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Re: [Chicken-users] canvas-draw and colors

From: Matt Welland
Subject: Re: [Chicken-users] canvas-draw and colors
Date: Sat, 16 Jul 2016 13:39:34 -0700

On Sat, Jul 16, 2016 at 1:04 PM, Thomas Chust <address@hidden> wrote:
On 2016-07-16 21:31, Matt Welland wrote:
> [...]
> (define (vg:rgb->number r g b #!key (a 0)) (u32vector-ref
> (blob->u32vector (u8vector->blob (list->u8vector (list a r g b)))) 0))
> [...]


this snippet seems somewhat sub-optimal to say the least. Apart from
being needlessly complicated, it may also be wrong because its result
depends on the endianness of the host platform.

I would suggest to simply use bitwise arithmetic:

    (arithmetic-shift a 24)
    (arithmetic-shift r 16)
    (arithmetic-shift g 8)

Nice. Blindingly obvious now that it has been pointed out. Interestingly enough it also appears to be 4x faster.

Or perhaps just normal arithmetic replacing bitwise-ior by + and
arithmetic-shift by (lambda (x n) (* x (expt 2 n))). This is probably no
less efficient, as the result is going to be a bignum anyway.


When C++ is your hammer, every problem looks like your thumb.

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