LSU EE 4702-1 Homework 5 Due: 27 November 2017
Problem 0: Follow the instruction on the
http://www.ece.lsu.edu/koppel/gpup/proc.html
page for account setup and programming homework work
flow. Compile and run the homework code unmodified.
It should initially show Scene 1, a cloud of green rectan-
gles, tiles, with balls dropping on them. (The familiar
water wheel is in scene 2.)
Scene Interaction and User Interface
Pressing d (drip) will toggle ball dripping on and off.
Pressing x will toggle a shower of balls. Pressing digits
1 through 2 will initialize different scenes, the program
starts with scene 1. Pressing t starts the lots of balls
scene (t is for test). Pressing T drops fewer balls.
The scenes differ in the number of objects, which
include spheres, tiles, and the platform (which for this
assignment we’ll consider one object). The rendering of
objects by type can be toggled on and off by pressing !,
@, #, for spheres, tiles, and the platform. See the green
text line starting with Hide. The scene includes shadow
and mirror effects.
These can be toggled by pressing o and r, their state is shown next to Effect.
Display of Performance-Related Data
The top green text line shows performance. XF shows the number of display updates per frame
buffer update. An ideal number is 1. A 2 means that two display updates were done in the time
needed to update the frame buffer, presumably because the code could not update the frame buffer
fast enough. GPU.GL shows how long the GPU spends updating the frame buffer (per frame),
GPU.CU shows how long the computational accelerator takes per frame. The computational accel-
erator computes physics in this assignment. On the lab computers the computational accelerator
GPU is different than the one performing graphics. CPU GR is the amount of time that the CPU
spends on graphics, and CPU PH is the amount of time that the CPU spends on physics.
The second line, the one starting with Vertices, shows the number of items being sent down
the rendering pipeline per frame. Clip Prim shows the number of primitives before clipping (in) and
after clipping (out).
User Interface for Navigation and Interaction
Initially the arrow keys, PageUp, and PageDown can be used to move around the scene. Press
(lower-case) b and then use the arrow and page keys to move the first ball around. Press l to move
the light around and e to move the eye (which is what the arrow keys do when the program starts).
Pressing Ctrl+ increases the size of the green text and Ctrl+ decreases it. The + and -
keys can be used to change the value of certain variables to change things like the light intensity,
spring constant, and variables needed for this assignment. The variable currently affected by the
+ and - keys is shown in the bottom line of green text. Pressing Tab cycles through the different
variables. Those who want to increase the spring constant to the point that the scene explodes
may be disappointed to learn that there is a protection mechanism that increases the mass of balls
when the spring constant is high enough to make the system go unstable, such balls turn red.
1
Look at the comments in the file hw05.cc and hw05-shdr.cc for documentation on other keys.
Switching Between Shaders
The program can use three different sets of shaders for the spheres, PLAIN (tiles_0), PROBLEM 1
(tiles_1), and PROBLEM 2 (tiles_2). Pressing v switches between them.
Description of Rendering
Scene 1 is filled with 8000 light green tiles arranged randomly in some volume, but all facing one
point. The CPU code rendering tiles (all tiles, not just the light green ones described above) is
in routine World::render_tiles in file hw05.cc. This routine launches one of the three shaders
for rendering tiles. Shader s_hw05_tiles_0, which will be called tiles_0 here, is used with a
rendering pass in which the vertices are grouped as individual triangles. The shaders for this
version are in file hw05-shdr.cc and have names ending in tiles_0. The tiles_0 work correctly
in the unmodified code, and they provide one special effect: a tile changes to red when touced by a
ball and remains red for two seconds. The tiles_0 shaders are for at most experimentation, they
don’t need to be modified for the homework. The tiles_1 shaders are used in a rendering pass
that also groups vertices as individual triangles, but the values used for glVertex and glColor are
unconventional (see the code). The tiles_2 shaders are used in a rendering pass in which vertices
are not grouped (meaning points). For tiles_2 the vertex data is placed in buffer objects, and the
buffer objects are only updated when the underlying data changes. The only vertex shader input
for the tiles_2 is gl_VertexID. Initially the tiles_1 and tiles_2 shaders just call their tiles_0
counterparts and so they won’t render properly.
Debugging Help
Boolean variables opt_tryout1 and opt_tryout2 can be used for debugging on the CPU. Pressing
y and Y toggles their state. In the shader code use tryout.x and tryout.y for these two variables.
To see the difference between your code and the original code issue the Emacs command Ctrl
x v = or from the shell the command git diff.
Problem 1: Modify the tiles_1 shaders in hw05-shdr.cc so that they correctly render the tiles.
They must operate with the data provided by World::render_tiles case 1, unconventional though
it is. Do not change what data is provided by the CPU for this rendering pass.
• Work with the data provided from the CPU, don’t send additional information.
• The code must be reasonably efficient.
• Pay attention to the geometry shader layout declarations including max vertices.
Problem 2: Modify the code in tiles_2 so that it renders the tiles correctly. They must operate
with the data provided by World::render_tiles case 2, which is much more straightforward than
case 1. Note that case 2 uses points as the primitive.
• Work with the data provided from the CPU, don’t send additional information.
• The code must be reasonably efficient.
• Pay attention to the geometry shader layout declarations including max vertices.
Problem 3: Modify the code in any of the shaders so that when a tile is struck by a ball an
expanding red ring is drawn centered at the point of contact. The screenshot at the top of the
assignment shows one ring in the center of the image, and some larger faded regions below.
2
Each tile has a tact_pos member that holds the coordinates of the point on the tile that
the ball has struck, that in the x, y, and z components, and the time that the contact occurred,
in the w component. All three rendering passes provide this data in one way or another, see
World::render_tiles. Also, notice that world_time is provided to shaders.
The ring should be easily visible and extend past the edges in a few seconds.
• The width of the ring should be some fixed object-space size, don’t make it one pixel wide.
• Try to reduce the amount of work done by the fragment shader by doing extra work in the
vertex or geometry shaders.
• Also try reducing the amount of work done by the rasterization stage.
