diff --git a/grayscale_filter/build.zig b/grayscale_filter/build.zig
new file mode 100644
index 0000000..93e6629
--- /dev/null
+++ b/grayscale_filter/build.zig
@@ -0,0 +1,66 @@
+const std = @import("std");
+
+// Although this function looks imperative, note that its job is to
+// declaratively construct a build graph that will be executed by an external
+// runner.
+pub fn build(b: *std.Build) void {
+    // Standard target options allows the person running `zig build` to choose
+    // what target to build for. Here we do not override the defaults, which
+    // means any target is allowed, and the default is native. Other options
+    // for restricting supported target set are available.
+    const target = b.standardTargetOptions(.{});
+
+    // Standard optimization options allow the person running `zig build` to select
+    // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall. Here we do not
+    // set a preferred release mode, allowing the user to decide how to optimize.
+    const optimize = b.standardOptimizeOption(.{});
+
+    // We will also create a module for our other entry point, 'main.zig'.
+    const exe_mod = b.createModule(.{
+        // `root_source_file` is the Zig "entry point" of the module. If a module
+        // only contains e.g. external object files, you can make this `null`.
+        // In this case the main source file is merely a path, however, in more
+        // complicated build scripts, this could be a generated file.
+        .root_source_file = b.path("src/main.zig"),
+        .target = target,
+        .optimize = optimize,
+    });
+
+    // This creates another `std.Build.Step.Compile`, but this one builds an executable
+    // rather than a static library.
+    const exe = b.addExecutable(.{
+        .name = "grayscale_filter",
+        .root_module = exe_mod,
+    });
+
+    exe.linkLibC();
+    exe.linkSystemLibrary("spng");
+
+    // This declares intent for the executable to be installed into the
+    // standard location when the user invokes the "install" step (the default
+    // step when running `zig build`).
+    b.installArtifact(exe);
+
+    // This *creates* a Run step in the build graph, to be executed when another
+    // step is evaluated that depends on it. The next line below will establish
+    // such a dependency.
+    const run_cmd = b.addRunArtifact(exe);
+
+    // By making the run step depend on the install step, it will be run from the
+    // installation directory rather than directly from within the cache directory.
+    // This is not necessary, however, if the application depends on other installed
+    // files, this ensures they will be present and in the expected location.
+    run_cmd.step.dependOn(b.getInstallStep());
+
+    // This allows the user to pass arguments to the application in the build
+    // command itself, like this: `zig build run -- arg1 arg2 etc`
+    if (b.args) |args| {
+        run_cmd.addArgs(args);
+    }
+
+    // This creates a build step. It will be visible in the `zig build --help` menu,
+    // and can be selected like this: `zig build run`
+    // This will evaluate the `run` step rather than the default, which is "install".
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+}
diff --git a/grayscale_filter/build.zig.zon b/grayscale_filter/build.zig.zon
new file mode 100644
index 0000000..47704ca
--- /dev/null
+++ b/grayscale_filter/build.zig.zon
@@ -0,0 +1,86 @@
+.{
+    // This is the default name used by packages depending on this one. For
+    // example, when a user runs `zig fetch --save <url>`, this field is used
+    // as the key in the `dependencies` table. Although the user can choose a
+    // different name, most users will stick with this provided value.
+    //
+    // It is redundant to include "zig" in this name because it is already
+    // within the Zig package namespace.
+    .name = .grayscale_filter,
+
+    // This is a [Semantic Version](https://semver.org/).
+    // In a future version of Zig it will be used for package deduplication.
+    .version = "0.0.0",
+
+    // Together with name, this represents a globally unique package
+    // identifier. This field is generated by the Zig toolchain when the
+    // package is first created, and then *never changes*. This allows
+    // unambiguous detection of one package being an updated version of
+    // another.
+    //
+    // When forking a Zig project, this id should be regenerated (delete the
+    // field and run `zig build`) if the upstream project is still maintained.
