pbakaus/impeccable · error

comp-diff: cannot read comp {comp_path}: {e}

Error message

comp-diff: cannot read comp {comp_path}: {e}

What it means

After argument validation, comp-diff reads the comp (reference) PNG via read_png. If the file cannot be read or decoded as a PNG, it reports the path plus the underlying io error and exits 1. This isolates which of the two images failed so the user is not left guessing.

Source

Thrown at crates/comp-verbs/src/comp_diff.rs:698

    } else {
        io.cwd.join(path)
    }
}

// ---- CLI -------------------------------------------------------------------

/// `impeccable comp-diff --comp <png> --build <png> ...`
pub fn run(argv: &[String], io: &mut Io) -> i32 {
    let comp_path = arg(argv, "comp");
    let build_path = arg(argv, "build");
    let (Some(comp_path), Some(build_path)) = (comp_path, build_path) else {
        io.err("usage: comp-diff.mjs --comp <png> --build <png> [--spec spec.json] [--out-dir dir] [--align top|stretch] [--label name] [--threshold 0.75] [--json]\n");
        return 1;
    };
    let comp = match read_png(io, comp_path) {
        Ok(v) => v,
        Err(e) => {
            io.err(&format!("comp-diff: cannot read comp {comp_path}: {e}\n"));
            return 1;
        }
    };
    let build = match read_png(io, build_path) {
        Ok(v) => v,
        Err(e) => {
            io.err(&format!("comp-diff: cannot read build {build_path}: {e}\n"));
            return 1;
        }
    };
    let mut spec: Option<Value> = None;
    let spec_path = arg(argv, "spec");
    if let Some(sp) = spec_path {
        match std::fs::read_to_string(resolve(io, sp)) {
            Ok(raw) => match serde_json::from_str::<Value>(&raw) {
                Ok(v) => spec = Some(v),
                Err(e) => {
                    io.err(&format!("comp-diff: cannot read spec {sp}: {e}\n"));

View on GitHub (pinned to 2bc2879276)

Solutions

  1. Verify the comp path exists: `ls -l <path>` (from the same working directory comp-diff runs in).
  2. Confirm the file is a valid PNG (`file <path>` shows 'PNG image data').
  3. Re-capture or re-export the reference screenshot as a real PNG.
  4. Fix filesystem permissions if read access is denied.

Example fix

// before
$ impeccable comp-diff --comp ./missing-ref.png --build ./build.png
comp-diff: cannot read comp ./missing-ref.png: No such file or directory
// after
$ impeccable comp-diff --comp ./.impeccable/comp/ref.png --build ./build.png
Defensive patterns

Strategy: try-catch

When it happens

Trigger: `--comp <path>` points at a nonexistent file, an unreadable path (permissions), a directory, a non-PNG file (JPEG/WebP/SVG renamed or not), or a corrupted/truncated PNG.

Common situations: Wrong working directory so relative paths don't resolve; the reference screenshot was never captured; the file was saved as JPEG but given a .png name; a partial download/transfer truncated the PNG.

Understand the failure class

Background: "failed to read file", EACCES, ENOENT and "could not read <path>" errors: when a program can't read a file from disk — this error's family across 49 libraries.

Related errors

AI-assisted analysis of pbakaus/impeccable@2bc2879276 (2026-09-08). Data as JSON: /api/errors/4e28dc3be650ec8d. Report an issue: GitHub.