In this lab, we’re going to learn about texture mapping — what texture coordinates do, how to set up the shaders to use textures, and how to manipulate the texture with the texture matrix.
Please download the code for the lab and go over the code.
Look though the code, paying particular attention to how the textures
are being loaded. We’ll be editing main.cpp and the two
.glsl files.
When you compile and run the code, you’ll get a blank square composed
of two triangles. You can press the w key to toggle on and
off the “wireframe” mode in order to see the two triangles. The square
is white because even though the texture is being loaded, the texture
coordinates are all set to zero.
To see the texture properly, fix the texture coordinates of the four
vertices of the square. Remember that the texture coordinates go from
0.0 to 1.0 in both \(S\) and \(T\) directions. (Many people use \(U\) and \(V\) for texture coordinates, but GLSL uses
\(S\) and \(T\).) Look at the two shaders to see how
the texture coordinates are being passed from main.cpp to the vertex
shader to the fragment shader. Once the texture coordinates are fixed,
you should see the TAMU logo as shown here.
We are now going to add another texture to the same square. Edit the
fragment shader so that the Reveille texture is overlaid on top. Assume
here that the background color of the reveille texture is white
(1.0, 1.0, 1.0). If the color of the Reveille texture is
white, or close to white, then use the color from the original TAMU
texture. Otherwise, use the color from the new Reveille texture.
if Reville texture color is white
Use TAMU texture color
else
Use Reville texture color
We now want to make the Reveille look twice as small. There are three
potential ways to do this. (1) Change the texture file. (2) Change the
texture coordinates. (3) Use the texture matrix. The first approach is
undesirable, since we don’t want to have to recreate the texture file
every time we want to scale the Reveille. The second approach would be
great, if we want fine-grained control, but this manual approach is very
tedious. The third approach is the best, if we want a simple transform,
such as a scale. So let’s take this approach. To do this, we have to
modify main.cpp and the two shaders.
First, modify the vertex shader so that vTex1 is
multiplied by the texture matrix, T1, rather than being
passed straight through to the fragment shader. Note that the texture
matrix is a 3x3 transformation matrix. Texture coordinates are points
(not vectors), so you’ll need to add a \(1.0\) as the homogeneous coordinate to form
a vec3. Also modify the fragment shader to use this new
texture coordinates for the Reveille texture. Currently, the texture
matrix is the identity matrix, so it does not have any effect on the
texture coordinates.
In main.cpp, modify the contents of the texture matrix
in the init() function. The scaling factor we need to make
the Reveille appear twice as small will not be what you expect. Make
sure you understand why this is. Since the origin of the texture
coordinates is at the lower left corner of the square, the Reveille will
shrink to the lower left. This is ok for this lab. Also, don’t worry if
you get a weird shade of gray in the rest of the square. We’ll fix this
in the next step.
Make multiple Reveilles show up on the square by changing the texture
wrap modes from GL_CLAMP_TO_EDGE to GL_REPEAT.
This makes OpenGL repeat the texture outside of the \(0\) to \(1\) range, rather than clamping it.
Finally, add a keyboard hook to make Reveille move to the right. You
should do this by adding some code in the char_callback()
function that modifies the texture matrix. The translation factors that
you add should be small, around \(0.05\).