+    // Otherwise, the fork is *hostile*, attempting to take control over the
+    // original project's identity. Thus it is recommended to leave the comment
+    // on the following line intact, so that it shows up in code reviews that
+    // modify the field.
+    .fingerprint = 0x376a3a558a44c1a7, // Changing this has security and trust implications.
+
+    // Tracks the earliest Zig version that the package considers to be a
+    // supported use case.
+    .minimum_zig_version = "0.14.1",
+
+    // This field is optional.
+    // Each dependency must either provide a `url` and `hash`, or a `path`.
+    // `zig build --fetch` can be used to fetch all dependencies of a package, recursively.
+    // Once all dependencies are fetched, `zig build` no longer requires
+    // internet connectivity.
+    .dependencies = .{
+        // See `zig fetch --save <url>` for a command-line interface for adding dependencies.
+        //.example = .{
+        //    // When updating this field to a new URL, be sure to delete the corresponding
+        //    // `hash`, otherwise you are communicating that you expect to find the old hash at
+        //    // the new URL. If the contents of a URL change this will result in a hash mismatch
+        //    // which will prevent zig from using it.
+        //    .url = "https://example.com/foo.tar.gz",
+        //
+        //    // This is computed from the file contents of the directory of files that is
+        //    // obtained after fetching `url` and applying the inclusion rules given by
+        //    // `paths`.
+        //    //
+        //    // This field is the source of truth; packages do not come from a `url`; they
+        //    // come from a `hash`. `url` is just one of many possible mirrors for how to
+        //    // obtain a package matching this `hash`.
+        //    //
+        //    // Uses the [multihash](https://multiformats.io/multihash/) format.
+        //    .hash = "...",
+        //
+        //    // When this is provided, the package is found in a directory relative to the
+        //    // build root. In this case the package's hash is irrelevant and therefore not
+        //    // computed. This field and `url` are mutually exclusive.
+        //    .path = "foo",
+        //
+        //    // When this is set to `true`, a package is declared to be lazily
+        //    // fetched. This makes the dependency only get fetched if it is
+        //    // actually used.
+        //    .lazy = false,
+        //},
+    },
+
+    // Specifies the set of files and directories that are included in this package.
+    // Only files and directories listed here are included in the `hash` that
+    // is computed for this package. Only files listed here will remain on disk
+    // when using the zig package manager. As a rule of thumb, one should list
+    // files required for compilation plus any license(s).
+    // Paths are relative to the build root. Use the empty string (`""`) to refer to
+    // the build root itself.
+    // A directory listed here means that all files within, recursively, are included.
+    .paths = .{
+        "build.zig",
+        "build.zig.zon",
+        "src",
+        // For example...
+        //"LICENSE",
+        //"README.md",
+    },
+}
diff --git a/grayscale_filter/pedro_pascal.png b/grayscale_filter/pedro_pascal.png
new file mode 100644
index 0000000..2c0dacf
Binary files /dev/null and b/grayscale_filter/pedro_pascal.png differ
diff --git a/grayscale_filter/pedro_pascal_grayscale.png b/grayscale_filter/pedro_pascal_grayscale.png
