Question

My objective: given some index i and some UIColor c, make the face at that index turn into that color.

let vertices = // all of my vertices in the format of SCNVector3

let vertexSource = SCNGeometrySource(data: verticesData,
                                    semantic: .vertex,
                                    vectorCount: vertices.count,
                                    usesFloatComponents: true,
                                    componentsPerVector: 3,
                                    bytesPerComponent: MemoryLayout<Float>.size,
                                    dataOffset: 0,
                                    dataStride: MemoryLayout<SCNVector3>.size)


var colors: [SCNVector3] = vertices.map { SCNVector3(1, 1, 1) } // Make it all white color at first
colors[10] = SCNVector3(1, 0, 0)                                // Pick one at random and make it red
let colorSource = SCNGeometrySource(data: NSData(bytes: colors, length: MemoryLayout<SCNVector3>.size * colors.count) as Data,
                                    semantic: .color,
                                    vectorCount: colors.count,
                                    usesFloatComponents: true,
                                    componentsPerVector: 3,
                                    bytesPerComponent: MemoryLayout<Float>.size,
                                    dataOffset: 0,
                                    dataStride: MemoryLayout<SCNVector3>.size)

let elements = SCNGeometryElement(data: NSData(bytes: indices, length: MemoryLayout<Int32>.size * indices.count) as Data,
                                primitiveType: .polygon,
                                primitiveCount: indices.count / 4,
                                bytesPerIndex: MemoryLayout<Int32>.size)

let g = SCNGeometry(sources: [vertexSource, colorSource], elements: [element])

My 3D object renders correctly. It is all white color, as expected. The red face, however, doesn't appear. I see a hint of red, but it looks like SceneKit is trying to color around the vertices rather than the face those vertices form. How can I force it to just color the face/polygon created by those vertices?

Answer 1

这里发生的事情最好用一个简单的例子来解释。这是一个由两个三角形组成的平面：

let vertices: [SCNVector3] = [
    SCNVector3(-1, 1, 0),
    SCNVector3( 1, 1, 0),
    SCNVector3( 1, -1, 0),
    SCNVector3(-1, -1, 0)
]
let indices: [UInt16] = [
    0, 3, 1,
    1, 3, 2
]
var colors: [SCNVector3] = vertices.map { _ in SCNVector3(1, 1, 1) }

现在，当您将红色分配给colors数组中的一项时，您将得到以下结果：

这很有意义，因为您是将颜色分配给单个顶点，而不是整个面。需要为脸部(0, 3, 1)的每个索引分配红色，以确保整个脸部都是红色的：

colors[0] = SCNVector3(1, 0, 0)
colors[3] = SCNVector3(1, 0, 0)
colors[1] = SCNVector3(1, 0, 0)

这将导致：

之所以存在红白色渐变，是因为颜色是在顶点之间插值的。现在具有红色的顶点也属于第二个面，该面也具有一个白色的顶点。如果要实现此结果：

然后，您必须复制面之间共享的顶点：

let vertices: [SCNVector3] = [
    SCNVector3(-1, 1, 0),
    SCNVector3(-1, -1, 0),
    SCNVector3( 1, 1, 0),

    SCNVector3( 1, 1, 0),
    SCNVector3(-1, -1, 0),
    SCNVector3( 1, -1, 0)
    ]

let indices: [UInt16] = [
    0, 1, 2,
    3, 4, 5
]

colors[0] = SCNVector3(1, 0, 0)
colors[1] = SCNVector3(1, 0, 0)
colors[2] = SCNVector3(1, 0, 0)

SceneKit selectively color a face by a certain index

1 个答案: