QMatrix4x4 multiply order

[QT-static-prgm]

Hi,

I have a question about the QMatrix4x4 class.

This example code:

QMatrix4x4 test;
test.perspective(...); // projection
test.lookat(...); // view

What kind of matrix is test at the end? is it something like that:
identity * projection * view
or is it like that:
view * projection * identity

[kshegunov]

@QT-static-prgm said in QMatrix4x4 multiply order:

Since matrices don't commute in the general case and operate on the right-hand term I'd say the latter.

[VRonin]

It's quite easy just to test it...
run

QMatrix4x4 test;
test.perspective(...); // projection

check what is in test.

QMatrix4x4 test;
test.lookat(...); // view

check what is in test.
pre and post multiply these two by hand (you can use wolframalpha) and compare it to the result you get in

QMatrix4x4 test;
test.perspective(...); // projection
test.lookat(...); // view

[QT-static-prgm]

QMatrix4x4 tmp = ...;
tmp.translate(10,0,0);

results in

tmp = tmp * translationmatrix

So the correct answer seams to be

1. identity * projection * view