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//! `CBDT` — Color Bitmap Data Table.
//!
//! The CBDT table holds the actual per-glyph image data; the `CBLC`
//! sibling table maps `(glyph_id, ppem)` → byte range here. Per
//! Microsoft OpenType spec, the table starts with a 4-byte header
//! (`majorVersion = 3`, `minorVersion = 0`) followed by an opaque blob
//! of glyph entries whose layout depends on the per-strike `imageFormat`
//! recorded in CBLC.
//!
//! We support the three PNG-based entry formats Noto Color Emoji and
//! every other Google "embedded PNG" colour-emoji font use:
//!
//! ```text
//! Format 17: SmallGlyphMetrics (5 B) + u32 dataLen + PNG[dataLen]
//! Format 18: BigGlyphMetrics (8 B) + u32 dataLen + PNG[dataLen]
//! Format 19: u32 dataLen + PNG[dataLen]
//! (metrics live in CBLC IndexSubtable's BigGlyphMetrics)
//! ```
//!
//! Format 19's metrics are recovered via `CblcEntry::fixed_metrics`
//! which the CBLC walker populates from the IndexSubtable Format 2 / 5
//! BigGlyphMetrics field.
//!
//! Other CBDT formats (1/2/5/6/7/8/9 inherited from EBDT — monochrome
//! / grayscale; 32 BGRA uncompressed) are not supported in this round.
//! They're rare in the wild compared to PNG and the consumer crate
//! doesn't have a native BGRA path yet.
use crate::parser::read_u32;
use crate::tables::cblc::{BigGlyphMetrics, CblcEntry, SmallGlyphMetrics};
use crate::Error;
/// One glyph's worth of data resolved out of CBDT, ready for PNG decode.
#[derive(Debug, Clone, Copy)]
pub struct ColorBitmap<'a> {
/// Width of the bitmap in pixels (from the per-glyph metrics).
pub width: u8,
/// Height in pixels.
pub height: u8,
/// Distance from the horizontal pen origin to the LEFT edge of the
/// bitmap, in pixels.
pub bearing_x: i8,
/// Distance from the horizontal pen origin to the TOP edge of the
/// bitmap, in pixels.
pub bearing_y: i8,
/// Horizontal advance in pixels.
pub advance: u8,
/// Strike pixels-per-em (the size at which this glyph was authored).
pub ppem: u8,
/// Raw PNG byte stream — pass to `oxideav_png::decode_png_to_frame`
/// in the consumer crate. Borrowed from the parent CBDT slice.
pub png_bytes: &'a [u8],
}
/// Parsed CBDT table.
#[derive(Debug, Clone)]
pub struct CbdtTable<'a> {
bytes: &'a [u8],
}
impl<'a> CbdtTable<'a> {
/// Wrap a CBDT byte slice. Validates the header version only — per-
/// glyph entry layout is parsed lazily via `lookup`.
pub fn parse(bytes: &'a [u8]) -> Result<Self, Error> {
if bytes.len() < 4 {
return Err(Error::UnexpectedEof);
}
let major = u16::from_be_bytes([bytes[0], bytes[1]]);
// CBDT = 3, EBDT = 2. We only emit `ColorBitmap` entries
// (Formats 17-19 are CBDT-only) but we accept either header
// since EBDT's bytes are layout-compatible for everything we
// touch (zero, in practice).
if major != 2 && major != 3 {
return Err(Error::BadStructure("CBDT: unknown major version"));
}
Ok(Self { bytes })
}
/// Decode a per-glyph entry given the CBLC-resolved descriptor.
///
/// Returns `None` if `entry.image_format` is not one of 17/18/19
/// (we don't support uncompressed BGRA or monochrome formats yet).
/// Returns `Err(_)` only on structural damage (truncated PNG range).
pub fn lookup(&self, entry: &CblcEntry) -> Result<Option<ColorBitmap<'a>>, Error> {
let off = entry.image_data_offset as usize;
let end = off
.checked_add(entry.data_len as usize)
.ok_or(Error::BadStructure("CBDT: entry overflow"))?;
if end > self.bytes.len() {
return Err(Error::BadOffset);
}
let blob = &self.bytes[off..end];
match entry.image_format {
17 => {
// SmallGlyphMetrics (5) + u32 dataLen + PNG.
let metrics = SmallGlyphMetrics::parse(blob, 0)?;
if blob.len() < 5 + 4 {
return Err(Error::UnexpectedEof);
}
let data_len = read_u32(blob, 5)? as usize;
let png = blob.get(9..9 + data_len).ok_or(Error::BadOffset)?;
Ok(Some(ColorBitmap {
width: metrics.width,
height: metrics.height,
bearing_x: metrics.bearing_x,
bearing_y: metrics.bearing_y,
advance: metrics.advance,
ppem: entry.ppem_y,
png_bytes: png,
}))
}
18 => {
// BigGlyphMetrics (8) + u32 dataLen + PNG.
