Arrays in GLSL You can use arrays in GLSL to store more than one object. You can declare an array outside of your main() function something like this: const int ns = 2; // MUST BE CONSTANT vec4 Sph[ns]; // YOU CANNOT ASSIGN VALUES TO ARRAY ELEMENTS HERE For now, we assign values inside the main() function, something like this: main() { ... Sph[0] = vec4(....); Sph[1] = vec4(....); ... } This is wasteful, because we are assigning values for every call to main(). In the next class we will learn how to send values down from the CPU to the GPU, so that array values can be assigned just once per animation frame. Also, when you loop over the values of an array, the number of times you will be looping must be known at compile time (since loops are actually unrolled by the compiler). So this, for example, is valid, because we have declared ns to be a constant: for (int is = 0 ; is < ns ; is++) { ... vec4 sph = Sph[is]; ... } In contrast, the following code is not valid, because the number of iterations of the loop cannot be known at compile time: void function myFunction(int n) { for (int is = 0 ; is < n ; is++) { ... vec4 sph = Sph[is]; ... } } In fact, the above code will produce the following compiler error: Loop index cannot be compared with non-constant expression Storing values in matrices We also learned in class that we can use the mat4 data type to store up to 16 values. For example, we might store the ambient, diffuse and specular values for a surface within a mat4 as follows: mat4 M = mat4(.1,.0,.0,.0, .5,.0,.0,.0, 1.,1.,1.,20., .0,.0,.0,.0); Note that the mat4(...) constructor needs to have exactly 16 floating point arguments, which is why we pad it on the end with zeroes. We can access the columns of a mat4 as M[0],M[1],M[2],M[3]. So, for example, to retrieve the ambient, diffuse and specular rgb color values, as well as the specular power, we could specify: vec3 ambient = M[0].rgb; vec3 diffuse = M[1].rgb; vec3 specular = M[2].rgb; float p = M[2].a ; // get the 4th element of column 2 of the matrix.