feat: Implement Unicode block renderer

.envrc

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+use flake

.gitignore

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+/target
+.direnv

Cargo.toml

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+[package]
+name = "picsl"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+clap = { version = "4.5.31", features = ["derive"] }
+image = "0.25.5"
+termion = "4.0.4"

flake.lock

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+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1739736696,
+        "narHash": "sha256-zON2GNBkzsIyALlOCFiEBcIjI4w38GYOb+P+R4S8Jsw=",
+        "rev": "d74a2335ac9c133d6bbec9fc98d91a77f1604c1f",
+        "revCount": 754461,
+        "type": "tarball",
+        "url": "https://api.flakehub.com/f/pinned/NixOS/nixpkgs/0.1.754461%2Brev-d74a2335ac9c133d6bbec9fc98d91a77f1604c1f/01951426-5a87-7b75-8413-1a0d9ec5ff04/source.tar.gz"
+      },
+      "original": {
+        "type": "tarball",
+        "url": "https://flakehub.com/f/NixOS/nixpkgs/0.1.%2A.tar.gz"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs",
+        "rust-overlay": "rust-overlay"
+      }
+    },
+    "rust-overlay": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1739845646,
+        "narHash": "sha256-UGQVBU/yDn6u0kAE4z1PYrOaaf3wl+gAAv5rui2TkFQ=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "ab2cd2b8b25ab3f65b8ce4aa701a6d69fbb0210f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

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+{
+  description = "A Nix-flake-based Rust development environment";
+
+  inputs = {
+    nixpkgs.url = "https://flakehub.com/f/NixOS/nixpkgs/0.1.*.tar.gz";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { self, nixpkgs, rust-overlay }:
+    let
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forEachSupportedSystem = f: nixpkgs.lib.genAttrs supportedSystems (system: f {
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [ rust-overlay.overlays.default self.overlays.default ];
+        };
+      });
+    in
+    {
+      overlays.default = final: prev: {
+        rustToolchain =
+          let
+            rust = prev.rust-bin;
+          in
+          if builtins.pathExists ./rust-toolchain.toml then
+            rust.fromRustupToolchainFile ./rust-toolchain.toml
+          else if builtins.pathExists ./rust-toolchain then
+            rust.fromRustupToolchainFile ./rust-toolchain
+          else
+            rust.stable.latest.default.override {
+              extensions = [ "rust-src" "rustfmt" ];
+            };
+      };
+
+      devShells = forEachSupportedSystem ({ pkgs }: {
+        default = pkgs.mkShell {
+          packages = with pkgs; [
+            rustToolchain
+            openssl
+            pkg-config
+            cargo-deny
+            cargo-edit
+            cargo-watch
+            rust-analyzer
+          ];
+
+          env = {
+            # Required by rust-analyzer
+            RUST_SRC_PATH = "${pkgs.rustToolchain}/lib/rustlib/src/rust/library";
+          };
+        };
+      });
+    };
+}

src/main.rs

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+use clap::{Parser, Subcommand};
+use image::{imageops, DynamicImage, GenericImageView, ImageReader, Pixel};
+use termion::{color, terminal_size};
+
+trait TextImageRenderer {
+    fn render(&self, image: DynamicImage, width: u32, height: u32) -> String;
+}
+
+struct UnicodeBlockRendererConfig {
+    alpha_threshold: u8,
+    block_char: String,
+}
+
+struct UnicodeBlockRenderer {
+    config: UnicodeBlockRendererConfig,
+}
+impl TextImageRenderer for UnicodeBlockRenderer {
+    fn render(&self, image: DynamicImage, width: u32, height: u32) -> String {
+        let scaled_image = image.resize(width, height, imageops::FilterType::Gaussian);
+
+        // This might not match `(environment.width, environment.height)` because resize preserves
+        // aspect ratio on the scaled image
+        let (scaled_x, scaled_y) = scaled_image.dimensions();
+
+        let mut out = String::new();
+        for y in 0..scaled_y {
+            for x in 0..scaled_x {
+                let pix = scaled_image.get_pixel(x.into(), y.into()).to_rgba();
+                if pix[3] >= self.config.alpha_threshold {
+                    out.push_str(&format!(
+                        "{}{}",
+                        color::Fg(color::Rgb(pix[0], pix[1], pix[2])),
+                        self.config.block_char,
+                    ));
+                } else {
+                    out.push_str(&format!("{} ", color::Fg(color::Reset)));
+                }
+            }
+            out.push_str("\n");
+        }
+
+        out
+    }
+}
+
+#[derive(Parser, Debug)]
+#[command(name = "picsl")]
+#[command(version = "0.1")]
+#[command(about = "Render images in the terminal", long_about = None)]
+struct Cli {
+    /// Path to image
+    #[arg(short, long)]
+    image: String,
+
+    #[command(subcommand)]
+    renderer: RendererCommands,
+}
+
+#[derive(Subcommand, Debug)]
+enum RendererCommands {
+    UnicodeBlock {
+        /// A pixel's alpha channel must be higher than this threshold to be rendered.
+        #[arg(long, default_value_t = 0)]
+        alpha_threshold: u8,
+
+        /// The (potentially unicode) character to use for filled-in values
+        #[arg(short, long, default_value_t = String::from("█"))]
+        block_char: String,
+    },
+}
+
+fn get_renderer(renderer_command: RendererCommands) -> impl TextImageRenderer {
+    match renderer_command {
+        RendererCommands::UnicodeBlock {
+            alpha_threshold,
+            block_char,
+        } => UnicodeBlockRenderer {
+            config: UnicodeBlockRendererConfig {
+                alpha_threshold,
+                block_char,
+            },
+        },
+    }
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let args = Cli::parse();
+    let image_path = args.image;
+
+    // Load the image and resize it to terminal size
+    let (term_x, term_y) = terminal_size().expect("Terminal size");
+    let image_buf = ImageReader::open(image_path)?.decode()?;
+    let renderer = get_renderer(args.renderer);
+    let rendered_image_text = renderer.render(image_buf, term_x.into(), term_y.into());
+
+    print!(
+        "{}{}{}{}{}",
+        // Blank canvas
+        termion::clear::All,
+        // Go to top
+        termion::cursor::Goto(1, 1),
+        // Display image
+        rendered_image_text,
+        // Reset colors in case we do something else later
+        color::Fg(color::Reset),
+        color::Bg(color::Reset),
+    );
+
+    Ok(())
+}