In the OpenGL ES 2.0 introduction, found here: http://www.webreference.com/programming/opengl_es/2.html a vertex shader is defined:
GLbyte vShaderStr[] =
"attribute vec4 vPosition; \n"
"void main() \n"
"{ \n"
" gl_Position = vPosition; \n"
"}; \n";
The vPosition attribute is a four component vector.
Later in the text the application will compile the vertex shader, together with the fragment shader. With the glBindAttribLocation a handle to pass the applications vertex data to the shader is established:
// Bind vPosition to attribute 0
glBindAttribLocation(programObject, 0, "vPosition");
The 2 shaders are now linked and the program is ready to use.
The use case is this:
GLfloat vVertices[] = {0.0f, 0.5f, 0.0f,
-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 0.0f};
...
// Load the vertex data
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, vVertices);
glEnableVertexAttribArray(0);
glDrawArrays(GL_TRIANGLES, 0, 3);
Which means:
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, vVertices): pass the vertex data to the program, use location 0, each vertex has 3 components, type is float, no normalisation shall be used.glEnableVertexAttribArray(0): The shader will use the passed data.glDrawArrays(GL_TRIANGLES, 0, 3);: The shader will execute the code to draw a single triangle, using the first (and only) three vertices found in the specified array.
My Questions are:
The vertex shader is expecting a four component vertex position (x,y,z,w), the program passes only three components (x,y,z) data Why is this working?
The shader needs a vec4, the w-component must be 1 to work as expected. Which step is responsible for adding the w-component correctly?
