CGContextDrawRadialGradient produces a very visible ‘cross’ at the centre of the gradient:
Code (reduced):
- (void)drawRect:(NSRect)dirtyRect {
CGContextRef context = [[NSGraphicsContext currentContext] graphicsPort];
size_t numberOfGradientLocations = 2;
CGFloat startRadius = 0.0f;
CGFloat endRadius = 30.0f;
CGPoint centre = CGPointMake(floorf(self.bounds.size.width / 2), floorf(self.bounds.size.height / 2));
CGFloat gradientColours[8] = {0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f};
CGFloat gradientLocations[2] = {0.0f, 1.0f};
CGColorSpaceRef colourspace = CGColorSpaceCreateDeviceRGB();
CGGradientRef gradient = CGGradientCreateWithColorComponents(colourspace, gradientColours, gradientLocations, numberOfGradientLocations);
CGContextDrawRadialGradient(context, gradient, centre, startRadius, centre, endRadius, kCGGradientDrawsBeforeStartLocation);
}
This happens on both macOS and iOS. Meanwhile, the same kind of gradient renders perfectly in WebKit with CSS (so it’s not some ‘bad display’ issue).
What am I doing wrong? Is there a known way around this?
Here's what I get on iOS; I used the iPhone 7 Plus simulator at 100% so as to make every pixel visible, and it seems completely smooth:
EDIT After some experimentation, I suspect you're seeing a moiré effect caused by misalignment between points and pixels.
Related
It should be very simple to create a black to white radial gradient using Core Graphics, but even after many tries I am not able to figure where I am going wrong.
I want something like this:
This is my code:
CGFloat radius = shapeRect.size.width/2.0;
CGPoint center = CGPointMake(shapeRect.origin.x+shapeRect.size.width/2, shapeRect.origin.y+shapeRect.size.height/2);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
CGContextRef context = CGBitmapContextCreate(NULL, shapeRect.size.width, shapeRect.size.height, 8, shapeRect.size.width*4, colorSpace, kCGImageAlphaNone);
CGFloat components[] = {0.0, 1.0};
CGFloat locations[] = {0.0, 1.0};
CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, components, locations, 2);
CGContextDrawRadialGradient(context, gradient, center, 10, center, radius, 0);
CGImageRef img = CGBitmapContextCreateImage(context);
[maskedView setImage:[UIImage imageWithCGImage:img]];
CGGradientRelease(gradient);
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
ShapeRect here has self.view bounds.
I am getting a whole black screen with no gradient as output.
I am using iPhone 7 simulator.
Your suggestions can save my day
Thank you.
Please check this snippet. You can change components and locations values to get what you want and to understand how the drawing works. Refer to CGGradientCreateWithColorComponentsdescription
#import "TestView.h"
#implementation TestView {
CGGradientRef _gradient;
}
-(void)drawRect:(CGRect)rect
{
CGFloat radius = self.frame.size.width / 2.0;
CGPoint center = CGPointMake(CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds));
CGContextRef context = UIGraphicsGetCurrentContext();
CGGradientRef gradient = [self createGradient];
CGContextDrawRadialGradient(context, gradient, center, 10, center, radius, kCGGradientDrawsBeforeStartLocation);
}
-(CGGradientRef)createGradient
{
if (!_gradient) {
CGFloat components[] = { 0.0, 1.0, 0.5, 0.5, 1.0, 0.3 };
CGFloat locations[] = { 0.0, 0.6, 1.0 };
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
_gradient = CGGradientCreateWithColorComponents(colorSpace, components, locations, 3);
CGColorSpaceRelease(colorSpace);
}
return _gradient;
}
- (void)layoutSubviews
{
self.backgroundColor = [UIColor whiteColor];
self.layer.borderColor = [UIColor lightGrayColor].CGColor;
self.layer.borderWidth = 1.;
}
#end
Result:
I want to accomplish a function just like a Brush. The area where finger swipes changes to trasparent with gradually changed border.
