Texturing and Shading - Learn OpenGL ES: For Mobile Game and Graphics Development - page 183

▪ GL_STREAM_DRAW
These values are defined as static constants in the android.opengl.GLES20
class. They provide a hint to OpenGL ES on how the application is going to
use the data stored in the buffer object. Of these values, GL_STATIC_DRAW and
GL_DYNAMIC_DRAW are most commonly used.
As the name suggests, GL_STATIC_DRAW is used when the application does not
modify the data stored in the buffer object, whereas GL_DYNAMIC_DRAW is used
when the buffer object data is specified repeatedly by the application (and used many
times to draw primitives). In the context of the Tank Fence game, only missiles use
the GL_DYNAMIC_DRAW buffer usage. The buffer object corresponding to missiles
undergoes rewrites in a repeated manner, so as to update the vertex data (and the cor-
responding indices) representing the centers of missiles. The remaining objects in
this game (that is, the plane, enemy, and player) use the GL_STATIC_DRAW buffer
usage.
Note We will work with the missile objects in Chapter 6 . They are
introduced here so you can understand the GL_DYNAMIC_DRAW
buffer usage.
So far, we've only looked at how buffer objects are generated and then filled with
corresponding data. We still have not discussed the way in which buffer objects' data
is passed to the shaders (to be specific, a vertex shader). Listing 5-2 shows that doing
so is quite easy and almost similar to how you have been passing data (per-vertex
and index data) without the use of buffer objects.
Passing a buffer object's data (to the shaders ) also requires you to make this buffer
object the current buffer object, by calling glBindBuffer , as shown in Listing
5-2 . Then, call glVertexAttribPointer to tell OpenGL about the format and
source of our vertex array data (as discussed in Chapter 3 ) . Finally, call glEn-
ableVertexAttribArray to activate the given attribute location. You may
have noticed that the call to glVertexAttribPointer ( Listing 5-2 ) no longer
requires you to explicitly specify the FloatBuffer
( java.nio.FloatBuffer ). This is because calling glBindBuffer makes
the specified buffer object (as in this case, array buffer object ) the current buffer
object. Similarly, glDrawElements does not require you to explicitly specify