One Quickie

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CGShading (Graphics->General)

CGShading lets you do color ramps. Setting one up is a little complicated, juggling a couple of CG data structures and writing the ramping function.

    float domain[2] = { 0.0, 1.0 };     // 1-in function
    float range[8] = { 0.0, 1.0,        // N-out, RGBA
                       0.0, 1.0,
                       0.0, 1.0,
                       0.0, 1.0 };
    CGFunctionCallbacks callbacks = { 0, evaluate, NULL };

    CGFunctionRef shaderFunction;
    shaderFunction = CGFunctionCreate (self,    // info / rock / context
                                       1,       // # of inputs for domain
                                       domain,  // domain
                                       4,       // # of inputs for range
                                       range,   // range
                                       &callbacks);

    CGColorSpaceRef deviceRGB;
    deviceRGB = CGColorSpaceCreateDeviceRGB ();

    CGShadingRef shader;
    shader = CGShadingCreateAxial (deviceRGB,       // colorspace
                                   cgpoint(start),  // start of axis
                                   cgpoint(end),    // end of axis
                                   shaderFunction,  // shader, 1-n, n-out
                                   NO,              // extend start
                                   NO);             // extend end

    CGContextSaveGState (context); {
        NSRect bounds = [self bounds];
        CGContextClipToRect (context, cgrect(bounds));
        CGContextDrawShading (context, shader);
    } CGContextRestoreGState (context);

    [self drawSpotAt: start  size: 4];
    [self drawSpotAt: end  size: 4];

    CGFunctionRelease (shaderFunction);
    CGColorSpaceRelease (deviceRGB);
    CGShadingRelease (shader);

And the evaluator function is given an array of inputs and outputs. Use the in value(s) (which run from your domain's start-to-finish values) to generate the out values (which should be in the range's start-to-finish values):

static void evaluate (void *info, const float *in, float *out)
{
    float thing;
    thing = in[0];

    out[0] = thing;
    out[1] = thing;
    out[2] = thing;
    out[3] = 1.0;

} // evaluate