Cogl 2.0 Reference Manual  

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IndicesIndices — Describe vertex indices stored in a CoglIndexBuffer. 
CoglIndices; CoglBool cogl_is_indices (void *object
); enum 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
);
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 reuseable indices
for drawing quads as above.
CoglBool cogl_is_indices (void *object
);
Gets whether the given object references a CoglIndices.

A CoglObject pointer 
Returns : 
TRUE if the object references a CoglIndices
and FALSE otherwise. 
Since 1.10
Stability Level: Unstable
typedef enum { COGL_INDICES_TYPE_UNSIGNED_BYTE, COGL_INDICES_TYPE_UNSIGNED_SHORT, COGL_INDICES_TYPE_UNSIGNED_INT } CoglIndicesType;
You should aim to use the smallest data type that gives you enough range, since it reduces the size of your index array and can help reduce the demand on memory bandwidth.
Note that COGL_INDICES_TYPE_UNSIGNED_INT
is only supported if the
COGL_FEATURE_ID_UNSIGNED_INT_INDICES
feature is available. This
should always be available on OpenGL but on OpenGL ES it will only
be available if the GL_OES_element_index_uint extension is
advertized.
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
);