Scheme Raytracer Optimized

Marc Feeley, the creator of Gambit Scheme, tweaked my raytracer and got it to run twice as fast. Here's what he said about it:

"The main thing that helped performance is the avoidance of mixed type arithmetic, that is cases when an operator is called with numbers of different types, such as

   (let ((x (sqrt 1.5))) (* x 2))

These mixed-type calls are not handled efficiently. To properly
implement the semantics of Scheme, they are implemented with an
actual function call to the library. Here the (* x 2) will actually
call the "*" function in the library. One transformation I applied
to your code is to convert integer constants to a floating point
value, when the other operand was also a floating point number. For
example:

   (let ((x (sqrt 1.5))) (* x 2.0))

"

Simply changing the numeric operations to use explicit types indeed increases the speed 2.2x. The second and third image in my previous post now only take ~186s to render instead of ~415s. Impressive! If I optimized the math and tweaked it even further, we'll have a pretty fast raytracer on our hands.

Edit: I meant to post the new source code, you can find it here: schemeray-0.2.tar.gz