Texturing and Shading - Learn OpenGL ES: For Mobile Game and Graphics Development - page 209

▪ Inside the vertex shader, store the current (transformed) normal in
a variable of type vec3 using the following code. This normal
corresponds to the N vector, shown in Figure 5-8 . vec3 normal
= normalize(vec3(uNormal * aNormal));
▪ Here, uNormal is the uniform variable (of type mat3 ) to store
the normal matrix, and aNormal is an attribute variable that re-
ceives vertex normals. Similar to how we pass vertices using
glVertexAttribPointer , we also pass the vertex normals.
Recall from Chapter 4 (“Parsing Objects for OpenGL ES”) that
you can access the vertex normals from the “normal:” block in the
parser's output text file.
▪ The normalize function, as the name suggests, is a built-in
function in ES 2.0 that helps normalize the given vector. Normal-
ization is necessary to avoid any scaling of the vectors.
▪ To pass the 3x3 normal matrix to the vertex shader, use the ES 2.0
function glUniformMatrix3fv , and call it as:
GLES20.glUniformMatrix3fv(_tankUNormalLocation,
1, false, _tankNormalMatrix, 0);
▪ Here, _tankNormalMatrix ( float[9] ) is the normal mat-
rix, obtained by copying the upper-left 3x3 portion of the MV
matrix corresponding to the object ( Listing 5-14 ) . You do not
need to create a separate MV matrix to copy the required values;
instead, copy the values from the MVP matrix, before it is com-
bined with the projection transformation, as shown in the follow-
ing code.
Listing 5-14. VERTEX POINT LIGHTING/src/com/apress/android/vertexpoint-
lighting/GLES20Renderer.java
_tankNormalMatrix[0] = _tankMVPMatrix[0];
_tankNormalMatrix[1] = _tankMVPMatrix[1];
_tankNormalMatrix[2] = _tankMVPMatrix[2]; // from 1st
column, ending at [3]
_tankNormalMatrix[3] = _tankMVPMatrix[4];
_tankNormalMatrix[4] = _tankMVPMatrix[5];