Extracting vertices from scenekit

Question

I'm having a problem with understanding scenekit geometery.

I have the default cube from Blender, and I export as collada (DAE), and can bring it into scenekit.... all good.

Now I want to see the vertices for the cube. In the DAE I can see the following for the "Cube-mesh-positions-array",

"1 1 -1 1 -1 -1 -1 -0.9999998 -1 -0.9999997 1 -1 1 0.9999995 1 0.9999994 -1.000001 1 -1 -0.9999997 1 -1 1 1"

Now what I'd like to do in scenekit, is get the vertices back, using something like the following:

SCNGeometrySource *vertexBuffer = [[cubeNode.geometry geometrySourcesForSemantic:SCNGeometrySourceSemanticVertex] objectAtIndex:0];

If I process the vertexBuffer (I've tried numerous methods of looking at the data), it doesn't seem correct.

Can somebody explain what "SCNGeometrySourceSemanticVertex" is giving me, and how to extract the vertex data properly? What I'd like to see is:

X = "float"
Y = "float"
Z = "float"

Also I was investigating the following class / methods, which looked promising (some good data values here), but the data from gmpe appears empty, is anybody able to explain what the data property of "SCNGeometryElement" contains?

SCNGeometryElement *gmpe = [theCurrentNode.geometry geometryElementAtIndex:0];

Thanks, assistance much appreciated,

D

Answer 1

The geometry source

When you call geometrySourcesForSemantic: you are given back an array of SCNGeometrySource objects with the given semantic in your case the sources for the vertex data).

This data could have been encoded in many different ways and a multiple sources can use the same data with a different stride and offset. The source itself has a bunch of properties for you to be able to decode the data like for example

• dataStride
• dataOffset
• vectorCount
• componentsPerVector
• bytesPerComponent

You can use combinations of these to figure out which parts of the data to read and make vertices out of them.

Decoding

The stride tells you how many bytes you should step to get to the next vector and the offset tells you how many bytes offset from the start of that vector you should offset before getting to the relevant pars of the data for that vector. The number of bytes you should read for each vector is componentsPerVector * bytesPerComponent

Code to read out all the vertices for a single geometry source would look something like this

// Get the vertex sources
NSArray *vertexSources = [geometry geometrySourcesForSemantic:SCNGeometrySourceSemanticVertex];

// Get the first source
SCNGeometrySource *vertexSource = vertexSources[0]; // TODO: Parse all the sources

NSInteger stride = vertexSource.dataStride; // in bytes
NSInteger offset = vertexSource.dataOffset; // in bytes

NSInteger componentsPerVector = vertexSource.componentsPerVector;
NSInteger bytesPerVector = componentsPerVector * vertexSource.bytesPerComponent;
NSInteger vectorCount = vertexSource.vectorCount;

SCNVector3 vertices[vectorCount]; // A new array for vertices

// for each vector, read the bytes
for (NSInteger i=0; i<vectorCount; i++) {

    // Assuming that bytes per component is 4 (a float)
    // If it was 8 then it would be a double (aka CGFloat)
    float vectorData[componentsPerVector];

    // The range of bytes for this vector
    NSRange byteRange = NSMakeRange(i*stride + offset, // Start at current stride + offset
                                    bytesPerVector);   // and read the lenght of one vector

    // Read into the vector data buffer
    [vertexSource.data getBytes:&vectorData range:byteRange];

    // At this point you can read the data from the float array
    float x = vectorData[0];
    float y = vectorData[1];
    float z = vectorData[2];

    // ... Maybe even save it as an SCNVector3 for later use ...
    vertices[i] = SCNVector3Make(x, y, z);

    // ... or just log it 
    NSLog(@"x:%f, y:%f, z:%f", x, y, z);
}

The geometry element

This will give you all the vertices but won't tell you how they are used to construct the geometry. For that you need the geometry element that manages the indices for the vertices.

You can get the number of geometry elements for a piece of geometry from the geometryElementCount property. Then you can get the different elements using geometryElementAtIndex:.

The element can tell you if the vertices are used a individual triangles or a triangle strip. It also tells you the bytes per index (the indices may have been ints or shorts which will be necessary to decode its data.

Answer 2

Here is an extension method if the data isn't contiguous (the vector size isn't equal to the stride) which can be the case when the geometry is loaded from a DAE file. It also doesn't use copyByte function.

extension  SCNGeometry{


    /**
     Get the vertices (3d points coordinates) of the geometry.

     - returns: An array of SCNVector3 containing the vertices of the geometry.
     */
    func vertices() -> [SCNVector3]? {

        let sources = self.sources(for: .vertex)

        guard let source  = sources.first else{return nil}

        let stride = source.dataStride / source.bytesPerComponent
        let offset = source.dataOffset / source.bytesPerComponent
        let vectorCount = source.vectorCount

        return source.data.withUnsafeBytes { (buffer : UnsafePointer<Float>) -> [SCNVector3] in

            var result = Array<SCNVector3>()
            for i in 0...vectorCount - 1 {
                let start = i * stride + offset
                let x = buffer[start]
                let y = buffer[start + 1]
                let z = buffer[start + 2]
                result.append(SCNVector3(x, y, z))
            }
            return result
        }
    }
}

Answer 3

The Swift Version

The Objective-C version and this are essentially identical.

let planeSources = _planeNode?.geometry?.geometrySourcesForSemantic(SCNGeometrySourceSemanticVertex)
if let planeSource = planeSources?.first {
    let stride = planeSource.dataStride
    let offset = planeSource.dataOffset
    let componentsPerVector = planeSource.componentsPerVector
    let bytesPerVector = componentsPerVector * planeSource.bytesPerComponent

    let vectors = [SCNVector3](count: planeSource.vectorCount, repeatedValue: SCNVector3Zero)
    let vertices = vectors.enumerate().map({
        (index: Int, element: SCNVector3) -> SCNVector3 in
        var vectorData = [Float](count: componentsPerVector, repeatedValue: 0)
        let byteRange = NSMakeRange(index * stride + offset, bytesPerVector)
        planeSource.data.getBytes(&vectorData, range: byteRange)
        return SCNVector3Make(vectorData[0], vectorData[1], vectorData[2])
    })

    // You have your vertices, now what?
}

Answer 4

// call this function _ = vertices(node: mySceneView.scene!.rootNode) // I have get the volume in Swift 4.2 :--- this function

func vertices(node:SCNNode) -> [SCNVector3] {

    let planeSources1 = node.childNodes.first?.geometry

    let planeSources = planeSources1?.sources(for: SCNGeometrySource.Semantic.vertex)
    if let planeSource = planeSources?.first {

        let stride = planeSource.dataStride
        let offset = planeSource.dataOffset
        let componentsPerVector = planeSource.componentsPerVector
        let bytesPerVector = componentsPerVector * planeSource.bytesPerComponent

        let vectors = [SCNVector3](repeating: SCNVector3Zero, count: planeSource.vectorCount)
        let vertices = vectors.enumerated().map({
            (index: Int, element: SCNVector3) -> SCNVector3 in
            let vectorData = UnsafeMutablePointer<Float>.allocate(capacity: componentsPerVector)
            let nsByteRange = NSMakeRange(index * stride + offset, bytesPerVector)
            let byteRange = Range(nsByteRange)

            let buffer = UnsafeMutableBufferPointer(start: vectorData, count: componentsPerVector)
            planeSource.data.copyBytes(to: buffer, from: byteRange)
            return SCNVector3Make(buffer[0], buffer[1], buffer[2])
        })
        var totalVolume = Float()
        var x1 = Float(),x2 = Float(),x3 = Float(),y1 = Float(),y2 = Float(),y3 = Float(),z1 = Float(),z2 = Float(),z3 = Float()
        var i = 0
        while i < vertices.count{

                x1 = vertices[i].x;
                y1 = vertices[i].y;
                z1 = vertices[i].z;

                x2 = vertices[i + 1].x;
                y2 = vertices[i + 1].y;
                z2 = vertices[i + 1].z;

                x3 = vertices[i + 2].x;
                y3 = vertices[i + 2].y;
                z3 = vertices[i + 2].z;

                totalVolume +=
                    (-x3 * y2 * z1 +
                        x2 * y3 * z1 +
                        x3 * y1 * z2 -
                        x1 * y3 * z2 -
                        x2 * y1 * z3 +
                        x1 * y2 * z3);

                        i = i + 3
        }
        totalVolume = totalVolume / 6;
        volume = "\(totalVolume)"
        print("Volume Volume Volume Volume Volume Volume Volume :\(totalVolume)")
        lbl_valume.text = "\(clean(String(totalVolume))) cubic mm"
    }
    return[]
}

Answer 5

Here's a Swift 5.3 version, based on the other answers, and that also supports a bytesPerComponent different from 4 (untested for size different from 4 though):

extension SCNGeometrySource {
    var vertices: [SCNVector3] {
        let stride = self.dataStride
        let offset = self.dataOffset
        let componentsPerVector = self.componentsPerVector
        let bytesPerVector = componentsPerVector * self.bytesPerComponent

        func vectorFromData<FloatingPoint: BinaryFloatingPoint>(_ float: FloatingPoint.Type, index: Int) -> SCNVector3 {
            assert(bytesPerComponent == MemoryLayout<FloatingPoint>.size)
            let vectorData = UnsafeMutablePointer<FloatingPoint>.allocate(capacity: componentsPerVector)
            let buffer = UnsafeMutableBufferPointer(start: vectorData, count: componentsPerVector)
            let rangeStart = index * stride + offset
            self.data.copyBytes(to: buffer, from: rangeStart..<(rangeStart + bytesPerVector))
            return SCNVector3(
                CGFloat.NativeType(vectorData[0]),
                CGFloat.NativeType(vectorData[1]),
                CGFloat.NativeType(vectorData[2])
            )
        }

        let vectors = [SCNVector3](repeating: SCNVector3Zero, count: self.vectorCount)
        return vectors.indices.map { index -> SCNVector3 in
            switch bytesPerComponent {
            case 4:
                return vectorFromData(Float32.self, index: index)
            case 8:
                return vectorFromData(Float64.self, index: index)
            case 16:
                return vectorFromData(Float80.self, index: index)
            default:
                return SCNVector3Zero
            }
        }
    }
}

