ARKIT - 它可以跟踪多少跟踪图像？

Question

So I understand that in order to track images, we need to create a AR Resource Folder and place all the images we intend to track there, as well as configuring thru the inspector their real world size properties.

Then we set the array of ARReferenceImages to the Session's World Config.

All good with that. But HOW MANY can we track ? 10? 100? 1000000? and would it be possible to download those images and create ARReferences on the fly, instead of having them in the bundle from the very beginning ?

Answer 1

Having a look at the Apple Docs it doesn't seem to specify a limit. As such it is likely to assume it would likely depend on memory management etc.

Regarding creating images on the fly, this is definitely possible.

According to the docs this can be done one of two ways:

1. Creating aa new reference image from a Core Graphics image object:

    init(CGImage, orientation: CGImagePropertyOrientation, physicalWidth: CGFloat) 

2. Creating a new reference image from a Core Video pixel buffer:

    init(CVPixelBuffer, orientation: CGImagePropertyOrientation, physicalWidth: CGFloat) 

Here is an example of creating a referenceImage on the fly using an image from the standard Assets Bundle , although this can easily be adapted for parsing an image from a URL etc:

// Create ARReference Images From Somewhere Other Than The Default Folder
func loadDynamicImageReferences(){

//1. Get The Image From The Folder
guard let imageFromBundle = UIImage(named: "moonTarget"),
//2. Convert It To A CIImage
let imageToCIImage = CIImage(image: imageFromBundle),
//3. Then Convert The CIImage To A CGImage
let cgImage = convertCIImageToCGImage(inputImage: imageToCIImage)else { return }

//4. Create An ARReference Image (Remembering Physical Width Is In Metres)
let arImage = ARReferenceImage(cgImage, orientation: CGImagePropertyOrientation.up, physicalWidth: 0.2)

//5. Name The Image
arImage.name = "CGImage Test"

//5. Set The ARWorldTrackingConfiguration Detection Images Assuming A Configuration Is Running
configuration.detectionImages = [arImage]

}


/// Converts A CIImage To A CGImage
///
/// - Parameter inputImage: CIImage
/// - Returns: CGImage
func convertCIImageToCGImage(inputImage: CIImage) -> CGImage? {

let context = CIContext(options: nil)
if let cgImage = context.createCGImage(inputImage, from: inputImage.extent) {

 return cgImage

}

return nil
}

We can then test this within ARSCNViewDelegate eg

func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {

//1. If Out Target Image Has Been Detected Than Get The Corresponding Anchor
guard let currentImageAnchor = anchor as? ARImageAnchor else { return }

let x = currentImageAnchor.transform
print(x.columns.3.x, x.columns.3.y , x.columns.3.z)

//2. Get The Targets Name
let name = currentImageAnchor.referenceImage.name!

//3. Get The Targets Width & Height In Meters
let width = currentImageAnchor.referenceImage.physicalSize.width
let height = currentImageAnchor.referenceImage.physicalSize.height

print("""
Image Name = \(name)
Image Width = \(width)
Image Height = \(height)
""")

//4. Create A Plane Geometry To Cover The ARImageAnchor
let planeNode = SCNNode()
let planeGeometry = SCNPlane(width: width, height: height)
planeGeometry.firstMaterial?.diffuse.contents = UIColor.white
planeNode.opacity = 0.25
planeNode.geometry = planeGeometry

//5. Rotate The PlaneNode To Horizontal
planeNode.eulerAngles.x = -.pi/2

//The Node Is Centered In The Anchor (0,0,0)
node.addChildNode(planeNode)

//6. Create AN SCNBox
let boxNode = SCNNode()
let boxGeometry = SCNBox(width: 0.1, height: 0.1, length: 0.1, chamferRadius: 0)

//7. Create A Different Colour For Each Face
let faceColours = [UIColor.red, UIColor.green, UIColor.blue, UIColor.cyan, UIColor.yellow, UIColor.gray]
var faceMaterials = [SCNMaterial]()

//8. Apply It To Each Face
for face in 0 ..< 5{
    let material = SCNMaterial()
    material.diffuse.contents = faceColours[face]
    faceMaterials.append(material)
}
boxGeometry.materials = faceMaterials
boxNode.geometry = boxGeometry

//9. Set The Boxes Position To Be Placed On The Plane (node.x + box.height)
boxNode.position = SCNVector3(0 , 0.05, 0)

//10. Add The Box To The Node
node.addChildNode(boxNode)

 }

As you can see the process if fairly easy. So in your case, you are probably more interested in the conversion function above which uses this method to create the dynamic images:

init(CGImage, orientation: CGImagePropertyOrientation, physicalWidth: CGFloat)

Answer 2

Paraphrasing the Human Interface Guidelines for AR... image detection performance/accuracy deteriorates as the number of images increases. So there's no hard limit in the API, but if you try to put more than around 25 images in the current detection set, it'll start getting to where it's too slow/inaccurate to be useful.

There are lots of other factors affecting performance/accuracy, too, so consider that a guideline, not a hard limit. Depending on scene conditions in the place where you're running the app, how much you're stressing the CPU with other tasks, how distinct your reference images are from one another, etc, you might manage a few more than 25... or start having detection problems with a few less than 25.