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              (unsigned int n_vertices);
unsigned int        cogl_vertex_buffer_get_n_vertices   (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);
enum                CoglVerticesMode;
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
                                                        (unsigned int n_indices);
gboolean            cogl_is_vertex_buffer_indices       (CoglHandle handle);

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              (unsigned int 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 ()

unsigned int        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_add ()

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

Adds an attribute to a buffer.

You either can 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 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 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);

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);

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);

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);

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

enum CoglVerticesMode

typedef enum {
  COGL_VERTICES_MODE_POINTS = GL_POINTS,
  COGL_VERTICES_MODE_LINE_STRIP = GL_LINE_STRIP,
  COGL_VERTICES_MODE_LINE_LOOP = GL_LINE_LOOP,
  COGL_VERTICES_MODE_LINES = GL_LINES,
  COGL_VERTICES_MODE_TRIANGLE_STRIP = GL_TRIANGLE_STRIP,
  COGL_VERTICES_MODE_TRIANGLE_FAN = GL_TRIANGLE_FAN,
  COGL_VERTICES_MODE_TRIANGLES = GL_TRIANGLES
} CoglVerticesMode;

How vertices passed to cogl_vertex_buffer_draw() and cogl_vertex_buffer_draw_elements() should be interpreted

COGL_VERTICES_MODE_POINTS

FIXME, equivalent to GL_POINTS

COGL_VERTICES_MODE_LINE_STRIP

FIXME, equivalent to GL_LINE_STRIP

COGL_VERTICES_MODE_LINE_LOOP

FIXME, equivalent to GL_LINE_LOOP

COGL_VERTICES_MODE_LINES

FIXME, equivalent to GL_LINES

COGL_VERTICES_MODE_TRIANGLE_STRIP

FIXME, equivalent to GL_TRIANGLE_STRIP

COGL_VERTICES_MODE_TRIANGLE_FAN

FIXME, equivalent to GL_TRIANGLE_FAN

COGL_VERTICES_MODE_TRIANGLES

FIXME, equivalent to GL_TRIANGLES

Since 1.0


cogl_vertex_buffer_draw ()

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

Allows you to 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 {
  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_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.

COGL_INDICES_TYPE_UNSIGNED_BYTE

Your indices are unsigned bytes

COGL_INDICES_TYPE_UNSIGNED_SHORT

Your indices are unsigned shorts

COGL_INDICES_TYPE_UNSIGNED_INT

Your indices are unsigned ints

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. [array length=indices_len]

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
                                                        (unsigned int 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 like that shown in Figure 4, “Example of vertices submitted to form a quad” then you can request 6 indices to render two triangles like those shown in Figure 5, “Illustration of the triangle indices that will be generated”.

Figure 4. Example of vertices submitted to form a quad

Example of vertices submitted to form a quad


Figure 5. Illustration of the triangle indices that will be generated

Illustration of the triangle indices that will be generated


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.

cogl_is_vertex_buffer_indices ()

gboolean            cogl_is_vertex_buffer_indices       (CoglHandle handle);

Checks whether handle is a handle to the indices for a vertex buffer object

handle :

a CoglHandle

Returns :

TRUE if the handle is indices, and FALSE otherwise

Since 1.4