On 2014-06-02, 4:59 AM, Robert O'Callahan wrote:
On Mon, Jun 2, 2014 at 3:19 PM, Rik Cabanier <caban...@gmail.com> wrote:
isIdentity() indeed suffers from rounding errors but since it's useful, I'm
hesitant to remove it.
In our rendering libraries at Adobe, we check if a matrix is *almost*
identity. Maybe we can do the same here?
One option would be to make "isIdentity" and "is2D" state bits in the
object rather than predicates on the matrix coefficients. Then for each
matrix operation, we would define how it affects the isIdentity and is2D
bits. For example we could say translate(tx, ty, tz)'s result isIdentity if
and only if the source matrix isIdentity and tx, ty and tz are all exactly
0.0, and the result is2D if and only if the source matrix is2D and tz is
exactly 0.0.
With that approach, isIdentity and is2D would be much less sensitive to
precision issues. In particular they'd be independent of the precision used
to compute and store store matrix elements, which would be helpful I think.
I agree that most mathematical ways of determining a matrix (as a
rotation, or a translation, etc) come with isIdentity for free; but are
most matrices derived from some underlying transformation, or are they
given as a list of coefficients?
If the latter, the isIdentity flag needs to be determined by the
constructor, or fed as a parameter. Exactly how does the constructor
determine the parameter? Exactly how does the user?
Nick
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