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:

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.

Since 1.4

Stability Level: Unstable

cogl_is_attribute ()

CoglBool            cogl_is_attribute                   (void *object);

Gets whether the given object references a CoglAttribute.

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.

Since 1.10

Stability Level: Unstable

cogl_attribute_get_normalized ()

CoglBool            cogl_attribute_get_normalized       (CoglAttribute *attribute);

Since 1.10

Stability Level: Unstable

cogl_attribute_get_buffer ()

CoglAttributeBuffer * cogl_attribute_get_buffer         (CoglAttribute *attribute);

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.

Since 1.10

Stability Level: Unstable