Graphics Reference
In-Depth Information
After setting the viewport, the next step is to clear the screen. In OpenGL
ES, multiple types of buffers are involved in drawing: color, depth, and
stencil. We cover these buffers in more detail in Chapter 11, “Fragment
Operations.” In the Hello Triangle example, only the color buffer is drawn
to. At the beginning of each frame, we clear the color buffer using the
glClear
function.
// Clear the color buffer
glClear(GL_COLOR_BUFFER_BIT);
The buffer will be cleared to the color specified with
glClearColor
. In the
example program at the end of
Init
, the clear color was set to (1.0, 1.0,
1.0, 1.0), so the screen is cleared to white. The clear color should be set by
the application prior to calling
glClear
on the color buffer.
Now that we have the color buffer cleared, viewport set, and program
object loaded, we need to specify the geometry for the triangle. The
vertices for the triangle are specified with three (
x
,
y
,
z
) coordinates in the
vVertices
array.
GLfloat vVertices[] = { O.Of, 0.5f, O.Of,
-0.5f, -0.5f, O.Of,
0.5f, -0.5f, O.Of};
…
// Load the vertex data
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, vVertices);
glEnableVertexAttribArray(O) ;
glDrawArrays(GL_TRIANGLES, 0, 3);
The vertex positions need to be loaded to the GL and connected to the
vPosition
attribute declared in the vertex shader. As you will remember,
earlier we bound the
vPosition
variable to the input attribute location
0. Each attribute in the vertex shader has a location that is uniquely
identified by an unsigned integer value. To load the data into vertex
attribute 0, we call the
glVertexAttribPointer
function. In Chapter 6,
“Vertex Attributes, Vertex Arrays, and Buffer Objects,” we cover how to
load vertex attributes and use vertex arrays in full.
The final step in drawing the triangle is to actually tell OpenGL ES to draw
the primitive. In this example, we use the function
glDrawArrays
for
