Four point gradient in iOS

Question

I am planing to create a four point gradient, pictured below, by drawing two linear gradients via core graphics and masking between them with a third black and white linear gradient.

Is there a more efficient way to draw a four point gradient using core graphics, or other?

Answer 1

Draw four circles:

Apply radial transparent gradient:

Result:

Notes:

• The gray lines represent the bitmap size.
• The diameter of the circles is twice the bitmap diameter.
• Each circle is centered at one of the bitmap corner.
• Effectively only the center part is drawn.
• The remaining parts are outside the bitmap.

Answer 2

You can save the mask gradient when you use a CGBlendMode. It's just harder to control the exact colors. But if that's not important for you, it could be a little more efficient in terms of lines of code and maybe also in terms of performance.

Here's an example with some random colors and CGBlendModeExclusion (CGBlendModeDifference gives you a similar effect)

- (void) drawRect:(CGRect)rect
{
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGContextSetBlendMode(ctx, kCGBlendModeExclusion);
    CGColorSpaceRef space = CGColorSpaceCreateDeviceRGB();

    CGFloat col1[8] = {
        1.0, 0.0, 0.0, 1.0,
        0.0, 0.0, 1.0, 1.0
    };
    CGGradientRef grad1 = CGGradientCreateWithColorComponents (space, col1, NULL, 2);
    CGContextDrawLinearGradient(ctx, grad1, CGPointMake(0, 0), CGPointMake(0, 320), 0);


    CGFloat col2[8] = {
        1.0, 0.5, 0.0, 1.0,
        0.0, 1.0, 0.0, 1.0
    };
    CGGradientRef grad2 = CGGradientCreateWithColorComponents (space, col2, NULL, 2);
    CGContextDrawLinearGradient(ctx, grad2, CGPointMake(0, 0), CGPointMake(320, 0), 0);

    CGGradientRelease(grad1);
    CGGradientRelease(grad2);
    CGColorSpaceRelease(space);
}