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

abtin
page 20: … protoc command… I had to additionally run the following go get commands in order to be able to compile protobuf code using go...
New
mikecargal
Title: Hands-on Rust: question about get_component (page 295) (feel free to respond. “You dug you’re own hole… good luck”) I have somet...
New
rmurray10127
Title: Intuitive Python: docker run… denied error (page 2) Attempted to run the docker command in both CLI and Powershell PS C:\Users\r...
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
s2k
Hi all, currently I wonder how the Tailwind colours work (or don’t work). For example, in app/views/layouts/application.html.erb I have...
New
tkhobbes
After some hassle, I was able to finally run bin/setup, now I have started the rails server but I get this error message right when I vis...
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
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
davetron5000
Hello faithful readers! If you have tried to follow along in the book, you are asked to start up the dev environment via dx/build and ar...
New

Other popular topics Top

PragmaticBookshelf
Machine learning can be intimidating, with its reliance on math and algorithms that most programmers don't encounter in their regular wor...
New
Exadra37
I am thinking in building or buy a desktop computer for programing, both professionally and on my free time, and my choice of OS is Linux...
New
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
PragmaticBookshelf
Learn different ways of writing concurrent code in Elixir and increase your application's performance, without sacrificing scalability or...
New
AstonJ
Continuing the discussion from Thinking about learning Crystal, let’s discuss - I was wondering which languages don’t GC - maybe we can c...
New
AstonJ
Saw this on TikTok of all places! :lol: Anyone heard of them before? Lite:
New
PragmaticBookshelf
Build efficient applications that exploit the unique benefits of a pure functional language, learning from an engineer who uses Haskell t...
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
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
PragmaticBookshelf
A concise guide to MySQL 9 database administration, covering fundamental concepts, techniques, and best practices. Neil Smyth MySQL...
New

Sub Categories: