collada - 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.