new file mode 100644
index 0000000..45bf4ed
Binary files /dev/null and b/grayscale_filter/pedro_pascal_grayscale.png differ
diff --git a/grayscale_filter/src/main.zig b/grayscale_filter/src/main.zig
new file mode 100644
index 0000000..d2ff9d7
--- /dev/null
+++ b/grayscale_filter/src/main.zig
@@ -0,0 +1,112 @@
+const std = @import("std");
+const c = @cImport({
+    @cDefine("_NO_CRT_STDIO_INLINE", "1");
+    @cInclude("stdio.h");
+    @cInclude("spng.h");
+});
+
+const PngError = error {
+    CouldNotOpenImage,
+    CouldNotCloseImage,
+    CouldNotGetImageHeader,
+    CouldNotCalcOutputSize,
+    CouldNotDecodeImage,
+    CouldNotEncodeImage,
+};
+
+pub fn main() !void {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    const allocator = arena.allocator();
+    defer arena.deinit();
+
+    const path = "pedro_pascal.png";
+    const fd = try open_image(path, "rb");
+    defer close_image(fd) catch @panic("Failed to close file");
+
+    const ctx = c.spng_ctx_new(0) orelse unreachable;
+    defer c.spng_ctx_free(ctx);
+
+    _ = c.spng_set_png_file(ctx, fd);
+    var header = try get_png_header(ctx);
+    const size = try calc_output_size(ctx);
+    var buffer = try allocator.alloc(u8, size);
+    @memset(buffer[0..], 0);
+
+    try read_data_to_buffer(ctx, buffer[0..]);
+    try apply_grayscale_filter(buffer[0..]);
+    try save_image(&header, buffer[0..]);
+}
+
+fn open_image(path: [*c]const u8, mode: [*c]const u8) !?*c.FILE {
+    const fd = c.fopen(path, mode);
+    if (fd == null) {
+        return error.CouldNotOpenImage;
+    }
+
+    return fd;
+}
+
+fn close_image(fd: ?*c.FILE) !void {
+    if (c.fclose(fd) != 0) {
+        return error.CouldNotCloseImage;
+    }
+}
+
+fn get_png_header(ctx: *c.spng_ctx) !c.spng_ihdr {
+    var header: c.spng_ihdr = undefined;
+    if (c.spng_get_ihdr(ctx, &header) != 0) {
+        return error.CouldNotGetImageHeader;
+    }
+
+    return header;
+}
+
+fn calc_output_size(ctx: *c.spng_ctx) !u64 {
+    var output_size: u64 = 0;
+    if (c.spng_decoded_image_size(ctx, c.SPNG_FMT_RGBA8, &output_size) != 0) {
+        return error.CouldNotCalcOutputSize;
+    }
+
+    return output_size;
+}
+
+fn read_data_to_buffer(ctx: *c.spng_ctx, buffer: []u8) !void {
+    if (c.spng_decode_image(ctx, buffer.ptr, buffer.len, c.SPNG_FMT_RGBA8, 0) != 0) {
+        return error.CouldNotDecodeImage;
+    }
+}
+
+fn apply_grayscale_filter(buffer: []u8) !void {
+    const rfactor: f16 = 0.2126;
+    const gfactor: f16 = 0.7152;
+    const bfactor: f16 = 0.0722;
+    var index: u64 = 0;
+
+    while (index < buffer.len) : (index += 4) {
+        const r: f16 = @floatFromInt(buffer[index]);
+        const g: f16 = @floatFromInt(buffer[index + 1]);
+        const b: f16 = @floatFromInt(buffer[index + 2]);
+
+        const y: f16 = r * rfactor + g * gfactor + b * bfactor;
+
+        buffer[index] = @intFromFloat(y);
+        buffer[index + 1] = @intFromFloat(y);
+        buffer[index + 2] = @intFromFloat(y);
+    }
+}
+
+fn save_image(header: *c.spng_ihdr, buffer: []u8) !void {
+    const path = "pedro_pascal_grayscale.png";
+    const fd = try open_image(path, "wb");
+    defer close_image(fd) catch @panic("Failed to close file");
+
+    const ctx = c.spng_ctx_new(c.SPNG_CTX_ENCODER) orelse unreachable;
+    defer c.spng_ctx_free(ctx);
+
+    _ = c.spng_set_png_file(ctx, fd);
+    _ = c.spng_set_ihdr(ctx, header);
+
+    if (c.spng_encode_image(ctx, buffer.ptr, buffer.len, c.SPNG_FMT_PNG, c.SPNG_ENCODE_FINALIZE) != 0) {
+        return error.CouldNotEncodeImage;
+    }
+}