ios - 什么是 OpenGL 中的切片？

Question

在下面的代码中，为什么我们需要切片？它有什么用？

//https://github.com/danginsburg/opengles-book-samples/blob/604a02cc84f9cc4369f7efe93d2a1d7f2cab2ba7/iPhone/Common/esUtil.h#L110
int esGenSphere(int numSlices, float radius, float **vertices,
                float **texCoords, uint16_t **indices, int *numVertices_out) {
    int numParallels = numSlices / 2;
    int numVertices = (numParallels + 1) * (numSlices + 1);
    int numIndices = numParallels * numSlices * 6;
    float angleStep = (2.0f * ES_PI) / ((float) numSlices);

    if (vertices != NULL) {
        *vertices = malloc(sizeof(float) * 3 * numVertices);
    }

    if (texCoords != NULL) {
        *texCoords = malloc(sizeof(float) * 2 * numVertices);
    }

    if (indices != NULL) {
        *indices = malloc(sizeof(uint16_t) * numIndices);
    }

    for (int i = 0; i < numParallels + 1; i++) {
        for (int j = 0; j < numSlices + 1; j++) {
            int vertex = (i * (numSlices + 1) + j) * 3;

            if (vertices) {
                (*vertices)[vertex + 0] = radius * sinf(angleStep * (float)i) * sinf(angleStep * (float)j);
                (*vertices)[vertex + 1] = radius * cosf(angleStep * (float)i);
                (*vertices)[vertex + 2] = radius * sinf(angleStep * (float)i) * cosf(angleStep * (float)j);
            }

            if (texCoords) {
                int texIndex = (i * (numSlices + 1) + j) * 2;
                (*texCoords)[texIndex + 0] = (float)j / (float)numSlices;
                (*texCoords)[texIndex + 1] = 1.0f - ((float)i / (float)numParallels);
            }
        }
    }

    // Generate the indices
    if (indices != NULL) {
        uint16_t *indexBuf = (*indices);
        for (int i = 0; i < numParallels ; i++) {
            for (int j = 0; j < numSlices; j++) {
                *indexBuf++ = i * (numSlices + 1) + j;
                *indexBuf++ = (i + 1) * (numSlices + 1) + j;
                *indexBuf++ = (i + 1) * (numSlices + 1) + (j + 1);

                *indexBuf++ = i * (numSlices + 1) + j;
                *indexBuf++ = (i + 1) * (numSlices + 1) + (j + 1);
                *indexBuf++ = i * (numSlices + 1) + (j + 1);
            }
        }
    }

    if (numVertices_out) {
        *numVertices_out = numVertices;
    }

    return numIndices;
}

Answer 1

该代码生成如下所示的球体网格:

来源:https://commons.wikimedia.org/wiki/File:Sphere_wireframe_10deg_6r.svg抄送 3.0

如图所示，有水平平行线和垂直线，它们都在两极相交。水平线通常称为平行线，而垂直线称为子午线。该代码的作者显然不知道这个术语，因此他们将其称为“切片”。