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Classes/HazardMapOverlayRenderer.m

/*

     File: HazardMapOverlayRenderer.m

 Abstract: MKOverlayRenderer subclass that renders a USGS earthquake hazard map represented by the

 HazardMap class.  This class demonstrates how to convert points represented as MKMapRects into

 the local coordinate system and then fill those rectangles to visualize earthquake hazards across

 the continental United States.

  Version: 1.2

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 MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION

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 OPERATION ALONE OR IN COMBINATION WITH YOUR PRODUCTS.

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*/

#import "HazardMapOverlayRenderer.h"

#import "HazardMap.h"

#define NUM_COLORS 14

@interface HazardMapOverlayRenderer ()

    CGColorRef *colors;

@end

#pragma mark -

@implementation HazardMapOverlayRenderer

// Create a table of possible colors to draw a grid cell with

- (void)initColors

    CGColorSpaceRef rgb = CGColorSpaceCreateDeviceRGB();

    colors = malloc(sizeof(CGColorRef) * NUM_COLORS);

    int i = 0;

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ .588, .294, .78, 1 }); // 1.00

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ .784, .471, .82, 1 }); // 0.77

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ 1, 0, 0, 1 }); // 0.59

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ 1, .392, 0, 1 }); // 0.46

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ 1, .392, 0, 1 }); // 0.35

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ 1, .784, 0, 1 }); // 0.27

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ 1, 1, .5, 1 }); // 0.21

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ .745, .941, .467, 1 }); // 0.16

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ .122, 1, .31, 1 }); // 0.12

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ .588, 1, .941, 1 }); // 0.10

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ .784, 1, 1, 1 }); // 0.08

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ .843, 1, 1, 1 }); // 0.06

    colors[i++] = CGColorCreate(rgb, (CGFloat[]){ .902, 1, 1, 1 }); // 0.04

    colors[i] = CGColorCreate(rgb, (CGFloat[]){ .784, .784, .784, 1 }); // 0.03

    CGColorSpaceRelease(rgb);

// Look up a color in the table of colors for a peak ground acceleration

- (CGColorRef)colorForAcceleration:(CGFloat)value

    if (value > 0.77) return colors[0];

    if (value > 0.59) return colors[1];

    if (value > 0.46) return colors[2];

    if (value > 0.35) return colors[3];

    if (value > 0.27) return colors[4];

    if (value > 0.21) return colors[5];

    if (value > 0.16) return colors[6];

    if (value > 0.12) return colors[7];

    if (value > 0.10) return colors[8];

    if (value > 0.08) return colors[9];

    if (value > 0.06) return colors[10];

    if (value > 0.04) return colors[11];

    if (value > 0.03) return colors[12];

    if (value > 0.02) return colors[13];

    return NULL;

- (id)initWithOverlay:(id <MKOverlay>)overlay

    if (self = [super initWithOverlay:overlay])

        [self initColors];

    return self;

- (void)dealloc

    int i;

    for (i = 0; i < NUM_COLORS; i++)

        CGColorRelease(colors[i]);

    free(colors);

- (void)drawMapRect:(MKMapRect)mapRect

          zoomScale:(MKZoomScale)zoomScale

          inContext:(CGContextRef)ctx

    HazardMap *hazards = (HazardMap *)self.overlay;

    float *values = NULL;

    MKMapRect *boundaries = NULL;

    int count = 0;

    // Fetch the grid values out of the model for this mapRect and zoomScale.

    [hazards hazardsInMapRect:mapRect

                      atScale:zoomScale

                       values:&values

                   boundaries:&boundaries

                        count:&count];

    CGContextSetAlpha(ctx, 0.5);

    // For each grid value that is colorable, color in its corresponding

    // boundary MKMapRect with the appropriate color.

    int i;

    for (i = 0; i < count; i++)

        float value = values[i];

        MKMapRect boundary = boundaries[i];

        CGColorRef color = [self colorForAcceleration:value];

        if (color) {

            CGContextSetFillColorWithColor(ctx, color);

            // Convert the MKMapRect (absolute points on the map proportional to screen points) to

            // a CGRect (points relative to the origin of this view) that can be drawn.

            CGRect boundaryCGRect = [self rectForMapRect:boundary];

            CGContextFillRect(ctx, boundaryCGRect);

    free(values);

    free(boundaries);

@end

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