I am developing a Mac app, I want to draw a view like a radar, I find no method to draw gradient color along the arc. The existing method only draw gradient towards one direction.
What you want is an angle gradient. I've written a Core Image filter that generates an angle gradient; give it an opaque color (e.g., green) for the start color and the completely-transparent version of that color for the middle and end colors.
(The filter's output is actually infinite in extent and centered at the origin, so you'll need to mask it out to a circle and use an affine transform at one level or another to get it into the right position.)
Extra credit: In the kernel code for that filter (near the start of the .m), there's a line that starts the gradient at straight-up (90°) rather than straight-right. You could change the code, both of the filter and of the kernel, to make this a parameter (like inputStartColor et al) that you could vary over time, using a CABasicAnimation or something similar.
You'll need to use a combination of axial and radial gradients and probably also some clipping paths. You can find all of this documented here.
You can also use colorWithPatternImage: to stroke any lines you're drawing.
Related
I'm writing an app that could make good use of the Apple Watch's fitness tracker design, here:
So far, I've created the basic outline which is just a CAShapeLayer with a CGPath of an ellipse. I use strokeStart and strokeEnd to animate the progress. My problem comes when applying a gradient to the outline. How do I apply a gradient like above to the stroke of a CGPath?
The cleanest way to do this without having to drop down to Core Graphics or GL is to create a layer containing the angle gradient that you want the ring filled with, mask it with a CAShapeLayer containing your circular path (with the appropriate line width and cap settings), then, as you’re currently doing, use the shape layer’s strokeEnd property to set the “fill” percentage. Note that there isn’t a built-in way to create an angle gradient—you can use one of the suggestions in this answer for that.
edit: Also, you’ll need a pair of semicircular “cap” images, one at each end of the ring—as the fill percentage gets close to 100%, the region at the top will reveal the discontinuity between the start and end color. In your example image above, you’d need a red semicircle oriented like this ( at the start, and a pink one oriented like this ) with a translation/rotation transform tracking the end.
additional edit: Also also, since the end-cap semicircle will be moving along the gradient, you’ll need it to change color, interpolating from the start color to the end color as the fill amount goes from 0% to 100%. Best way to do that is with a shape layer with a semicircular path, since you can set the fillColor of that without having to redraw image contents.
We did this for an iOS app.. but quickly stopped as it gets bogged down quickly.
I think Apple is using images.. as they do in the Lister example
Is it possible to get this kind of gradient in cocoa ?
I can use simple CGContextDrawRadialGradient with clip made by CGContextAddEllipseInRect but the effect will slightly different.
Is it any posibility to draw exact this shade/gradient?
I would use a radial gradient, and then apply a scale trasnformation such that the y coordinate is squiched, and the x remains.
There is an advanced topic where you can define your own gradients providing an callback that calculates the color or so. Ist stated in Core Graphics docu.
I have two instances of a CALayer subclass.
The only difference between them is this line:
[self setTransform:CATransform3DMakeScale(2, 2, 2)];
What else do I need so that the large layer looks good at scale 2x ?
PS: (to avoid any confusion) The layers also include a few control buttons, shadows and rounded corner to mimic the look of windows in a windowing system, but those are not NSWindows instances.
The short answer is, don't use transforms. Transforms scale the layer by magnifying it, without re-rendering.
You could get a very similar effect by using a CAShapeLayer and animating changes to the path. That would give you sharp rendering, however, because it path animation does re-render the pixels.
I say "similar" effect because CAShapeLayers use a lineWidth property for the whole layer. You can animate the line width between values, and use fractional values, but you'll have to do some fine-tuning to get the line thickness to animate up and down in proportion to the size of the shape. Another consideration is that the graphics system uses anti-aliasing to draw fractional width paths, so when the line width is not an integer value they will look slightly soft. You could turn off antialiasing, but then they would look really jaggy.
I am using OpenGL ES 1.1 to draw lines in my iPad app. I want to make sure that the drawn lines are always visible on the screen regardless of the background colors, and without allowing the user to choose a color. Is there a blend function that will create this effect? So the color of the line drawn will change based on the colors already drawn beneath it and therefore always be visible.
Sadly the final blending of fragments into the framebuffer is still fixed function. Furthermore glLogicOp isn't implemented in ES so you can't do something cheap like XOR drawing.
I think the net effect is that:
you want the output colour to be a custom function of the colour already in the frame buffer;
but the frame buffer can't be read in a shader (it'd break the pipeline and lead towards concurrency issues).
You're therefore going to have to implement a ping pong pipeline.
You have two off-screen buffers. One represents what you output last frame, the other represents what you output the frame before that.
To generate a new frame you render using the one that represents the frame before as an input. Because it's an input you can sample it wherever you want and make whatever calculations you like on it. You render to the other buffer that you have (ie, the even older one) because you no longer care about its contents.
Then you copy all that to the screen and swap the two over, meaning that what you just drew is still in a texture to refer to as what you drew last frame. What you just referred to becomes your next drawing target because it's something you conveniently already have lying around.
So you'll be immediately interested in rendering to a texture. You'll also need to decide what function you want to use to pick a suitable 'different' colour to the existing background. Maybe just inverting it will do?
I think this could work:
glBlendFunc(GL_ONE_MINUS_DST_COLOR, GL_ZERO);
Draw your lines with a white color, and then the result will be rendered as
[1,1,1,1] * ( 1 - [DstR, DstG, DstB, DstA]) + ([DstR, DstG, DstB, DstA] * 0)
This should render a black pixel where the background is white, a white pixel where the background is black, a yellow pixel where the background is blue, etc.
I've created a canvas within which I display an image that is clipped when it goes over the edges. I can do this fine with a square shaped frame, however the frame I want to use is the one below. Is there any way I can clip the image inside the frame without having to add a non transparent square border around the image, i.e. just using the black line that I've already drawn? (on iPad)
You'll need to use Core Graphics and Quartz to handle this sort of clipping/graphics manipulation.
http://developer.apple.com/library/ios/#documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/Introduction/Introduction.html#//apple_ref/doc/uid/TP30001066
If you're using UIBezierPath, you may be able to achieve the clipping you're after using the following process
http://developer.apple.com/library/ios/#documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_paths/dq_paths.html#//apple_ref/doc/uid/TP30001066-CH211-TPXREF101
Convert your UIBezierPath to a CGPath
Get your image into a CGContext
Add your CGPath to the context via CGContextAddPath
Clip your context using CGContextClip
Alternatively, if you don't want to be messing with paths (and depending on whether this technique is suitable for your situation, your description of the issue makes it hard to tell), it might be worth using image masking to achieve the effect you're after. See the first link and look under "Bitmap Images and Image Masks".