Graphics Reference
In-Depth Information
The width of a line can be specified using the
glLineWidth
API call.
void
glLineWidth
(GLfloat
width
)
width
specifies the width of the line in pixels; the default
width is 1.0
The width specified by
glLineWidth
will be clamped to the line width
range supported by the OpenGL ES 3.0 implementation. In addition, the
width specified will be remembered by OpenGL until updated by the
application. The supported line width range can be queried using the
following command. There is no requirement for lines with widths greater
than 1 to be supported.
GLfloat lineWidthRange[2];
glGetFloatv ( GL_ALIASED_LINE_WIDTH_RANGE, lineWidthRange );
The point sprite primitive supported by OpenGL ES is
GL_POINTS
. A point
sprite is drawn for each vertex specified. Point sprites are typically used for
rendering particle effects efficiently by drawing them as points instead of
quads. A point sprite is a screen-aligned quad specified as a
position
and a
radius
. The position describes the center of the square, and the radius is
then used to calculate the four coordinates of the quad that describes the
point sprite.
gl_PointSize
is the built-in variable that can be used to output the point
radius (or point size) in the vertex shader. It is important that a vertex
shader associated with the point primitive output
gl_PointSize
; otherwise,
the value of the point size is considered undefined and will most likely result
in drawing errors. The
gl_PointSize
value output by a vertex shader will
be clamped to the aliased point size range supported by the OpenGL ES 3.0
implementation. This range can be queried using the following command:
GLfloat pointSizeRange[2];
glGetFloatv ( GL_ALIASED_POINT_SIZE_RANGE, pointSizeRange );
By default, OpenGL ES 3.0 describes the window origin (0, 0) to be the
(left, bottom) region. However, for point sprites, the point coordinate
origin is (left, top).
gl_PointCoord
is a built-in variable available only inside a fragment
shader when the primitive being rendered is a point sprite. It is declared as
