# CSCE 441 Assignment 1 – Rasterizer solved

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## Description

5/5 - (1 vote)

Goal
Create a program that reads in and renders a triangle mesh (of type .obj) to an image via soware
rasterization. You may use existing resources to load in the mesh and to write out an image. You
must write your own rasterizer. In general the reuired steps for the program are:
Compute colors per vertex.
Convert triangles to image coordinates.
Rasterize each triangle using barycentric coordinates for linear interpolations and intriangle test.
Write interpolated color values per pixel using a z-buer test to resolve depth.
Associated Labs
Lab -1 (reuired): Setting Up Your Development Environment. For A1, only the compiler and
CMake are reuired, so only Lab Negative One is reuired, not Lab Zero.
Lab 1 (optional): Bounding Box Rasterizer. See this lab if you need an example of how the
Image class is used.
Lab 2 (optional): Single Triangle Rasterizer.
and run the code by following the same set of commands as in Lab 0.
writer is from http://github.com/nothings/stb.
When the mesh is loaded, the vertex positions are stored in the posBuf array as follows:
——————————————————————————————–

| x0 | y0 | z0 | x1 | y1 | z1 | x2 | y2 | z2 | x3 | y3 | z3 | x4 | y4 | z4 | x5 | y5 | z5 |
… <- posBuf array ——————————————————————————————– — | vertex 0 | vertex 1 | vertex 2 | vertex 3 | vertex 4 | vertex 5 | | triangle 1 | triangle 2 | Every three consecutive vertices in this array forms a triangle. In other words, every nine elements form the (x,y,z) coordinates of the three vertices of a triangle. Example mesh les are included in 1/13/2020 CSCE 441 – Assignment 1 courses.cs.tamu.edu/sueda/CSCE441/2020S/assignments/A1/index.html 2/7 the base code. In addition, there are numerous OB meshes on the web. For grading purposes, your program will be run using the provided Stanford Bunny and Utah Teapot. Ultimately you will want each triangle to be represented in a C/C++ structure/class, with 3 vertices and a color per vertex. In addition, your triangle data should include a 2D bounding box, which will represent the triangle’s extents in image coordinates. Add a command line argument to accept the following command line arguments. 1. Input lename of the .obj le to rasterize 2. Output image lename 3. Image width 4. Image height 5. Coloring mode (0, 1, or 2) For example, your program should be able to be run as follows: > ./A1 ../resources/bunny.obj output.png 512 512 1
In Xcode, the 1st argument should be ../../resources/bunny.obj . Add error checking to
specify the reuired command line arguments if an incorrect number are given. Your program
should not dump core if no input le is specied, or fail without an error message! Follow the
golden rule; treat your user/grader/instructor the way you’d like to be treated as a
Drawing Bounding Boxes
Write code to convert each 3D coordinates into 2D image coordinates. Assume the camera is at
the origin looking down the negative z axis. Make sure the object completely lls the image
without any distortion. To do so, you need to compute the scale and translation factors as we
discussed in class. Some tips for starting out:
You’ll need a bounding box for the whole mesh as well as for each triangle.
Color each triangle with a dierent random color.
First, write out the bounding box, rather than the triangles, to the image. If you do this with the
provided tri.obj , sphere.obj , teapot.obj , and bunny.obj , you should see blocky images like
below. Make sure the object takes up the whole image, is centered, and is undistorted (not
stretched).
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Drawing Triangles
Once the bounding boxes are being displayed correctly, add the barycentric test to write out the
triangles as in (optional) Lab 2. You should not see any gaps between the triangles.
Make sure you test nonuniform window sizes. As shown below, the aspect ratio of the object
should be preserved no matter what the image size is, and the object should ll out the image.
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Here is another image of a bunny and a teapot, from Alice in Wonderland [Wikimedia].
Interpolating Per-Vertex Colors
Instead of using random per-triangle colors, use random per-vertex colors. For each pixel inside
each triangle, you need to interpolate the pixel’s color from the three vertices using the pixel’s
barycentric coordinates. is should be the output when the “coloring mode” command line
argument is set to 0 .
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Optional information: Because of the way we are loading the mesh, the triangles do not share any
vertices. For example, if we were to load a suare consisting of four vertices and two triangles, we
end up with six vertices – three for each of the triangles. In other words, we end up duplicating
any shared vertices. erefore, when we assign a color to each vertex, triangles having a vertex at a
common position can have dierent colors assigned at this vertex position. For example, in the
sphere image above, the center vertex is incident to eight triangles, and so it has been duplicated
eight times, each time with a dierent random color. For further information check out indexed
drawing.
ZBuer
Now that you have interpolated colors, implement z-buer tests. First, create a data structure to
support z-buer tests. Your z-buer should be a separate buer from your image pixel buer, and
it should be the same size as your pixel buer. e z-buer contains the z-coordinate of each
pixel, which is interpolated from the z-coordinates of the three vertices using the pixel’s
barycentric coordinates. Once you have z-buer implemented, you should be able to render
tri2.obj properly – the two triangles should be intersecting.
Now, use the z value of the pixel as the color. (You can choose any color, not just red.) To do this,
you have to map the z-value to the range 0 to 255. If your z-buer test is not working, you’ll see
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some strange results, since some pixels that are farther from the camera may be drawn on top of
closer pixels. Use “color mode” 1 for this task.
Add an additional “color mode” 2 , where you use the y-value to linearly interpolate two colors of
your choice. For example, in the right gure below, I am interpolating between yellow and cyan.
Make sure to specify these two colors in your README. e color should vary smoothly from top
to bottom.
Important Note
Make sure to pass your std::vector by reference rather than by value. (E.g., void
foo(std::vector &bar) ) Otherwise, your program may become too slow. Since the
Image class has an std::vector inside it, it should also be passed by reference.
Point breakdown
10 points for image coordinate transforms for suare images.
5 points for image coordinate transforms for non-suare images.
20 points for correct rasterization of triangles.
15 points for color mode 0 : per-vertex colors.
20 points for correct z-buer implementation.
10 points for color mode 1 : depth colors.
10 points for color mode 2 : y colors.
10 points for coding style and general execution. For example, do not put everything in
main() , and do remember to pass big data by reference.
Total: 100 points
h h d
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What to hand in
Failing to follow these points may decrease your “general execution” score. Make sure that your
code compiles and runs by typing, for example:
> mkdir build
> cd build
> cmake ..
> make
> ./A1
Make sure the arguments are exactly as specied.
Include an README le (ascii or PDF) that includes: