compare-render

Testing & Quality

Visually verify graphviz2drawio conversion fidelity by rendering a graphviz source side-by-side as a native graphviz PNG and a converted draw.io PNG, then reading both images to compare. Use this whenever modifying conversion code in graphviz2drawio/ (nodes, edges, styles, text, layout), fixing a rendering or fidelity bug, or whenever the user asks if output "looks right" or "improved" — even if they don't explicitly ask for a visual check. Always verify visually before declaring a conversion change done.

QUICK START

How to use this skill

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Compare Render: Visual Fidelity Checking

graphviz2drawio converts Graphviz files into draw.io XML. Correctness here is ultimately visual: the draw.io render should look like what graphviz itself produces. Unit tests and spec diffs can pass while the output looks wrong, so any change to conversion logic should end with an actual side-by-side look.

Core workflow

Pick 1–3 relevant test inputs from test/ that exercise the feature you're changing (see the table below). Prefer a small file plus one stress-test file.
Render the "before" state first if you haven't made changes yet:
```
./scripts/compare_render.sh test/directed/hello.gv.txt tmp_render/before
```
Skip this if the change is already made — the graphviz PNG is the ground truth either way, so an after-only comparison is still meaningful.
Make your change, then render "after":
```
./scripts/compare_render.sh test/directed/hello.gv.txt tmp_render/after
```
This produces <base>_graphviz.png (ground truth, rendered by dot) and <base>_drawio.png (your conversion, rendered by the draw.io CLI), plus the intermediate <base>.xml.
Read both PNGs with the Read tool and compare them deliberately, checking each of:
- overall layout: node positions and relative arrangement
- node shapes, sizes, fill and border colors
- edge routing (straight vs curved, where they attach), arrowheads and their direction
- labels: text content, placement, font size; edge labels especially
- clusters/subgraphs: bounding boxes present and enclosing the right nodes
Report what you see honestly. Name specific differences ("the edge label sits on top of node B instead of beside the edge") rather than "looks good". If something regressed versus the before render or the graphviz reference, say so — a failed visual check is a finding, not a reason to skip the report.

Choosing test inputs by feature area

Feature being changed	Good inputs (under `test/`)
Minimal smoke test	`directed/hello.gv.txt`
Node shapes / polygons	`directed/fsm.gv.txt`, `directed/switch.gv.txt`
Clusters / subgraphs	`directed/cluster.gv.txt`, `directed/compound.gv.txt`, `directed/subgraph_multiple.gv.txt`
Colors	`directed/Twelve_colors.gv.txt`
Gradients	`gradient/colors.gv.txt`, `gradient/radial_angle.gv.txt`
Edge routing / curves	`undirected/polylines.gv.txt`, `undirected/polylines_curved.gv.txt`
Ports	`directed/port.gv.txt`
Labels / multi-labels	`directed/multilabel.gv.txt`, `directed/tooltip.gv.txt`
Records / HTML tables	`directed/datastruct.gv.txt`, `directed/UML_Class_diagram.gv.txt`, `directed/subgraph_with_tables.gv.txt`
Radial layouts	`twopi/twopi2.gv.txt`, `undirected/networkmap_twopi.gv.txt`
Large/stress graphs	`directed/Linux_kernel_diagram.gv.txt`, `directed/world.gv.txt`

Catching regressions beyond the file you're focused on

A fix for one feature frequently shifts output for many graphs. After the targeted comparison passes:

Textual spec check — converts every test input and diffs against the committed expected XML in specs/ (ignoring unstable ids):
```
uv run ./scripts/test_specs.sh test tmp_out
```
Differences are not automatically failures — if the new output is better, the specs should be updated (see scripts/generate_specs.sh). Decide by looking, not by diff size.
Visual spec check — renders any spec XMLs you've modified as _new.png / _old.png (HEAD version) / _reference.png (graphviz) trios:
```
./scripts/render_specs.sh test specs tmp_render
```
Read the trios for each changed spec to confirm new beats old.

Practicalities

Run scripts from the repo root. Output dirs (tmp_render/, tmp_out/) are gitignored — never commit renders.
The script needs dot (brew install graphviz) and the draw.io CLI (brew install --cask drawio); it tells you which is missing.
The first draw.io CLI invocation can take ~10s while the app cold-starts; later runs are faster.
If the draw.io PNG comes out blank or missing, the converted XML is likely malformed — inspect the intermediate <base>.xml in the output dir before blaming the renderer.