Answer 6

With swift 3.1 you can extract vertices from SCNGeometry in a much faster and shorter way:

func vertices(node:SCNNode) -> [SCNVector3] {
    let vertexSources = node.geometry?.getGeometrySources(for: SCNGeometrySource.Semantic.vertex)
    if let vertexSource = vertexSources?.first {
        let count = vertexSource.data.count / MemoryLayout<SCNVector3>.size
        return vertexSource.data.withUnsafeBytes {
            [SCNVector3](UnsafeBufferPointer<SCNVector3>(start: $0, count: count))
        }
    }
    return []
}

... Today i've noted that on osx this not going to work correct. This happens because on iOS SCNVector3 build with Float and on osx CGFloat (only apple good do smth simple so suffering). So I had to tweak the code for osx but this not gonna work as fast as on iOS.

func vertices() -> [SCNVector3] {

        let vertexSources = sources(for: SCNGeometrySource.Semantic.vertex)


        if let vertexSource = vertexSources.first {

        let count = vertexSource.vectorCount * 3
        let values = vertexSource.data.withUnsafeBytes {
            [Float](UnsafeBufferPointer<Float>(start: $0, count: count))
        }

        var vectors = [SCNVector3]()
        for i in 0..<vertexSource.vectorCount {
            let offset = i * 3
            vectors.append(SCNVector3Make(
                CGFloat(values[offset]),
                CGFloat(values[offset + 1]),
                CGFloat(values[offset + 2])
            ))
        }

        return vectors
    }
    return []
}

Answer 7

The Swift 3 version:

    // `plane` is some kind of `SCNGeometry`

    let planeSources = plane.geometry.sources(for: SCNGeometrySource.Semantic.vertex)
    if let planeSource = planeSources.first {
        let stride = planeSource.dataStride
        let offset = planeSource.dataOffset
        let componentsPerVector = planeSource.componentsPerVector
        let bytesPerVector = componentsPerVector * planeSource.bytesPerComponent

        let vectors = [SCNVector3](repeating: SCNVector3Zero, count: planeSource.vectorCount)
        let vertices = vectors.enumerated().map({
            (index: Int, element: SCNVector3) -> SCNVector3 in

            let vectorData = UnsafeMutablePointer<Float>.allocate(capacity: componentsPerVector)
            let nsByteRange = NSMakeRange(index * stride + offset, bytesPerVector)
            let byteRange = Range(nsByteRange)

            let buffer = UnsafeMutableBufferPointer(start: vectorData, count: componentsPerVector)
                planeSource.data.copyBytes(to: buffer, from: byteRange)
            let vector = SCNVector3Make(buffer[0], buffer[1], buffer[2])
        })

        // Use `vertices` here: vertices[0].x, vertices[0].y, vertices[0].z
    }

Answer 8

For someone like me want to extract data of face from SCNGeometryElement.

Notice I only consider primtive type is triangle and index size is 2 or 4.

void extractInfoFromGeoElement(NSString* scenePath){
    NSURL *url = [NSURL fileURLWithPath:scenePath];
    SCNScene *scene = [SCNScene sceneWithURL:url options:nil error:nil];
    SCNGeometry *geo = scene.rootNode.childNodes.firstObject.geometry;
    SCNGeometryElement *elem = geo.geometryElements.firstObject;
    NSInteger componentOfPrimitive = (elem.primitiveType == SCNGeometryPrimitiveTypeTriangles) ? 3 : 0;
    if (!componentOfPrimitive) {//TODO: Code deals with triangle primitive only
        return;
    }
    for (int i=0; i<elem.primitiveCount; i++) {
        void *idxsPtr = NULL;
        int stride = 3*i;
        if (elem.bytesPerIndex == 2) {
            short *idxsShort = malloc(sizeof(short)*3);
            idxsPtr = idxsShort;
        }else if (elem.bytesPerIndex == 4){
            int *idxsInt = malloc(sizeof(int)*3);
            idxsPtr = idxsInt;
        }else{
            NSLog(@"unknow index type");
            return;
        }
        [elem.data getBytes:idxsPtr range:NSMakeRange(stride*elem.bytesPerIndex, elem.bytesPerIndex*3)];
        if (elem.bytesPerIndex == 2) {
            NSLog(@"triangle %d : %d, %d, %d\n",i,*(short*)idxsPtr,*((short*)idxsPtr+1),*((short*)idxsPtr+2));
        }else{
            NSLog(@"triangle %d : %d, %d, %d\n",i,*(int*)idxsPtr,*((int*)idxsPtr+1),*((int*)idxsPtr+2));
        }
        //Free
        free(idxsPtr);
    }
}