Subject: Another excellent question reg'd sp4

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>> Dr. Westall,
>>
>> I'm having complications with my map_pix_to_world function. I'm
>> using a method similar to the one in your notes on page 61:
>>
>>    map_pix_to_world(proj_t proj*, int x, int y, double *world)
>>
>> However, the one defined in rayhdrs.h is as follows:
>>
>>    map_pix_to_world(int x, int y, double *world)
>>
>> Do I need to change my function so that it doesn't take proj_t
>> *proj as a parameter or is this an issue with rayhdrs.h? Thank
>> you for you time.

This little dilemma is one that is commonly encountered
in C programming -- whether to trade off good design principles
for a bit of extra performance.   The notes reflect good design.

The sample "rayhdrs.h" came from my own raytracer in which I
decided to try to make it run a bit faster.  There is no "right"
answer here and either way is fine.  The only thing that is not
fine here is compiler warnings!  So if you use the notes approach
you MUST change rayhdrs.h.

If you use the Westall implementation approach you obviously
still need access to the proj_t data structure that defines
the projection.  So how do you do that?  The way I did it was
to define

static proj_t *proj = NULL;  // static variable where we keep proj ptr

at the top of my projection.c module outside the body
of all functions... Some might say eeeewwwwwwwwweee "global variable".
But because its static its global ONLY to the functions in
projection.c --- more on this later in the course.

Anyhow,  in projection_init() I do

    proj = malloc(sizeof(proj_t));

but I DON"T create a local variable named proj in projection_init.

Thus when map_pix...() later refers to proj (it also DOESN"T
declare a local proj) it will get the value safely stored
there by projection_init()

mw