I can only change the color to crystal clear now with following codes:
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
if(self.eraser) return;
CGFloat scale = self.transform.a;
if (scale < 1) scale = 1;
CGPoint p = [[touches anyObject] locationInView: self];
CGPoint q = [[touches anyObject] previousLocationInView: self];
UIImage* image;
image = self.image;
CGSize size = self.frame.size;
UIGraphicsBeginImageContext(size);
CGRect rect;
rect.origin = CGPointZero;
rect.size = size;
[image drawInRect:rect];
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineCap(context, kCGLineCapRound);
CGContextBeginPath(context);
CGContextSaveGState( context );
CGContextSetLineWidth(context, (10.0 / scale) + 1);
CGContextSetBlendMode(context, kCGBlendModeClear);
CGContextMoveToPoint(context, q.x, q.y);
CGContextAddLineToPoint(context, p.x, p.y);
CGContextStrokePath(context);
CGContextRestoreGState( context );
UIImage* editedImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
[self setBounds:rect];
[self setImage:editedImage];
}
How can I get the edge with gradually change? Thanks in advance.
You can achieve this effect by drawing a radial gradient with a variable alpha in the kCGBlendModeDestinationIn mode in each spot the user passes.
This blend mode has the effect of only applying the layer's alpha to layers below. With the variable alpha of our gradient, we can achieve this effect.
const CGFloat kBrushSize = 10.f;
CGContextSaveGState(context);
// Make a radial gradient that goes from transparent black on the inside
// to opaque back on the outside.
size_t num_locations = 2;
CGFloat locations[2] = { 0.0, 1.0 };
CGFloat components[8] = { 1.0, 1.0, 1.0, 0.0,
1.0, 1.0, 1.0, 1.0 };
CGColorSpaceRef myColorspace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
CGGradientRef myGradient = CGGradientCreateWithColorComponents (myColorspace, components,
locations, num_locations);
CGColorSpaceRelease(myColorspace);
// Draw the gradient at the point using kCGBlendModeDestinationIn
// This mode only applies the new layer's alpha to the lower layer.
CGContextSetBlendMode(context, kCGBlendModeDestinationIn);
CGContextDrawRadialGradient(context, myGradient, p, 0.f, p, (kBrushSize / scale) + 1, kCGGradientDrawsAfterEndLocation);
CGGradientRelease(myGradient);
CGContextRestoreGState(context);
Here is a screenshot of this code in action:
Note: Using this technique, if the user is moving his/her finger very fast, you may see a spacing effect where discrete brush dots are visible. This is a feature of some graphics software, but if this is undesirable for you, you can add code to interpolate the points between the current and last to draw more brush points, creating a more continuous stroke.
Also, you should be able to adjust the gradient color stops to achieve any kind of brush softness you like.
Source: https://developer.apple.com/library/mac/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_shadings/dq_shadings.html
I'm ultimately trying to create a gradient overlay to fade from 60% black to clear, to 40% black. The result has a lot of banding, so I'm just trying to get two colours working properly first.
In this first case, I'm trying a gradient from solid black to solid white which draws pretty much perfectly.
In the second case, I'm placing a gradient from 100% black to clear, against a solid gray background. The banding is really awful.
The code to make this gradient looks like this:
// Draw the overlay gradient
CGContextTranslateCTM(context, 0.0, size.height);
CGContextScaleCTM(context, 1.0, -1.0);
UIColor *firstColor = [UIColor colorWithWhite:0.0 alpha:1.0];
UIColor *secondColor = [UIColor clearColor];
CGContextSetBlendMode(context, kCGBlendModeSourceAtop);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
NSArray *coreGraphicsColors = #[(id)firstColor.CGColor, (id)secondColor.CGColor];
CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (__bridge CFArrayRef)coreGraphicsColors, 0);
static const CGFloat standardDegree = 270;
const CGFloat degree = (standardDegree * M_PI / 180);
const CGPoint center = CGPointMake(size.width / 2, size.height / 2);
const CGPoint startPoint = CGPointMake(center.x - cos(degree) * size.width / 2, center.y - sin(degree) * size.height / 2);
const CGPoint endPoint = CGPointMake(center.x + cos(degree) * size.width / 2, center.y + sin(degree) * size.height / 2);
CGContextDrawLinearGradient(context, gradient, startPoint, endPoint, 0);
CGGradientRelease(gradient);
CGColorSpaceRelease(colorSpace);
Does anyone have some experience/tips with creating these gradients which are partly transparent? I've tried altering a few things like the positions and color space, all to no avail.
