Drawing gradient over image in ios

问题:

How to create gradient colour look like following image programatically.

回答1:

When you say "apply it over the image as a gradient", do you mean as a mask (revealing the image at the top, having it fade the image to transparent at the bottom)? If that's the case, you can apply that gradient as a mask, using CAGradientLayer:

CAGradientLayer *gradientMask = [CAGradientLayer layer];
gradientMask.frame = self.imageView.bounds;
gradientMask.colors = @[(id)[UIColor whiteColor].CGColor,
                        (id)[UIColor clearColor].CGColor];
self.imageView.layer.mask = gradientMask;

---

The above does a simple vertical gradient (because the default is vertical, linear gradient). But you asked about startPoint, endPoint, and locations. If for example, you wanted your mask applied horizontally, you would do:

gradientMask.startPoint = CGPointMake(0.0, 0.5);   // start at left middle
gradientMask.endPoint = CGPointMake(1.0, 0.5);     // end at right middle

If you wanted to have two gradients, one at the first 10% and another at the last 10%, you'd do:

gradientMask.colors = @[(id)[UIColor clearColor].CGColor,
                        (id)[UIColor whiteColor].CGColor,
                        (id)[UIColor whiteColor].CGColor,
                        (id)[UIColor clearColor].CGColor];
gradientMask.locations = @[@0.0, @0.10, @0.90, @1.0];

---

If you want a simple gradient by itself (not as a mask), you'd create a view and then add the gradient layer to it:

CAGradientLayer *gradient = [CAGradientLayer layer];
gradient.frame = view.bounds;
gradient.colors = @[(id)[UIColor whiteColor].CGColor,
                    (id)[UIColor blackColor].CGColor];
[view.layer addSublayer:gradient];

See the CAGradientLayer class reference.

回答2:

I just wrote an UIImage extension for Swift 2.0. Maybe it's of some use. You call it with an array of UIColor (any number) and a frame where the gradient should be drawn.

extension UIImage {

    class func convertGradientToImage(colors: [UIColor], frame: CGRect) -> UIImage {

        // start with a CAGradientLayer
        let gradientLayer = CAGradientLayer()
        gradientLayer.frame = frame

        // add colors as CGCologRef to a new array and calculate the distances
        var colorsRef = [CGColor]()
        var locations = [NSNumber]()

        for i in 0 ... colors.count-1 {
            colorsRef.append(colors[i].CGColor as CGColorRef)
            locations.append(Float(i)/Float(colors.count-1))
        }

        gradientLayer.colors = colorsRef
        gradientLayer.locations = locations

        // now build a UIImage from the gradient
        UIGraphicsBeginImageContext(gradientLayer.bounds.size)
        gradientLayer.renderInContext(UIGraphicsGetCurrentContext()!)
        let gradientImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        // return the gradient image
        return gradientImage
    }
}

Call it like this:

let colors = [
    UIColor.blueColor(),
    UIColor.greenColor()
    // and many more if you wish
]
let gradientImage = UIImage.convertGradientToImage(colors, frame: navigationBar.bounds)

and apply with:

.backgroundColor = UIColor(patternImage: gradientImage)

or

.setBackgroundImage(gradientImage, forBarMetrics: .Default)

回答3:

CAGradientLayer *gradient = [CAGradientLayer layer];
gradient.frame = self.view.bounds;


gradient.startPoint = CGPointMake(1.0, 1.0); //Dark From bottom
gradient.endPoint = CGPointMake(1.0, 0);


gradient.colors = [NSArray arrayWithObjects:
                   (id)[[UIColor blackColor] CGColor],
                   (id)[[UIColor clearColor] CGColor], nil];


[self.view.layer insertSublayer:gradient atIndex:0];

回答4:

This is the best approach, working for me as requirement

CAGradientLayer *gradientLayer = [CAGradientLayer layer];
gradientLayer.frame = yourImageView.layer.bounds;

gradientLayer.colors = [NSArray arrayWithObjects:
                        (id)[UIColor colorWithWhite:1.0f alpha:1.0f].CGColor,
                        (id)[UIColor colorWithWhite:0.0f alpha:0.9f].CGColor,
                        nil];

gradientLayer.locations = [NSArray arrayWithObjects:
                           [NSNumber numberWithFloat:0.0f],
                           [NSNumber numberWithFloat:1.0f],
                           nil];

[yourImageView.layer addSublayer:gradientLayer];

回答5:

-(void) drawGradientinBounds: (CGRect) currentBounds withColors:(NSArray*) colors andPercentages:(NSArray *)percentages andGradientDirectionIsVertical:(BOOL)isGradientDirectionVertical
{

    CGContextRef currentContext = UIGraphicsGetCurrentContext();

    CGGradientRef glossGradient;
    CGColorSpaceRef rgbColorspace;

    size_t num_locations = [percentages count];
    CGFloat locations[num_locations];
    for(int i=0;i<num_locations;i++)
        locations[i] = [[percentages objectAtIndex:i] floatValue];

    int comps = [colors count]*4;
    CGFloat components[comps];
    for(int i = [colors count]-1;i>=0;i--)
    {
        comps--;
        UIColor *c = [colors objectAtIndex:i];
        const CGFloat *cg = CGColorGetComponents([c CGColor]);
        components[comps] = cg[3];
        comps--;
        components[comps] = cg[2];
        comps--;
        components[comps] = cg[1];
        comps--;
        components[comps] = cg[0];
    }

    rgbColorspace = CGColorSpaceCreateDeviceRGB();
    glossGradient = CGGradientCreateWithColorComponents(rgbColorspace, components, locations, num_locations);

    CGPoint topCenter;
    CGPoint endCenter;

    if(isGradientDirectionVertical)
    {
        topCenter = CGPointMake(CGRectGetMidX(currentBounds), currentBounds.origin.y);
        endCenter = CGPointMake(CGRectGetMidX(currentBounds), CGRectGetHeight(currentBounds)+currentBounds.origin.y);

    }
    else
    {
        topCenter = CGPointMake( currentBounds.origin.x,CGRectGetMidY(currentBounds));
        endCenter = CGPointMake(CGRectGetWidth(currentBounds)+currentBounds.origin.x,CGRectGetMidY(currentBounds));
    }
    CGContextDrawLinearGradient(currentContext, glossGradient, topCenter, endCenter, 0);

    CGGradientRelease(glossGradient);
    CGColorSpaceRelease(rgbColorspace);
}

And the input to this method are bounds, colorArray:

[NSArray arrayWithObjects:[UIColor blackColor], [UIColor whiteColor], nil]

percentageArray:

[NSArray arrayWithObjects:[NSNumber numberWithFloat:0.0], [NSNumber numberWithFloat:1.0], nil]