# PNG Image Exporter — Design Spec

**Date:** 2026-03-23
**Status:** Approved

## Problem

Users generating Minecraft text structures need a visual reference showing which blocks to place and where. Current exporters (mcfunction, litematica) produce machine-readable files; there is no human-readable image guide.

## Solution

Add a `"png"` format to the exporter registry. The output is a PNG image with a color-coded grid — one cell per block position — plus a legend mapping colors to Minecraft block names.

## Visual Design

### Cell style
- Each cell: `cell_size × cell_size` pixels with 1px gaps between cells (gaps act as grid lines)
- Filled cells show a letter code in the center (B = Body, O = Outline, S = Shadow)
- Empty/air positions: dark background color, no letter

### Colors
| VoxelType | Color     | Hex       | Letter |
|-----------|-----------|-----------|--------|
| Body      | Blue      | `#4a90d9` | B      |
| Outline   | Gray      | `#888888` | O      |
| Shadow    | Tan/gold  | `#c0a060` | S      |
| Empty     | Dark      | `#222222` | —      |
| Background| Dark      | `#222222` | —      |

### Depth handling
All z-layers are stacked vertically in one PNG. Each layer has a "Layer N" header row above its grid.

### Legend
Below all layers: one row per **active** VoxelType. A VoxelType is active if both:
1. The palette field is `Some(_)` (i.e. `palette.blocks.outline.is_some()` for Outline, `palette.blocks.shadow.is_some()` for Shadow; Body is always active)
2. At least one cell of that type exists in the grid

Each legend row shows: color swatch, letter code, full Minecraft block name (e.g. `B  minecraft:stone`).

If a VoxelType has cells in the grid but its palette field is `None`, `BlockPalette::resolve` returns `"minecraft:air"`. Such cells are rendered in the grid with the type's color and letter (since the voxel exists), but the legend shows `"minecraft:air"` as the block name — no special suppression.

If the grid contains no voxels at all (all cells `None`), the legend is omitted and the image contains only the layer label rows on a dark background.

## CLI Interface

New flag added to `crates/bin`:

```
--cell-size <PIXELS>    Cell size in pixels for PNG export (16 or 32) [default: 16]
```

The `--format png` flag selects the image exporter. `--format all` includes PNG using whatever `--cell-size` the user specified (or the default 16).

The CLI always routes `"png"` through `build_png(&palette, cli.cell_size)` regardless of whether it arrived via `--format png` or `--format all`. The registry entry (16px default) exists solely so `available_names()` lists `"png"` and the help text is accurate; it is never used directly for rendering.

`--cell-size` is validated at parse time to the range `[8, 64]` via a `clap::value_parser` range constraint (matching the pattern used by `--outline-mode`). Values below 8 would produce a letter scale of 0 (invisible); values above 64 are unreasonably large. The flag is ignored for all non-PNG formats.

## Architecture

### New file: `crates/exporters/src/image_export.rs`

```rust
pub struct PngExporter {
    palette: BlockPalette,
    cell_size: u32,
}

impl PngExporter {
    pub fn new(palette: &BlockPalette, cell_size: u32) -> Self
}

impl StructureExporter for PngExporter {
    fn export(&self, grid: &VoxelGrid) -> anyhow::Result<Vec<u8>>;
    fn file_extension(&self) -> &'static str; // "png"
}
```

Private helpers:
- `image_dimensions(grid, palette, cell_size) -> (u32, u32)` — computes full image dimensions including legend; takes palette to determine number of active legend rows
- `draw_cell(img, px, py, cell_size, color)` — fill a rectangle
- `draw_letter(img, px, py, cell_size, ch, color)` — render one char from embedded bitmap font
- `draw_layer_label(img, y, z_index, cell_size)` — "Layer N" header row
- `draw_text_row(img, x, y, text, scale: u32)` — render a text string at a fixed pixel scale (1 = 5×7 px per char, 2 = 10×14, etc.); used at `scale=1` for legend labels and `scale = cell_size / 8` for layer labels
- `draw_legend(img, y_start, palette, cell_size)` — legend section

### Text rendering
Letters are rendered using an embedded 5×7 bitmap font (`const FONT_5X7: [[u8; 7]; 128]`). Each font pixel is scaled by `cell_size / 8` (so 2× at 16px, 4× at 32px). No external font files or font-rendering crates required.

### Registry integration

`crates/exporters/src/lib.rs` changes:
- Register `("png", |p| Box::new(PngExporter::new(p, 16)))` — enables `--format all`
- Add `pub fn build_png(palette: &BlockPalette, cell_size: u32) -> Box<dyn StructureExporter>` — used by CLI for user-specified cell size

The generic `ExporterFactory = fn(&BlockPalette) -> Box<dyn StructureExporter>` signature is unchanged. No changes to `McFunctionExporter` or `LitematicaExporter`.

### CLI dispatch (`crates/bin/src/cli.rs`)

When `"png"` appears in `format_names`, the CLI calls `exporters::build_png(&palette, cli.cell_size)` directly (to honour `--cell-size`) rather than the generic registry path.

### Image layout (per-pixel coordinates)

```
gap = 1
label_height       = cell_size + 4   // "Layer N" header row
per_layer_height   = height * (cell_size + gap) - gap   // 0 if height=0
layer_block_height = label_height + gap + per_layer_height
total_grid_height  = depth * layer_block_height + (depth - 1) * gap
active_types       = count of VoxelTypes active in palette+grid (0–3)
legend_height      = if active_types > 0 { active_types * (cell_size + gap) + gap } else { 0 }
img_width          = gap + width * (cell_size + gap)    // 1 if width=0
img_height         = total_grid_height + (if legend_height > 0 { gap + legend_height } else { 0 })
```

Row y=0 in `VoxelGrid` is the bottom row; the image renders row `height-1-y` from the top so text reads naturally top-to-bottom.

**Edge cases:**
- `width=0` or `height=0`: `img_width`/`per_layer_height` collapse gracefully to minimal values; no panic. The "empty grid" test case (#4) uses a grid with non-zero dimensions but all-`None` cells — this is the common case and must not panic.
- `depth=1`: `(depth-1)*gap = 0`, only one layer block rendered; correct.
- `--out result.mcfunction --format png`: `set_extension("png")` replaces the existing extension, producing `result.png`. This is existing CLI behavior, unchanged for PNG.

## Dependencies

Only `crates/exporters/Cargo.toml` gains a new dependency:

```toml
image = { version = "0.25", default-features = false, features = ["png"] }
```

No changes to `crates/lib` or `crates/bin` dependencies.

## Tests

All in `crates/exporters/src/image_export.rs`:

1. `dimensions_single_layer` — formula correct for 1 layer
2. `dimensions_multi_layer` — height increases per additional layer
3. `export_produces_valid_png` — output starts with PNG magic bytes `[137, 80, 78, 71, ...]`
4. `export_empty_grid_no_panic` — grid with all-`None` cells (non-zero dimensions) produces valid output without panic
5. `export_multi_layer_larger_than_single` — 3-layer > 1-layer byte count
6. `letter_for_each_voxel_type` — B/O/S mapping is correct
7. `export_palette_with_no_outline` — palette where `outline = None`; grid with Outline voxels still renders without panic; legend shows `"minecraft:air"` for Outline

## Verification

```bash
cargo build
cargo test -p exporters
cargo run -- --text "Hi" --font /path/to/font.ttf --format png --cell-size 32 --out test_out
# Verify: test_out.png is a valid image showing the block grid + legend
```