Description

Indices allow you to avoid duplicating vertices in your vertex data by virtualizing your data and instead providing a sequence of index values that tell the GPU which data should be used for each vertex.

If the GPU is given a sequence of indices it doesn't simply walk through each vertex of your data in order it will instead walk through the indices which can provide random access to the underlying data.

Since it's very common to have duplicate vertices when describing a shape as a list of triangles it can often be a significant space saving to describe geometry using indices. Reducing the size of your models can make it cheaper to map them into the GPU by reducing the demand on memory bandwidth and may help to make better use of your GPUs internal vertex caching.

For example, to describe a quadrilateral as 2 triangles for the GPU you could either provide data with 6 vertices or instead with indices you can provide vertex data for just 4 vertices and an index buffer that specfies the 6 vertices by indexing the shared vertices multiple times.

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CoglVertex2f quad_vertices[] = {
    {x0, y0}, //0 = top left
    {x1, y1}, //1 = bottom left
    {x2, y2}, //2 = bottom right
    {x3, y3}, //3 = top right
  };
  //tell the gpu how to interpret the quad as 2 triangles...
  unsigned char indices[] = {0, 1, 2, 0, 2, 3};

Even in the above illustration we see a saving of 10bytes for one quad compared to having data for 6 vertices and no indices but if you need to draw 100s or 1000s of quads then its really quite significant.

Something else to consider is that often indices can be defined once and remain static while the vertex data may change for animations perhaps. That means you may be able to ignore the negligable cost of mapping your indices into the GPU if they don't ever change.

The above illustration is actually a good example of static indices because it's really common that developers have quad mesh data that they need to display and we know exactly what that indices array needs to look like depending on the number of quads that need to be drawn. It doesn't matter how the quads might be animated and changed the indices will remain the same. Cogl even has a utility (cogl_get_rectangle_indices()) to get access to re-useable indices for drawing quads as above.

Details

typedef struct _CoglIndices CoglIndices;

CoglBool            cogl_is_indices                     (void *object);

typedef enum {
  COGL_INDICES_TYPE_UNSIGNED_BYTE,
  COGL_INDICES_TYPE_UNSIGNED_SHORT,
  COGL_INDICES_TYPE_UNSIGNED_INT
} CoglIndicesType;

CoglIndices *       cogl_indices_new                    (CoglContext *context,
                                                         CoglIndicesType type,
                                                         const void *indices_data,
                                                         int n_indices);

CoglIndices *       cogl_get_rectangle_indices          (CoglContext *context,
                                                         int n_rectangles);