Inner Shadow Ground Truth — PNG Pipeline (no PSD needed)

Problem

The Rust PSD writer (hcie-psd/src/psd_saver.rs) produces files that pass Psd::from_bytes internally but cannot be opened in Photoshop. We've spent multiple rounds fixing the PSD format (layer extra data, lfx2 padding, merged image layout, 8BIM blocks) without success.

Root cause: The PSD binary format is extremely complex (200+ page spec). Getting every field exactly right for Photoshop's parser is a long, error-prone process.

Solution: Skip PSD entirely

The Photopea JSX script at _tools/photopea/generate_inner_shadow_dataset.jsx already creates ground truth PNGs directly — no PSD intermediate needed. Photopea/Photoshop is the renderer; it applies inner shadow via its own engine and exports transparent PNGs.

This is the simplest, most reliable path to ground truth.

Plan

Step 1: Generate Ground Truth PNGs via Photopea

The user opens Photopea (https://www.photopea.com) in a browser and runs the existing JSX script. It will:

Create 256×256 transparent documents
Add a layer with a black centered rectangle (50% margin)
Apply Inner Shadow via ActionDescriptor (Photoshop's native effect engine)
Save transparent PNG

Parameter grid (192 images):

Param	Values
angle	0, 90, 180, 270
distance	5, 15, 30
size (blur)	10, 30
choke	0, 20
fx opacity	75, 100
fx blend mode	Normal, Multiply

Output: _tmp/is_ground_truth/ directory with PNGs like is_a0_d5_s10_c0_op75_bmnormal.png.

Step 2: Write Rust MAE Tuning Example Against PNGs

Create hcie-io/examples/tune_mae_inner_shadow_grid.rs that:

Loads ground truth PNGs from _tmp/is_ground_truth/
Parses the filename to extract parameters (angle, distance, size, choke, opacity, blend mode)
Recreates the same shape (256×256, black rectangle at 25% margin) in Rust
Calls generate_inner_shadow_tuned() with the same parameters
Blends the shadow with the shape using the specified blend mode and opacity
Computes MAE against the ground truth PNG
Grid-searches blur_factor, passes, choke_scale, mask_strength across the full dataset
Reports best global parameters and per-combo breakdown

Key differences from the existing tune_mae_inner_shadow.rs:

Works against multiple parameter combos (192 PNGs), not a single PSD
Loads PNG files directly (no PSD import needed)
Creates the shape geometry from scratch (no PSD layer needed)
Reports per-combo MAE for diagnostics

Step 3: Run and Report

cargo run --package hcie-io --example tune_mae_inner_shadow_grid

Expected output: best blur_factor, passes, choke_scale, mask_strength with average MAE across all 192 combos.

Step 4: Apply Best Params to `tuned.rs`

Update hcie-fx/src/tuned.rs generate_inner_shadow_tuned() if new best params differ from current (blur_factor=1.45, passes=3, choke_scale=0.0, mask_strength=1.0).

Files to Create/Modify

File	Action
`hcie-io/examples/tune_mae_inner_shadow_grid.rs`	Create — grid search against PNG ground truth
`hcie-fx/src/tuned.rs`	Modify (if params change)

Files NOT Modified

hcie-psd/src/psd_saver.rs — no more PSD fixes needed
_tools/photopea/generate_inner_shadow_dataset.jsx — already works as-is

User Action Required

Open https://www.photopea.com in a browser
Press Alt+Ctrl+I (or File → Automate → Script)
Paste contents of _tools/photopea/generate_inner_shadow_dataset.jsx
Select output folder: create/choose _tmp/is_ground_truth/
Wait for ~192 images to generate
Tell me when done — I'll run the Rust tuning example

Verification

After tuning, generate a test inner shadow with our engine and visually compare against one of the Photopea PNGs side-by-side.

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