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Functions
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 andcogl_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.
[constructor]
Parameters
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
|
|
offset |
The byte offset from the start of |
|
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
.
[transfer full]
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
);
Returns
the CoglAttributeBuffer that was
set with cogl_attribute_set_buffer()
or cogl_attribute_new()
.
[transfer none]
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