jiricodes

jiricodes

The Ray Tracer Challenge: Capped cone normal vector (page 190)

Hi @jamis,
I’m not sure if this has been discussed elsewhere, so apologies for a potential duplicate.

I refer to cone normal calculation at page 190, where author states "

Lastly, for the normal vector, compute the end cap normals just as you did for the cylinder, but …"

This unfortunately doesn’t work since the code for cylinder cap normal assumes constant diameter of 1.0. Instead one should calculate an actual distance from y axis (including sqrt) and check it against the currently considered y limit’s absolute value.

Pseudo codes and tests

Assumed original pseudo-code (combining cylinder and cone):

function local_normal_at(cone, point)
    #part from the cylinder caps check
    dist = point.x ^ 2 + point.z ^ 2
    # if squared distance is < 1.0 then distance is also < 1.0 therefore we don't need sqrt here
    if dist < 1 and point.y >= cone.maximum - EPSILON
         return vector(0, 1, 0)
    else if dist < 1 and point.y <= cone.minimum + EPSILON
       return vector(0, -1, 0)
    else
        y = sqrt(point.x ^ 2 + point.z ^ 2)
       if point.y > 0.0
           y = -y
       return (point.x, y, point.z)

Test that fails with above example:

Scenario Outline: Computing the normal vector on a capped cone
    Given shape = cone()
    And shape.minimum = -1
    And shape.maximum = 2
    And p  = point(1.5, 2, 0.0)
    When n = local_normal_at(shape, p)
    Then n = point(0, 1, 0)

This test assumes a point that should be located at the upper cone cap with distance from y axis larger than 1.0. In this case we expect the cap’s (plane) normal to be returned.\

With the above pseudocode, we get instead vector(1.5, -1.5, 0)

Pseudocode of a proposed fix:

function local_normal_at(cone, point)
    #calculate distance from y axis,
    dist = sqrt(point.x ^ 2 + point.z ^ 2)
    if dist < cone.maximum and point.y >= cone.maximum - EPSILON
         return vector(0, 1, 0)
    else if dist < cone.minimum and point.y <= cone.minimum + EPSILON
       return vector(0, -1, 0)
    else
        # note that y calculation is same as dist, we can potentially reuse the value here
        y = sqrt(point.x ^ 2 + point.z ^ 2)
       if point.y > 0.0
           y = -y
       return (point.x, y, point.z)

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
ianwillie
Hello Brian, I have some problems with running the code in your book. I like the style of the book very much and I have learnt a lot as...
New
Alexandr
Hi everyone! There is an error on the page 71 in the book “Programming machine learning from coding to depp learning” P. Perrotta. You c...
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
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
cro
I am working on the “Your Turn” for chapter one and building out the restart button talked about on page 27. It recommends looking into ...
New
leonW
I ran this command after installing the sample application: $ cards add do something --owner Brian And got a file not found error: Fil...
New
adamwoolhether
Is there any place where we can discuss the solutions to some of the exercises? I can figure most of them out, but am having trouble with...
New
dtonhofer
@parrt In the context of Chapter 4.3, the grammar Java.g4, meant to parse Java 6 compilation units, no longer passes ANTLR (currently 4....
New
New

Other popular topics Top

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
siddhant3030
I’m thinking of buying a monitor that I can rotate to use as a vertical monitor? Also, I want to know if someone is using it for program...
New
dasdom
No chair. I have a standing desk. This post was split into a dedicated thread from our thread about chairs :slight_smile:
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
Do the test and post your score :nerd_face: :keyboard: If possible, please add info such as the keyboard you’re using, the layout (Qw...
New
foxtrottwist
A few weeks ago I started using Warp a terminal written in rust. Though in it’s current state of development there are a few caveats (tab...
New
AstonJ
Was just curious to see if any were around, found this one: I got 51/100: Not sure if it was meant to buy I am sure at times the b...
New
New
New
sir.laksmana_wenk
I’m able to do the “artistic” part of game-development; character designing/modeling, music, environment modeling, etc. However, I don’t...
New

Sub Categories: