Book 2.8: Optional simplification of box() face construction

[xiaodoliu]

The box() helper constructs the six faces of the box by explicitly recombining coordinates, for example:

point3(min.x(), min.y(), max.z())

However, since the function already computes the axis-aligned edge vectors dx, dy, and dz, the six faces can alternatively be constructed symmetrically using only min, max, and these edge vectors:

sides->add(make_shared<quad>(min, dx, dy, mat)); // back
sides->add(make_shared<quad>(min, dx, dz, mat)); // bottom
sides->add(make_shared<quad>(min, dy, dz, mat)); // left
sides->add(make_shared<quad>(max, -dx, -dy, mat)); // front
sides->add(make_shared<quad>(max, -dx, -dz, mat)); // top
sides->add(make_shared<quad>(max, -dy, -dz, mat)); // right

[hollasch]

I like the idea. The proposed constructors, however, have several flipped faces. I believe the correct code would be

sides->add(make_shared<quad>(min, dy, dx, mat)); // back (corrected)
sides->add(make_shared<quad>(min, dx, dz, mat)); // bottom
sides->add(make_shared<quad>(min, dy, dz, mat)); // left (corrected)
sides->add(make_shared<quad>(max, -dx, -dy, mat)); // front
sides->add(make_shared<quad>(max, -dz, -dx, mat)); // top (corrected)
sides->add(make_shared<quad>(max, -dy, -dz, mat)); // right

Do you agree?

[xiaodoliu]

I like the idea. The proposed constructors, however, have several flipped faces. I believe the correct code would be

sides->add(make_shared<quad>(min, dy, dx, mat)); // back (corrected)
sides->add(make_shared<quad>(min, dx, dz, mat)); // bottom
sides->add(make_shared<quad>(min, dy, dz, mat)); // left (corrected)
sides->add(make_shared<quad>(max, -dx, -dy, mat)); // front
sides->add(make_shared<quad>(max, -dz, -dx, mat)); // top (corrected)
sides->add(make_shared<quad>(max, -dy, -dz, mat)); // right

Do you agree?

I agree with that.
In particular, for the left face it should be:
sides->add(make_shared<quad>(min, dz, dy, mat)); // left

[hollasch]

Good catch! I somehow fumbled the correction.

[hollasch]

Final version:

sides->add(make_shared<quad>(min, dy, dx, mat)); // back
sides->add(make_shared<quad>(min, dx, dz, mat)); // bottom
sides->add(make_shared<quad>(min, dz, dy, mat)); // left
sides->add(make_shared<quad>(max, -dx, -dy, mat)); // front
sides->add(make_shared<quad>(max, -dz, -dx, mat)); // top
sides->add(make_shared<quad>(max, -dy, -dz, mat)); // right

[xiaodoliu]

@hollasch. I’d be happy to submit a PR for this change if you’d like me to proceed.

[hollasch]

Sure. Make sure to change all affected code listings (all books), plus any associated text, plus add an entry to the changelog.

[hollasch]

Also add an entry in books/acknowledgments.html if you're not already there.