## Vertex Buffers

Vertex Buffers — An API for submitting extensible arrays of vertex attributes to be mapped into the GPU for fast drawing.

## Synopsis

    CoglHandle          cogl_vertex_buffer_new              (guint n_vertices);
    guint               cogl_vertex_buffer_get_n_vertices   (CoglHandle handle);
    CoglHandle          cogl_vertex_buffer_ref              (CoglHandle handle);
    void                cogl_vertex_buffer_unref            (CoglHandle handle);
    void                cogl_vertex_buffer_add              (CoglHandle handle,
                                                             const char *attribute_name,
                                                             guint8 n_components,
                                                             CoglAttributeType type,
                                                             gboolean normalized,
                                                             guint16 stride,
                                                             const void *pointer);
    void                cogl_vertex_buffer_delete           (CoglHandle handle,
                                                             const char *attribute_name);
    void                cogl_vertex_buffer_submit           (CoglHandle handle);
    void                cogl_vertex_buffer_disable          (CoglHandle handle,
                                                             const char *attribute_name);
    void                cogl_vertex_buffer_enable           (CoglHandle handle,
                                                             const char *attribute_name);
    void                cogl_vertex_buffer_draw             (CoglHandle handle,
                                                             CoglVerticesMode mode,
                                                             int first,
                                                             int count);
    gboolean            cogl_is_vertex_buffer               (CoglHandle handle);

    enum                CoglIndicesType;
    CoglHandle          cogl_vertex_buffer_indices_new      (CoglIndicesType indices_type,
                                                             const void *indices_array,
                                                             int indices_len);
    void                cogl_vertex_buffer_draw_elements    (CoglHandle handle,
                                                             CoglVerticesMode mode,
                                                             CoglHandle indices,
                                                             int min_index,
                                                             int max_index,
                                                             int indices_offset,
                                                             int count);
    CoglHandle          cogl_vertex_buffer_indices_get_for_quads
                                                            (guint n_indices);

## Description

For example to describe a textured triangle, you could create a new cogl vertex buffer with 3 vertices, and then you might add 2 attributes for each vertex:

1. a "gl_Position" describing the (x,y,z) position for each vertex.
2. a "gl_MultiTexCoord0" describing the (tx,ty) texture coordinates for each vertex.

The Vertex Buffer API is designed to be a fairly raw mechanism for developers to be able to submit geometry to Cogl in a format that can be directly consumed by an OpenGL driver and mapped into your GPU for fast re-use. It is designed to avoid repeated validation of the attributes by the driver; to minimize transport costs (e.g. considering indirect GLX use-cases) and to potentially avoid repeated format conversions when attributes are supplied in a format that is not natively supported by the GPU.

Although this API does allow you to modify attributes after they have been submitted to the GPU you should be aware that modification is not that cheap, since it implies validating the new data and potentially the OpenGL driver will need to reformat it for the GPU.

If at all possible think of tricks that let you re-use static attributes, and if you do need to repeatedly update attributes (e.g. for some kind of morphing geometry) then only update and re-submit the specific attributes that have changed.

## Details

### cogl_vertex_buffer_new ()

    CoglHandle          cogl_vertex_buffer_new              (guint n_vertices);

Creates a new vertex buffer that you can use to add attributes.

`n_vertices` : | The number of vertices that your attributes will correspond to.
Returns : | a new CoglHandle

### cogl_vertex_buffer_get_n_vertices ()

    guint               cogl_vertex_buffer_get_n_vertices   (CoglHandle handle);

Retrieves the number of vertices that `handle` represents

`handle` : | A vertex buffer handle
Returns : | the number of vertices

### cogl_vertex_buffer_ref ()

    CoglHandle          cogl_vertex_buffer_ref              (CoglHandle handle);

Increment the reference count for a vertex buffer

`handle` : | a `CoglHandle`.
Returns : | the `handle`.

### cogl_vertex_buffer_unref ()

    void                cogl_vertex_buffer_unref            (CoglHandle handle);

Decrement the reference count for a vertex buffer

`handle` : | a `CoglHandle`.

### cogl_vertex_buffer_add ()

    void                cogl_vertex_buffer_add              (CoglHandle handle,
                                                             const char *attribute_name,
                                                             guint8 n_components,
                                                             CoglAttributeType type,
                                                             gboolean normalized,
                                                             guint16 stride,
                                                             const void *pointer);

This function lets you add an attribute to a buffer. You either use one of the built-in names such as "gl_Vertex", or "gl_MultiTexCoord0" to add standard attributes, like positions, colors and normals or you can add custom attributes for use in shaders.

The number of vertices declared when calling `cogl_vertex_buffer_new()` determines how many attribute values will be read from the supplied pointer.

The data for your attribute isn't copied anywhere until you call `cogl_vertex_buffer_submit()`, (or issue a draw call which automatically submits pending attribute changes) so the supplied pointer must remain valid until then. If you are updating an existing attribute (done by re-adding it) then you still need to re-call `cogl_vertex_buffer_submit()` to commit the changes to the GPU. (Be carefull to minimize the number of calls to cogl_vertex_buffer_submit though.)

Note: If you are interleving attributes it is assumed that each interleaved attribute starts no farther than +- stride bytes from the other attributes it is interleved with. I.e. this is ok:

    |-0-0-0-0-0-0-0-0-0-0|

This is not ok:

    |- - - - -0-0-0-0-0-0 0 0 0 0|

(Though you can have multiple groups of interleved attributes)

`handle` : | A vertex buffer handle

`attribute_name` :

The name of your attribute. It should be a valid GLSL variable name and standard attribute types must use one of following built-in names: (Note: they correspond to the built-in names of GLSL)

- "gl_Color"
- "gl_Normal"
- "gl_MultiTexCoord0, gl_MultiTexCoord1, ..."
- "gl_Vertex"

To support adding multiple variations of the same attribute the name can have a detail component, E.g. "gl_Color::active" or "gl_Color::inactive"

`n_components` : | The number of components per attribute and must be 1,2,3 or 4
`type` : | a CoglAttributeType specifying the data type of each component.
`normalized` : | If GL_TRUE, this specifies that values stored in an integer format should be mapped into the range [-1.0, 1.0] or [0.0, 1.0] for unsigned values. If GL_FALSE they are converted to floats directly.
`stride` : | This specifies the number of bytes from the start of one attribute value to the start of the next value (for the same attribute). So for example with a position interleved with color like this: XYRGBAXYRGBAXYRGBA, then if each letter represents a byte, the stride for both attributes is 6. The special value 0 means the values are stored sequentially in memory.
`pointer` : | This addresses the first attribute in the vertex array. (This must remain valid until you either call `cogl_vertex_buffer_submit()` or issue a draw call.)

### cogl_vertex_buffer_delete ()

    void                cogl_vertex_buffer_delete           (CoglHandle handle,
                                                             const char *attribute_name);

This function deletes an attribute from a buffer. You will need to call `cogl_vertex_buffer_submit()` or issue a draw call to commit this change to the GPU.

`handle` : | A vertex buffer handle
`attribute_name` : | The name of a previously added attribute

### cogl_vertex_buffer_submit ()

    void                cogl_vertex_buffer_submit           (CoglHandle handle);

This function submits all the user added attributes to the GPU; once submitted the attributes can be used for drawing.

You should aim to minimize calls to this function since it implies validating your data; it potentially incurs a transport cost (especially if you are using GLX indirect rendering) and potentially a format conversion cost if the GPU doesn't natively support any of the given attribute formats.

`handle` : | A vertex buffer handle

### cogl_vertex_buffer_disable ()

    void                cogl_vertex_buffer_disable          (CoglHandle handle,
                                                             const char *attribute_name);

This function disables a previosuly added attribute.

Since it can be costly to add and remove new attributes to buffers; to make individual buffers more reuseable it is possible to enable and disable attributes before using a buffer for drawing.

You don't need to call `cogl_vertex_buffer_submit()` after using this function.

`handle` : | A vertex buffer handle
`attribute_name` : | The name of the attribute you want to disable

### cogl_vertex_buffer_enable ()

    void                cogl_vertex_buffer_enable           (CoglHandle handle,
                                                             const char *attribute_name);

This function enables a previosuly disabled attribute.

Since it can be costly to add and remove new attributes to buffers; to make individual buffers more reuseable it is possible to enable and disable attributes before using a buffer for drawing.

You don't need to call `cogl_vertex_buffer_submit()` after using this function

`handle` : | A vertex buffer handle
`attribute_name` : | The name of the attribute you want to enable

### cogl_vertex_buffer_draw ()

    void                cogl_vertex_buffer_draw             (CoglHandle handle,
                                                             CoglVerticesMode mode,
                                                             int first,
                                                             int count);

This function lets you draw geometry using all or a subset of the vertices in a vertex buffer.

Any un-submitted attribute changes are automatically submitted before drawing.

`handle` : | A vertex buffer handle
`mode` : | A CoglVerticesMode specifying how the vertices should be interpreted.
`first` : | Specifies the index of the first vertex you want to draw with
`count` : | Specifies the number of vertices you want to draw.

### cogl_is_vertex_buffer ()

    gboolean            cogl_is_vertex_buffer               (CoglHandle handle);

Checks whether `handle` is a Vertex Buffer Object

`handle` : | a CoglHandle for a vertex buffer object
Returns : | `TRUE` if the handle is a VBO, and `FALSE` otherwise

Since 1.0

### enum CoglIndicesType

    typedef enum _CoglIndicesType
    {
      COGL_INDICES_TYPE_UNSIGNED_BYTE,
      COGL_INDICES_TYPE_UNSIGNED_SHORT,
    } 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.

`COGL_INDICES_TYPE_UNSIGNED_BYTE` | Your indices are unsigned bytes
`COGL_INDICES_TYPE_UNSIGNED_SHORT` | Your indices are unsigned shorts

### cogl_vertex_buffer_indices_new ()

    CoglHandle          cogl_vertex_buffer_indices_new      (CoglIndicesType indices_type,
                                                             const void *indices_array,
                                                             int indices_len);

Depending on how much geometry you are submitting it can be worthwhile optimizing the number of redundant vertices you submit. Using an index array allows you to reference vertices multiple times, for example during triangle strips.

`indices_type` : | a CoglIndicesType specifying the data type used for the indices.
`indices_array` : | Specifies the address of your array of indices
`indices_len` : | The number of indices in indices_array
Returns : | A CoglHandle for the indices which you can pass to `cogl_vertex_buffer_draw_elements()`.

### cogl_vertex_buffer_draw_elements ()

    void                cogl_vertex_buffer_draw_elements    (CoglHandle handle,
                                                             CoglVerticesMode mode,
                                                             CoglHandle indices,
                                                             int min_index,
                                                             int max_index,
                                                             int indices_offset,
                                                             int count);

This function lets you use an array of indices to specify the vertices within your vertex buffer that you want to draw. The indices themselves are created by calling `cogl_vertex_buffer_indices_new()`

Any un-submitted attribute changes are automatically submitted before drawing.

`handle` : | A vertex buffer handle
`mode` : | A CoglVerticesMode specifying how the vertices should be interpreted.
`indices` : | A CoglHandle for a set of indices allocated via `cogl_vertex_buffer_indices_new()`
`min_index` : | Specifies the minimum vertex index contained in indices
`max_index` : | Specifies the maximum vertex index contained in indices
`indices_offset` : | An offset into named indices. The offset marks the first index to use for drawing.
`count` : | Specifies the number of vertices you want to draw.

### cogl_vertex_buffer_indices_get_for_quads ()

    CoglHandle          cogl_vertex_buffer_indices_get_for_quads
                                                            (guint n_indices);

Creates a vertex buffer containing the indices needed to draw pairs of triangles from a list of vertices grouped as quads. There will be at least `n_indices` entries in the buffer (but there may be more).

The indices will follow this pattern:

0, 1, 2, 0, 2, 3, 4, 5, 6, 4, 6, 7 ... etc

For example, if you submit vertices for a quad like this:

       0        3
        ########
        #      #
        #      #
        ########
       1        2

Then you can request 6 indices to render two triangles like this:

       0           0        3
        ##          ########
        # ##          ##   #
        #   ##          ## #
        ########          ##
       1        2           2

`n_indices` : | the number of indices in the vertex buffer.
Returns : | A `CoglHandle` containing the indices. The handled is owned by Cogl and should not be modified or unref'd.