If I understand it correctly, setting CALayer.shouldRasterize = YES generates a bitmap with the contents of the layer and its whole hierarchy of sublayers.
The documentation says:
When the value of this property is YES, the layer is rendered as a bitmap in its local coordinate space and then composited to the destination with any other content.
Is there a way to access this bitmap's data? I would like to use it to generate a CIImage and apply a CIFilter without the need of drawing the whole layer into a new separate bitmap context and using the layer's method -renderInContext:.
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I've set up a custom iOS UIView that displays a list of layers, one of which is a CAMetalLayer. I was trying to paint that same content to an image in a CGContext. I tried to extract the layer's contents in order to build a CGImage, but couldn't find a good path.
First, I've tried to asynchronously extract the contents of the framebuffer with a addCompletedHandler() callback, but the async nature of the callback didn't behave nicely with CGContext.
I've also tried extracting the contents of the CAMetalLayer by using the layer.contents property, but apparently the type of the contents is CAImageQueue, which has no documentation or exposed through the API.
I've also tried rendering the layer directly to the CGContext like so:
layer.render(in: cgContext)
but that didn't bear any results either.
Lastly all I'd need to do is retrieve the bytes making up the layer texture, and I could build my own CGImage from scratch.
I am having different Texture Brushes I am having the image of those texture of each pixel of the stroke I want to assign the texture to the CGPath and change the width .
I need to assign the texture image and change the width to the slider response.
You achieve this in the following order:
Use CGLayerCreateWithContext to create a CGLayer, during this stage, the slider value is obtained and used as the CGSize in the initializer.
Use CGLayerGetContext to get the context of the CGLayer created, then you render your brush texture with the context, for example, CGContextDrawImage.
Use the completed CGLayer as a texture and draw it on screen with CGContextDrawLayerAtPoint, since you got your CGPath, you need to manually calculate the density of your drawing, and generate an array of CGPoint to be used as a parameter in the drawing function.
You may reference the Quartz 2D Programming Guide by Apple.
In Apple's Core Animation documentation, it says there are two rendering paths involved. From what I know, CALayer caches bitmap data of a UIView content. There are two ways of providing content of a CALayer. One is implementing drawRect: or other CALayer drawing methods, the other is setting a bitmap to the property contents of CALayer.
Here I'm wondering, what happens behind the scene if neither of the above two things is done? I believe there is a private drawing path UIView uses in this situation. What is this private drawing path consists of? How does it work?
The crux of CALayer is that it's GPU-backed. In modern graphics and animation, you want to minimize the number of times your bitmap data crosses between the CPU and the GPU. These operations are costly.
CALayer always uses a private drawing path, whether you use setContents: or drawRect:. In fact, the underlying plumbing of CALayer handles both of these in essentially the same way. When you setContents: , CALayer takes the image you gave it, and uploads it to the GPU via OpenGL (probably nowadays Metal) calls. When you drawRect: the CALayer gives you a context into which you can draw, and then it does the same thing with the bitmap data.
If you don't set contents or implement drawRect, you can still do things like set the layer's background color, border, corner radius, etc. This is being rendered by CALayer's GPU-based private drawing path.
CALayer does not draw any of its content using drawRect:. Only view based drawing techniques like Core Graphics use drawRect: but the only problem with this ways of things is that it is done on the CPU and the main thread making it an expensive process. So instead, Core Animation manipulates a cached bitmap of your apps content in the graphics hardware directly which is far much more optimised. You update or provide the initial contents for a Core Animation layer through one of its delegates (displayLayer: or drawLayer:inContext) or using the contents property as you have mentioned. All layer objects in Core Animation are derived from CALayer.
CALayer is simply a layer object belonging to Core Animation which in itself is simply a support system for UIView or subclasses of UIView. Core Animation is not a drawing technology in the sense it cannot create primitive shapes like Quartz, Open GL ES and Metal can. Instead, Core Animation allows you to manipulate an already existing view and it does this by caching the bitmap data of a UIView and sending this off to the graphics hardware to be manipulated. We say Core Animation is a support system and all its work relies on using layer objects of which CALayer is the main type. It can only do this of course if a view has a layer and a view does not actually need a layer to exist. However, in iOS all views come with a layer attached by default. We say views in iOS are "layer backed". In MacOS, you need to actually add Core Animation support for views.
The actual drawing of the contents of a CALayer happens in a few ways. The first is changing the contents property of the CALayer as you have mentioned and you do this by giving it a CGImageRef. The second is by implementing or overriding in a subclass the CALayer delegate displayLayer: which creates a bitmap and sets it to the content property of a layer. The third is by implementing or overriding in a subclass the CALayer delegate drawLayer:inContext which creates a bitmap, creates a graphics context to draw into that bitmap, and then calls your delegate method to fill the bitmap.
In iOS we do not usually worry about how the content of our view's layers are rendered. Since all views are layer backed, iOS manages how to render these views in the most efficient way possible using the methods I've just described. This is an optimisation to save you time and it makes layers very easy to use. You'll usually worry about overriding these delegates or subclassing them if you are developing for MacOS where views are not always layer backed. You might also worry about this if you decide not to use the default CALayer in iOS, for example you might change a view's layer from CALayer to CAMetalLayer. Or perhaps if you are looking for a performance optimisation and even this in a small number of cases.
There are three ways to provide content to a layer.
- Assign an image object directly to the layer object’s contents property.
- Assign a delegate object to the layer and let the delegate draw the layer’s content.
- Define a layer subclass and override one of its drawing methods to provide the layer contents yourself.
If we don't implement drawRect: or setting the contents property or subclass the layer, it will use the default way which is the second one to provide content to a Layer-backed views's layer. The layer will capture the content of the view and render it.
I have a question about the underlying implementation of the Core Image system. I'm adding some CIImages on top of each other. Not that much, about 5 or 6 of them. To save memory and performance, they all have their transparent pixels cropped. They are then drawn at offsets, so I'm using a #"CIAffineTransform" filter to position them.
CIFilter* moveFilter = [CIFilter filterWithName:#"CIAffineTransform"];
My question is: does the moveFilter.outputImage REALLY generate a new image, or does it generate "render settings" that are later on used to draw the actual image?
(If it is the first, that would mean I'm effectively rendering the image twice. It would be a huge flaw in the Core Image API and hard to believe Apple created it this way.)
Filters do not generate anything. outputImage does not generate anything. CIImage does not generate anything. All you are doing is constructing a chain of filters.
Rendering to a bitmap doesn't happen until you explicitly ask for it to happen. You do this in one of two ways:
Call CIContext createCGImage:fromRect:.
Actually draw a CIImage-based UIImage into a graphics context.
I got an image I want to mask on the fly. The mask is basically shaped like a part-circle and changes in volume from time to time. Therefor I need to create an in-memory image, draw the mask circle stuff to it and do the masking on the original image like described in How to Mask an UIImageView.
The thing is that I got no idea how to create an in-memory image I may use for masking and I can apply basic drawing opperations on.t
If your mask is only a semi-circle, it might be easier to create a clipping path with CGContext* calls and draw your image as a CGImage with the clipping path applied. See the documentation for CGContextClip() for details.