FPChris

FPChris

The Ray Tracer Challenge: Page 37 4x4 cofactor?

On Page 37. How to calculate the cofactor for a 4x4 matrix is never shown. Only the 3x3.
How is the author getting the cofactor(A,0,0) = 690 section? and then the determinant -4071?

Marked As Solved

FPChris

FPChris

I finally resolved this. The book is really unclear. Terrible in fact.
If anyone needs hep on this. Here is my solution below.
This YT video was a great help Finding the determinant of a 3x3 matrix method 2 | Matrices | Precalculus | Khan Academy - YouTube

//-----------------------------------------------------------------------------------
function RCCalcCofactor4x4(A,row,col){

let B = RCGetSubmatrix4x3(A,row,col); //4x4 > 3x3
let s,a;
s = RCGetSubmatrix3x2(B,0,0); //3x3 to 2x2
a = B[0][0] * RCCalcDeterminant2x2(s);
s = RCGetSubmatrix3x2(B,0,1);
a += -B[0][1] * RCCalcDeterminant2x2(s); //Note: -B
s = RCGetSubmatrix3x2(B,0,2);
a += B[0][2] * RCCalcDeterminant2x2(s);

if(isOdd(row + col)){ return -a; } //odd so negate
return a; //even - leave alone. just return it

}
//-----------------------------------------------------------------------------------
function isOdd(num) { return num % 2;}
//-----------------------------------------------------------------------------------
function RCCalcDeterminant4x4(A){
// Ch 3 - page 37
/*
A[0][0] = -2; A[0][1] = -8; A[0][2] = 3; A[0][3] = 5;
A[1][0] = -3; A[1][1] = 1; A[1][2] = 7; A[1][3] = 3;
A[2][0] = 1; A[2][1] = 2; A[2][2] = -9; A[2][3] = 6;
A[3][0] = -6; A[3][1] = 7; A[3][2] = 7; A[3][3] = -9;
*/

let d0 = RCCalcCofactor4x4(A,0,0); //690
let d1 = RCCalcCofactor4x4(A,0,1); //447
let d2 = RCCalcCofactor4x4(A,0,2); //210
let d3 = RCCalcCofactor4x4(A,0,3); //51

let d = (A[0][0] * d0) + (A[0][1] * d1) + (A[0][2] * d2) + (A[0][3] * d3); //-4071

return d;

}

Also Liked

FPChris

FPChris

Thanks. I think I struggled mostly throughout with being presented with a test and not realizing they would fail initially and that solutions where sometimes pages away. I eventually caught on to this and began reading through several more pages before coding a new topic.

Where Next?

Popular Pragmatic Bookshelf topics Top

jimmykiang
This test is broken right out of the box… — FAIL: TestAgent (7.82s) agent_test.go:77: Error Trace: agent_test.go:77 agent_test.go:...
New
iPaul
page 37 ANTLRInputStream input = new ANTLRInputStream(is); as of ANTLR 4 .8 should be: CharStream stream = CharStreams.fromStream(i...
New
simonpeter
When I try the command to create a pair of migration files I get an error. user=> (create-migration "guestbook") Execution error (Ill...
New
mikecargal
Title: Hands-On Rust (Chapter 11: prefab) Just played a couple of amulet-less games. With a bit of debugging, I believe that your can_p...
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
curtosis
Running mix deps.get in the sensor_hub directory fails with the following error: ** (Mix) No SSH public keys found in ~/.ssh. An ssh aut...
New
nicoatridge
Hi, I have just acquired Michael Fazio’s “Kotlin and Android Development” to learn about game programming for Android. I have a game in p...
New
hgkjshegfskef
The test is as follows: Scenario: Intersecting a scaled sphere with a ray Given r ← ray(point(0, 0, -5), vector(0, 0, 1)) And s ← sphere...
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
bjnord
Hello @herbert ! Trying to get the very first “Hello, Bracket Terminal!" example to run (p. 53). I develop on an Amazon EC2 instance runn...
New

Other popular topics Top

PragmaticBookshelf
Brace yourself for a fun challenge: build a photorealistic 3D renderer from scratch! In just a couple of weeks, build a ray tracer that r...
New
wolf4earth
@AstonJ prompted me to open this topic after I mentioned in the lockdown thread how I started to do a lot more for my fitness. https://f...
New
DevotionGeo
The V Programming Language Simple language for building maintainable programs V is already mentioned couple of times in the forum, but I...
New
PragmaticBookshelf
Create efficient, elegant software tests in pytest, Python's most powerful testing framework. Brian Okken @brianokken Edited by Kat...
New
rustkas
Intensively researching Erlang books and additional resources on it, I have found that the topic of using Regular Expressions is either c...
New
AstonJ
Biggest jackpot ever apparently! :upside_down_face: I don’t (usually) gamble/play the lottery, but working on a program to predict the...
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
PragmaticBookshelf
Get the comprehensive, insider information you need for Rails 8 with the new edition of this award-winning classic. Sam Ruby @rubys ...
New
AstonJ
Curious what kind of results others are getting, I think actually prefer the 7B model to the 32B model, not only is it faster but the qua...
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: