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

iPaul
page 37 ANTLRInputStream input = new ANTLRInputStream(is); as of ANTLR 4 .8 should be: CharStream stream = CharStreams.fromStream(i...
New
johnp
Hi Brian, Looks like the api for tinydb has changed a little. Noticed while working on chapter 7 that the .purge() call to the db throws...
New
jamis
The following is cross-posted from the original Ray Tracer Challenge forum, from a post by garfieldnate. I’m cross-posting it so that the...
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
herminiotorres
Hi! I know not the intentions behind this narrative when called, on page XI: mount() |&gt; handle_event() |&gt; render() but the correc...
New
alanq
This isn’t directly about the book contents so maybe not the right forum…but in some of the code apps (e.g. turbo/06) it sends a TURBO_ST...
New
patoncrispy
I’m new to Rust and am using this book to learn more as well as to feed my interest in game dev. I’ve just finished the flappy dragon exa...
New
AndyDavis3416
@noelrappin Running the webpack dev server, I receive the following warning: ERROR in tsconfig.json TS18003: No inputs were found in c...
New
mert
AWDWR 7, page 152, page 153: Hello everyone, I’m a little bit lost on the hotwire part. I didn’t fully understand it. On page 152 @rub...
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

Other popular topics Top

AstonJ
If it’s a mechanical keyboard, which switches do you have? Would you recommend it? Why? What will your next keyboard be? Pics always w...
New
Exadra37
Please tell us what is your preferred monitor setup for programming(not gaming) and why you have chosen it. Does your monitor have eye p...
New
AstonJ
Curious to know which languages and frameworks you’re all thinking about learning next :upside_down_face: Perhaps if there’s enough peop...
New
AstonJ
You might be thinking we should just ask who’s not using VSCode :joy: however there are some new additions in the space that might give V...
New
AstonJ
Inspired by this post from @Carter, which languages, frameworks or other tech or tools do you think is killing it right now? :upside_down...
New
dimitarvp
Small essay with thoughts on macOS vs. Linux: I know @Exadra37 is just waiting around the corner to scream at me “I TOLD YOU SO!!!” but I...
New
AstonJ
If you are experiencing Rails console using 100% CPU on your dev machine, then updating your development and test gems might fix the issu...
New
PragmaticBookshelf
Use WebRTC to build web applications that stream media and data in real time directly from one user to another, all in the browser. ...
New
PragmaticBookshelf
Rails 7 completely redefines what it means to produce fantastic user experiences and provides a way to achieve all the benefits of single...
New
AstonJ
This is a very quick guide, you just need to: Download LM Studio: https://lmstudio.ai/ Click on search Type DeepSeek, then select the o...
New

Sub Categories: