How to create UIImage with vertical gradient using “from-color” and “to-color”

Question

How to create new image just with gradient colors, using \"from-color\" and \"to-color\"?

Answer 1

Here's how I implemented Simon's suggestion (elsewhere in this thread) to create a reusable "hot" and "cold" UIImage like below:

Objective-C code:

-(UIImage*)createHotOrColdImage:(BOOL)bHot
{
    //  Create a UIImage with either a "Hot" or "Cold" gradient background
    //
    const int WIDTH = 75;
    const int HEIGHT = 44;

    //  Do we want our UIImage to fade from black-to-red or black-to-blue ?
    UIColor* color = (bHot) ? [UIColor redColor] : [UIColor colorWithRed:0.3 green:0.3 blue:1.0 alpha:1.0];

    CGSize size = CGSizeMake(WIDTH, HEIGHT);
    UIGraphicsBeginImageContextWithOptions(size, NO, 0);


    const CGFloat *components = CGColorGetComponents(color.CGColor);
    CGFloat red = components[0];
    CGFloat green = components[1];
    CGFloat blue = components[2];

    size_t gradientNumberOfLocations = 4;
    CGFloat gradientLocations[4] = { 0.0, 0.3, 0.7, 1.0 };
    CGFloat gradientComponents[16] = { red, green, blue, 0,
        red, green, blue, 1,
        red, green, blue, 1,
        red, green, blue, 0 }; 

    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
    CGGradientRef gradient = CGGradientCreateWithColorComponents (colorspace, gradientComponents, gradientLocations, gradientNumberOfLocations);

    //  Create a UIImage containing this gradient
    CGContextRef context = UIGraphicsGetCurrentContext();

    //  Make sure the gradient is vertical
    CGContextDrawLinearGradient(context, gradient, CGPointMake(0, 0), CGPointMake(0, HEIGHT), 0);

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    CGGradientRelease(gradient);
    CGColorSpaceRelease(colorspace);
    UIGraphicsEndImageContext();

    return image;
}

To use:

UIImage* hotImage = [self createHotOrColdImage:TRUE];
UIImage* coldImage = [self createHotOrColdImage:FALSE];

Answer 2

UPDATED TO Swift 3

I wrote UIImage extension for Swift but you can also use it from Objective-C:

import UIKit

private let ChannelDivider: CGFloat = 255

public class RGBA: NSObject {
    var red: CGFloat
    var green: CGFloat
    var blue: CGFloat
    var alpha: CGFloat

    init(red: CGFloat, green: CGFloat, blue: CGFloat, alpha: CGFloat) {
        self.red = red
        self.green = green
        self.blue = blue
        self.alpha = alpha
    }

    init(intRed: Int, green: Int, blue: Int, alpha: Int) {
        self.red = CGFloat(intRed)/ChannelDivider
        self.green = CGFloat(green)/ChannelDivider
        self.blue = CGFloat(blue)/ChannelDivider
        self.alpha = CGFloat(alpha)/ChannelDivider
    }
}

public class Grayscale: NSObject {
    var white: CGFloat
    var alpha: CGFloat

    init(white: CGFloat, alpha: CGFloat) {
        self.white = white
        self.alpha = alpha
    }
}

public class GradientPoint<C>: NSObject {
    var location: CGFloat
    var color: C

    init(location: CGFloat, color: C) {
        self.location = location
        self.color = color
    }
}

extension UIImage {

    public class func image(withGradientPoints gradientPoints: [GradientPoint<[CGFloat]>], colorSpace: CGColorSpace, size: CGSize) -> UIImage? {
        UIGraphicsBeginImageContextWithOptions(size, false, 0);
        guard
            let context = UIGraphicsGetCurrentContext(),
            let gradient = CGGradient(colorSpace: colorSpace,
                                      colorComponents: gradientPoints.flatMap { $0.color },
                                      locations: gradientPoints.map { $0.location }, count: gradientPoints.count) else {
                                        return nil
        }

        context.drawLinearGradient(gradient, start: CGPoint.zero, end: CGPoint(x: 0, y: size.height), options: CGGradientDrawingOptions())
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return image
    }

    public class func image(withRGBAGradientPoints gradientPoints: [GradientPoint<RGBA>], size: CGSize) -> UIImage? {
        return image(withGradientPoints: gradientPoints.map {
            GradientPoint(location: $0.location, color: [$0.color.red, $0.color.green, $0.color.blue, $0.color.alpha])
        }, colorSpace: CGColorSpaceCreateDeviceRGB(), size: size)
    }

    public class func image(withRGBAGradientColors gradientColors: [CGFloat: RGBA], size: CGSize) -> UIImage? {
        return image(withRGBAGradientPoints: gradientColors.map {  GradientPoint(location: $0, color: $1)}, size: size)
    }

    public class func image(withGrayscaleGradientPoints gradientPoints: [GradientPoint<Grayscale>], size: CGSize) -> UIImage? {
        return image(withGradientPoints: gradientPoints.map {
            GradientPoint(location: $0.location, color: [$0.color.white, $0.color.alpha]) },
                     colorSpace: CGColorSpaceCreateDeviceGray(), size: size)
    }

    public class func image(withGrayscaleGradientColors gradientColors: [CGFloat: Grayscale], size: CGSize) -> UIImage? {
        return image(withGrayscaleGradientPoints: gradientColors.map { GradientPoint(location: $0, color: $1) }, size: size)
    }
}

You can create gradient image using RGBA colors:

// Objective-C
RGBA *startColor = [[RGBA alloc] initWithRed:1 green:0 blue:0 alpha:1];
RGBA *endColor = [[RGBA alloc] initWithIntRed:0 green:255 blue:0 alpha:255];
UIImage *gradientImage = [UIImage imageWithRGBAGradient: @{ @0: startColor, @1: endColor} size: CGSizeMake(32, 64)];

// Swift
let startColor = RGBA(red: 1, green: 0, blue: 0, alpha: 1)
let endColor = RGBA(intRed: 0, green: 255, blue: 0, alpha: 255)
let gradientImage = UIImage.image(withRGBAGradientPoints: [0: startColor, 1: endColor], size: CGSizeMake(32, 64))

And grayscale colors:

// Objective-C
Grayscale *startColor = [[Grayscale alloc] initWithWhite:1 alpha:1];
Grayscale *endColor = [[Grayscale alloc] initWithWhite:0 alpha: 0.5];
UIImage *gradientImage = [UIImage imageWithGrayscaleGradient: @{ @0: startColor, @1: endColor} size: CGSizeMake(32, 64)];

// Swift
let startColor = Grayscale(white: 1, alpha: 1)
let endColor = Grayscale(white:0, alpha: 0.5)
let gradientImage = UIImage.image(withGrayscaleGradientPoints: [0: startColor, 1: endColor], size: CGSizeMake(32, 64))

If you are not going to use this code from Objective-C than you can remove NSObject inheritance from RGBA, Grayscale and GradientPoint.

Answer 3

Xamarin solution:

    private UIImage GenerateBackgroundGradient(CGSize size, CGColor[] colors)
    {
        UIImage backgroundGradient = null;

        var layer = new CAGradientLayer
        {
            Frame = View.Frame,
            Colors = colors
        };

        UIGraphics.BeginImageContext(size);
        layer.RenderInContext(UIGraphics.GetCurrentContext());
        backgroundGradient = UIGraphics.GetImageFromCurrentImageContext();
        UIGraphics.EndImageContext();

        return backgroundGradient;
    }