What is slices in OpenGL? - ios

Question

In the code bellow , Why we need slices ? and what does it for ?
//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

That code generates a sphere mesh that looks like this:
Source: https://commons.wikimedia.org/wiki/File:Sphere_wireframe_10deg_6r.svg CC BY 3.0
As you can see in the picture, there are horizontal parallel lines, and vertical lines which all meet at the poles. The horizontal lines are typically called parallels whereas the vertical ones are called meridians. The author of that code apparently didn't know this term, so they called it "slices" instead.