This might help you AlphaGradientView
I am a new ios developer and here's my problem.I draw a shape with this code :
-(void)drawFirstShape:(float)xcoord :(float)ycoord :(CGContextRef)c {
float cote = 70.0f;
float x = 60.0f;
float y = 35.0f;
CGFloat strokecolor[4] = {1.0f, 1.0f, 1.0f, 1.0f};
CGContextSetStrokeColor(c, strokecolor);
CGContextSetLineWidth(c, 4.0f);
CGContextBeginPath(c);
CGContextMoveToPoint(c, xcoord, ycoord);
CGContextAddLineToPoint(c, xcoord+x, ycoord+y);
CGContextAddLineToPoint(c, xcoord+x, ycoord+cote+y);
CGContextAddLineToPoint(c, xcoord, ycoord+cote+2*y);
CGContextClosePath(c);
CGContextDrawPath(c, kCGPathStroke);
}
- (void)drawRect:(CGRect)rect
{
CGContextRef c = UIGraphicsGetCurrentContext();
[self drawFirstShape:160.0f :100.0f:c];
}
I want to create relative coordinates instead of absolute ones.
The aim is to stabilize this code to use it with a 3.5inch and 4inch screen. Can someone help me please?
If you want to use relative coordinates then use relative coordinates :)
All positions and width/height should be defined as float or double from 0 to 1.
Then to get real coordinate - just multiply these relative coordinates to frame sizes (or to rect in your case) and you will get absolute coordinates or sizes.
Example (I just used your code with small changes. keep in mind that I changed sizes to relative and you will need to change them to be correct ones):
-(void)drawFirstShape:(float)xcoord :(float)ycoord :(CGContextRef)c inRect:(CGRect)rect{
xcoord = xcoord*rect.size.width;
ycoord = ycoord*rect.size.height;
float cote = 0.07f*rect.size.width;
float x = 0.05f*rect.size.width;
float y = 0.025f*rect.size.height;
CGFloat strokecolor[4] = {1.0f, 1.0f, 1.0f, 1.0f};
CGContextSetStrokeColor(c, strokecolor);
CGContextSetLineWidth(c, 4.0f);
CGContextBeginPath(c);
CGContextMoveToPoint(c, xcoord, ycoord);
CGContextAddLineToPoint(c, xcoord+x, ycoord+y);
CGContextAddLineToPoint(c, xcoord+x, ycoord+cote+y);
CGContextAddLineToPoint(c, xcoord, ycoord+cote+2*y);
CGContextClosePath(c);
CGContextDrawPath(c, kCGPathStroke);
}
- (void)drawRect:(CGRect)rect {
CGContextRef c = UIGraphicsGetCurrentContext();
[self drawFirstShape:0.2 :0.1 :c inRect:rect];
}
I am using radial gradient to draw circle. It is working fine but now I want to set border color and border width to that. I tried it but it was not working.
Here is my code,
CGGradientRef gradient;
CGColorSpaceRef colorSpace;
CGFloat locations[] = {0.0,1.0};
CGFloat components[] = { red2,green2,blue2,1.0,red1,green1,blue1,1.0 };
colorSpace = CGColorSpaceCreateDeviceRGB();
gradient = CGGradientCreateWithColorComponents(colorSpace,components,locations,
sizeof(locations)/sizeof(CGFloat));
CGPoint start = CGPointMake(self.bounds.size.width/2, self.bounds.size.height/2 ), end = CGPointMake(self.bounds.size.width/2 , self.bounds.size.height/2);
CGFloat startRadius = 0.0, endRadius = radius;
CGContextDrawRadialGradient(contextRef,gradient,start,startRadius,end,endRadius,0);
CGContextSetLineWidth(contextRef, 5.0);
CGGradientRelease(gradient);
CGColorSpaceRelease(colorSpace);
How can I set border color and width to it?
First add QuartzCore Framework and then try this code...
[[Firstbtn layer]setBorderColor:[[UIColor blackColor]CGColor]];
[[self.Firstbtn layer]setBorderWidth:2.3];
[[self.Firstbtn layer]setCornerRadius:15];