ES 2.0 Fundamentals - Learn OpenGL ES: For Mobile Game and Graphics Development - page 101

GLES20.glDrawArrays(GLES20.GL_POINTS, 0, 4);

}

The call to ES 2.0 functions— glVertexAttribPointer and glEnableVer-
texAttribArray (and others explained in the following section)—inside
onDrawFrame is sandwiched between function calls for using an ES 2.0 program
( glUseProgram in Listing 3-20 ) and drawing primitives ( glDrawArrays or
glDrawElements ). This must be done every time we render an object (that is, a
primitive or a combination of same type of primitives).

I'll deal with the glDrawElements function in Chapter 4 . Here, I explain
glDrawArrays . This method takes three arguments— mode , first , and
count —all of type int . Using mode , we specify the type of primitive we want
to render. We work with modes GL_POINTS , GL_LINES , and GL_TRIANGLES ,
but please be aware that ES 2.0 also provides alternate modes ( GL_LINE_STRIP ,
GL_LINE_LOOP , GL_TRIANGLE_STRIP , and GL_TRIANGLE_FAN ).
GL_POINTS mode is very straightforward. By calling glDrawAr-
rays(GLES20.GL_POINTS, 0, 4) , we notify the vertex shader that we want
to render four points (point sprites) on the OpenGL surface. The second argument
is '0,' because we want to render from the first vertex inside pointVFA (0.1f,
0.1f, 0.0f) in Listing 3-17 .

Similarly, for rendering a line primitive, we call glDrawAr-
rays(GLES20.GL_LINES, 0, 2) , and for a triangle primitive, we call
glDrawArrays(GLES20.GL_TRIANGLES, 0, 3) . Next, I'll show you how
to render a line, triangle, and rectangle using glDrawArrays .

Drawing Line and Triangle Primitives

For the following applications, you may use the GL POINT ADVANCED application
( glpointadvanced.zip ) as a template. To render a line primitive ( Figure 3-11 )
using glDrawArrays method, we must make sure we set its mode argument as
GLES20.GL_LINES . Since a line requires two points, while rendering a line prim-
itive, the count argument (the last argument in the glDrawArrays method) should
be at least '2.'