Programming Phoenix LiveView, page 142 (157/378), file lib/pento_web/live/product_live/form_component.ex, in the function below: d...
New
andreheijstek
After running /bin/setup, the first error was: The foreman' command exists in these Ruby versions: That was easy to fix: gem install fore...
New
a.zampa
@mfazio23 I’m following the indications of the book and arriver ad chapter 10, but the app cannot be compiled due to an error in the Bas...
New
roadbike
From page 13: On Python 3.7, you can install the libraries with pip by running these commands inside a Python venv using Visual Studio ...
New

Other popular topics Top

DevotionGeo
I know that -t flag is used along with -i flag for getting an interactive shell. But I cannot digest what the man page for docker run com...
New
New
AstonJ
Just done a fresh install of macOS Big Sur and on installing Erlang I am getting: asdf install erlang 23.1.2 Configure failed. checking ...
New
Maartz
Hi folks, I don’t know if I saw this here but, here’s a new programming language, called Roc Reminds me a bit of Elm and thus Haskell. ...
New
mafinar
This is going to be a long an frequently posted thread. While talking to a friend of mine who has taken data structure and algorithm cou...
New
PragmaticBookshelf
Author Spotlight James Stanier @jstanier James Stanier, author of Effective Remote Work , discusses how to rethink the office as we e...
New
PragmaticBookshelf
Author Spotlight Mike Riley @mriley This month, we turn the spotlight on Mike Riley, author of Portable Python Projects. Mike’s book ...
New
New
DevotionGeo
I have always used antique keyboards like Cherry MX 1800 or Cherry MX 8100 and almost always have modified the switches in some way, like...
New
PragmaticBookshelf
Fight complexity and reclaim the original spirit of agility by learning to simplify how you develop software. The result: a more humane a...
New

Sub Categories: