Vertex Attributes

Vertex Attributes — Functions for declaring and drawing vertex attributes

Synopsis

                    CoglAttribute;
CoglAttribute *     cogl_attribute_new                  (CoglAttributeBuffer *attribute_buffer,
                                                         const char *name,
                                                         size_t stride,
                                                         size_t offset,
                                                         int components,
                                                         CoglAttributeType type);
CoglBool            cogl_is_attribute                   (void *object);
void                cogl_attribute_set_normalized       (CoglAttribute *attribute,
                                                         CoglBool normalized);
CoglBool            cogl_attribute_get_normalized       (CoglAttribute *attribute);
CoglAttributeBuffer * cogl_attribute_get_buffer         (CoglAttribute *attribute);
void                cogl_attribute_set_buffer           (CoglAttribute *attribute,
                                                         CoglAttributeBuffer *attribute_buffer);

Description

FIXME

Details

CoglAttribute

typedef struct _CoglAttribute CoglAttribute;

cogl_attribute_new ()

CoglAttribute *     cogl_attribute_new                  (CoglAttributeBuffer *attribute_buffer,
                                                         const char *name,
                                                         size_t stride,
                                                         size_t offset,
                                                         int components,
                                                         CoglAttributeType type);

Describes the layout for a list of vertex attribute values (For example, a list of texture coordinates or colors).

The name is used to access the attribute inside a GLSL vertex shader and there are some special names you should use if they are applicable:

  • "cogl_position_in" (used for vertex positions)
  • "cogl_color_in" (used for vertex colors)
  • "cogl_tex_coord0_in", "cogl_tex_coord1", ... (used for vertex texture coordinates)
  • "cogl_normal_in" (used for vertex normals)
  • "cogl_point_size_in" (used to set the size of points per-vertex. Note this can only be used if COGL_FEATURE_ID_POINT_SIZE_ATTRIBUTE is advertised and cogl_pipeline_set_per_vertex_point_size() is called on the pipeline.

The attribute values corresponding to different vertices can either be tightly packed or interleaved with other attribute values. For example it's common to define a structure for a single vertex like:

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typedef struct
{
  float x, y, z; /* position attribute */
  float s, t; /* texture coordinate attribute */
} MyVertex;

And then create an array of vertex data something like:

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MyVertex vertices[100] = { .... }

In this case, to describe either the position or texture coordinate attribute you have to move sizeof (MyVertex) bytes to move from one vertex to the next. This is called the attribute stride. If you weren't interleving attributes and you instead had a packed array of float x, y pairs then the attribute stride would be (2 * sizeof (float)). So the stride is the number of bytes to move to find the attribute value of the next vertex.

Normally a list of attributes starts at the beginning of an array. So for the MyVertex example above the offset is the offset inside the MyVertex structure to the first component of the attribute. For the texture coordinate attribute the offset would be offsetof (MyVertex, s) or instead of using the offsetof macro you could use sizeof (float) * 3. If you've divided your array into blocks of non-interleved attributes then you will need to calculate the offset as the number of bytes in blocks preceding the attribute you're describing.

An attribute often has more than one component. For example a color is often comprised of 4 red, green, blue and alpha components, and a position may be comprised of 2 x and y components. You should aim to keep the number of components to a minimum as more components means more data needs to be mapped into the GPU which can be a bottlneck when dealing with a large number of vertices.

Finally you need to specify the component data type. Here you should aim to use the smallest type that meets your precision requirements. Again the larger the type then more data needs to be mapped into the GPU which can be a bottlneck when dealing with a large number of vertices.

attribute_buffer :

The CoglAttributeBuffer containing the actual attribute data

name :

The name of the attribute (used to reference it from GLSL)

stride :

The number of bytes to jump to get to the next attribute value for the next vertex. (Usually sizeof (MyVertex))

offset :

The byte offset from the start of attribute_buffer for the first attribute value. (Usually offsetof (MyVertex, component0)

components :

The number of components (e.g. 4 for an rgba color or 3 for and (x,y,z) position)

type :

FIXME

Returns :

A newly allocated CoglAttribute describing the layout for a list of attribute values stored in array.

Since 1.4

Stability Level: Unstable


cogl_is_attribute ()

CoglBool            cogl_is_attribute                   (void *object);

Gets whether the given object references a CoglAttribute.

object :

A CoglObject

Returns :

TRUE if the object references a CoglAttribute, FALSE otherwise

cogl_attribute_set_normalized ()

void                cogl_attribute_set_normalized       (CoglAttribute *attribute,
                                                         CoglBool normalized);

Sets whether fixed point attribute types are mapped to the range 0→1. For example when this property is TRUE and a COGL_ATTRIBUTE_TYPE_UNSIGNED_BYTE type is used then the value 255 will be mapped to 1.0.

The default value of this property depends on the name of the attribute. For the builtin properties cogl_color_in and cogl_normal_in it will default to TRUE and for all other names it will default to FALSE.

attribute :

A CoglAttribute

normalized :

The new value for the normalized property.

Since 1.10

Stability Level: Unstable


cogl_attribute_get_normalized ()

CoglBool            cogl_attribute_get_normalized       (CoglAttribute *attribute);

attribute :

A CoglAttribute

Returns :

the value of the normalized property set with cogl_attribute_set_normalized().

Since 1.10

Stability Level: Unstable


cogl_attribute_get_buffer ()

CoglAttributeBuffer * cogl_attribute_get_buffer         (CoglAttribute *attribute);

attribute :

A CoglAttribute

Returns :

the CoglAttributeBuffer that was set with cogl_attribute_set_buffer() or cogl_attribute_new().

Since 1.10

Stability Level: Unstable


cogl_attribute_set_buffer ()

void                cogl_attribute_set_buffer           (CoglAttribute *attribute,
                                                         CoglAttributeBuffer *attribute_buffer);

Sets a new CoglAttributeBuffer for the attribute.

attribute :

A CoglAttribute

attribute_buffer :

A CoglAttributeBuffer

Since 1.10

Stability Level: Unstable