Add writeup

assignment-1/.gitignore

@@ -1,3 +1,5 @@
 /target
+/assignment-1
 *.ppm
 *.zip
+*.pdf

assignment-1/Makefile

@@ -1,13 +1,18 @@
-.PHONY: clean
+.PHONY: all clean
+
+DOCKER := docker
+ZIP := zip
+PANDOC := pandoc
 
 HANDIN := hw1a.michael.zhang.zip
-
-BINARY := target/release/assignment-1
+BINARY := assignment-1
+WRITEUP := writeup.pdf
+SOURCES := $(shell find -name "*.rs")
 
 all: $(HANDIN)
 
-$(BINARY):
-	docker run \
+$(BINARY): $(SOURCES)
+	$(DOCKER) run \
 		--rm \
 		-v "$(shell pwd)":/usr/src/myapp \
 		-v cargo-registry:/usr/local/cargo \
@@ -15,9 +20,13 @@ $(BINARY):
 		-w /usr/src/myapp \
 		rust \
 		cargo build --release
+	mv target/release/assignment-1 $@
 
-$(HANDIN): $(BINARY) Cargo.toml Cargo.lock README.md
-	zip -r $@ src $<
+$(HANDIN): $(BINARY) $(WRITEUP) Cargo.toml Cargo.lock README.md
+	$(ZIP) -r $@ src $<
+
+writeup.pdf: writeup.md
+	$(PANDOC) -o $@ $<
 
 clean:
-	rm -f $(HANDIN) $(BINARY)
+	rm -f $(HANDIN) $(BINARY) $(WRITEUP)

assignment-1/README.md

@@ -1 +0,0 @@
-# Raycaster

assignment-1/src/main.rs

@@ -59,7 +59,13 @@ fn main() -> Result<()> {
     move |px: usize, py: usize| {
       let x_component = pixel_base_x * px as f64;
       let y_component = pixel_base_y * py as f64;
-      view_window.upper_left + x_component + y_component
+
+      // Without adding this, we would be getting the top-left of the pixel's
+      // rectangle. We want the center, so add half of the pixel size as
+      // well.
+      let center_offset = (pixel_base_x + pixel_base_y) / 2.0;
+
+      view_window.upper_left + x_component + y_component + center_offset
     }
   };
 

assignment-1/writeup.md

@@ -0,0 +1,73 @@
+---
+geometry: margin=2cm
+output: pdf_document
+---
+
+# Raycaster
+
+Determining the viewing window for the raycaster for this assignment involved
+creating a "virtual" screen in world coordinates, mapping image pixels into that
+virtual screen, and then casting a ray through each pixel's world coordinate to
+see where it would intersect objects.
+
+### Creating a virtual screen
+
+The virtual screen is determined first using the eye's position and where it's
+looking. This gives us a single 3d vector, but it doesn't give us a 2d screen in
+the world. This is where the field of view (FOV) comes in; the FOV determines
+how many degrees the screen should take up.
+
+![Field of view](doc/fov.jpg){width=180px}
+
+Changing the angle of the field of view would result in a wider or narrower
+screen, which when paired with the aspect ratio (width / height), would produce
+a bigger or smaller viewing screen, like the orange box in the above diagram
+shows. Simply put, FOV affects how _much_ of the frame you're able to see.
+
+Curiously, distance from the eye actually doesn't really affect the viewing
+screen very much. The reason is the screen is only used to determine how to
+project rays. As the two black rectangles in the diagram above demonstrates,
+changing the distance would still allow the viewer to see the same amount of the
+scene. (using the word _amount_ very loosely here to mean percentage of the
+landscape, rather than # of pixels, which is determined by the actual image
+dimensions)
+
+The up-direction vector controls the rotation of the scene. Without the
+up-direction, it would not be possible to tell which rotation the screen should
+be in:
+
+![Rotation determined by up direction](doc/rot.jpg){width=240px}
+
+Together, all of these parameters can uniquely determine a virtual screen
+location, that we can use to cast rays through and fill pixels. We can change
+any of these to produce an image with a more exaggerated view of the scene for
+example; simply move the eye position to be incredibly close to the object that
+we are observing, and increase the field of view to cover the entire object.
+
+Because the rays are going in much different directions and travelling different
+distances, the corners of the image will seem more stretched than if we were
+observing the object from afar and all the rays are in approximately the same
+part of the virtual screen.
+
+One other point to make is that we're currently using a rectangle for our
+virtual screen, which automatically does a bit of the distortion. If instead we
+were to use a curved lens-like shape, then the rays pointing to any pixel of the
+screen would be travelling the same distance. Moving the eye position closer to
+the object would still generate distortion, but to a lesser extent.
+
+### Mapping image pixels
+
+After the rectangle has been determined, we can simply pick one corner to start
+as an anchor, and then find out what pixel values would correspond to it. For
+example, in the image below:
+
+![Mapping image pixels](doc/map.jpg){width=240px}
+
+I would pick a starting point like $A$, and then take the vector $B-A$ and
+subdivide it into 4 pieces, letting $\Delta x = \frac{B-A}{4}$. Then, same thing
+for the $y$ direction, I would set $\Delta y = \frac{D-A}{4}$. Taking $A +
+x_i \times \Delta x + y_i \times \Delta y$ yields the precise coordinate
+location for any pixel.
+
+(Technically really we would want the middle of the pixel, so just add
+$\frac{\Delta x + \Delta y}{2}$ to the point to get that)

flake.nix

@@ -12,16 +12,22 @@
         toolchain = pkgs.fenix.stable;
       in rec {
         devShell = pkgs.mkShell {
-          packages =
-            (with pkgs; [ cargo-watch cargo-deny cargo-edit zip unzip ])
-            ++ (with toolchain; [
-              cargo
-              rustc
-              clippy
+          packages = (with pkgs; [
+            cargo-watch
+            cargo-deny
+            cargo-edit
+            pandoc
+            zip
+            unzip
+            texlive.combined.scheme-full
+          ]) ++ (with toolchain; [
+            cargo
+            rustc
+            clippy
 
-              # Get the nightly version of rustfmt so we can wrap comments
-              pkgs.fenix.default.rustfmt
-            ]);
+            # Get the nightly version of rustfmt so we can wrap comments
+            pkgs.fenix.default.rustfmt
+          ]);
           CARGO_UNSTABLE_SPARSE_REGISTRY = "true";
         };
       });