let metrics = BigGlyphMetrics::parse(blob, 0)?;
if blob.len() < 8 + 4 {
return Err(Error::UnexpectedEof);
}
let data_len = read_u32(blob, 8)? as usize;
let png = blob.get(12..12 + data_len).ok_or(Error::BadOffset)?;
Ok(Some(ColorBitmap {
width: metrics.width,
height: metrics.height,
bearing_x: metrics.hori_bearing_x,
bearing_y: metrics.hori_bearing_y,
advance: metrics.hori_advance,
ppem: entry.ppem_y,
png_bytes: png,
}))
}
19 => {
// u32 dataLen + PNG. Metrics come from CBLC.
let metrics = entry.fixed_metrics.ok_or(Error::BadStructure(
"CBDT format 19 needs CBLC fixed metrics",
))?;
if blob.len() < 4 {
return Err(Error::UnexpectedEof);
}
let data_len = read_u32(blob, 0)? as usize;
let png = blob.get(4..4 + data_len).ok_or(Error::BadOffset)?;
Ok(Some(ColorBitmap {
width: metrics.width,
height: metrics.height,
bearing_x: metrics.hori_bearing_x,
bearing_y: metrics.hori_bearing_y,
advance: metrics.hori_advance,
ppem: entry.ppem_y,
png_bytes: png,
}))
}
_ => Ok(None),
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn parses_format17_entry() {
// Build a CBDT byte slice with a single Format 17 entry at
// offset 16:
// [00..04] header (major=3, minor=0)
// [04..16] padding zeros
// [16..21] SmallGlyphMetrics (h=10, w=12, bx=-1, by=8, adv=14)
// [21..25] dataLen = 5
// [25..30] PNG bytes (fake) [0x89, 'P', 'N', 'G', 0x0D]
let mut bytes = vec![0u8; 64];
bytes[0..2].copy_from_slice(&3u16.to_be_bytes());
bytes[16] = 10; // height
bytes[17] = 12; // width
bytes[18] = (-1i8) as u8; // bearingX
bytes[19] = 8; // bearingY
bytes[20] = 14; // advance
bytes[21..25].copy_from_slice(&5u32.to_be_bytes()); // dataLen
bytes[25..30].copy_from_slice(&[0x89, b'P', b'N', b'G', 0x0D]);
let cbdt = CbdtTable::parse(&bytes).expect("parse");
let entry = CblcEntry {
image_format: 17,
image_data_offset: 16,
data_len: 30 - 16,
ppem_x: 96,
ppem_y: 96,
bit_depth: 32,
fixed_metrics: None,
};
let cb = cbdt.lookup(&entry).expect("lookup ok").expect("entry");
assert_eq!(cb.width, 12);
assert_eq!(cb.height, 10);
assert_eq!(cb.bearing_x, -1);
assert_eq!(cb.bearing_y, 8);
assert_eq!(cb.advance, 14);
assert_eq!(cb.png_bytes.len(), 5);
assert_eq!(cb.png_bytes[0], 0x89);
}
#[test]
fn parses_format18_entry() {
// BigGlyphMetrics is 8 bytes: h, w, hbx, hby, hadv, vbx, vby, vadv.
let mut bytes = vec![0u8; 64];
bytes[0..2].copy_from_slice(&3u16.to_be_bytes());
// Entry at offset 12.
bytes[12] = 20; // height
bytes[13] = 24; // width
bytes[14] = 2; // hori_bearing_x
bytes[15] = 18; // hori_bearing_y
bytes[16] = 26; // hori_advance
bytes[17..20].copy_from_slice(&[0; 3]); // vert metrics ignored
bytes[20..24].copy_from_slice(&3u32.to_be_bytes()); // dataLen
bytes[24..27].copy_from_slice(&[0xA1, 0xB2, 0xC3]);
let cbdt = CbdtTable::parse(&bytes).expect("parse");
let entry = CblcEntry {
image_format: 18,
image_data_offset: 12,
data_len: 27 - 12,
ppem_x: 109,
ppem_y: 109,
bit_depth: 32,
fixed_metrics: None,
};
let cb = cbdt.lookup(&entry).expect("lookup ok").expect("entry");
assert_eq!(cb.width, 24);
assert_eq!(cb.height, 20);
assert_eq!(cb.bearing_x, 2);
assert_eq!(cb.bearing_y, 18);
assert_eq!(cb.advance, 26);
assert_eq!(cb.ppem, 109);
assert_eq!(cb.png_bytes, &[0xA1, 0xB2, 0xC3]);
}
#[test]
fn returns_none_for_unsupported_format() {
let mut bytes = vec![0u8; 16];
bytes[0..2].copy_from_slice(&3u16.to_be_bytes());
let cbdt = CbdtTable::parse(&bytes).expect("parse");
let entry = CblcEntry {
image_format: 1, // monochrome — not supported
image_data_offset: 4,
data_len: 4,
ppem_x: 32,
ppem_y: 32,
bit_depth: 1,
fixed_metrics: None,
};
assert!(cbdt.lookup(&entry).expect("lookup ok").is_none());
}
#[test]
fn parses_format19_with_cblc_metrics() {
let mut bytes = vec![0u8; 32];
bytes[0..2].copy_from_slice(&3u16.to_be_bytes());
// Entry at offset 8: just dataLen + PNG.
bytes[8..12].copy_from_slice(&4u32.to_be_bytes());
bytes[12..16].copy_from_slice(&[0xDE, 0xAD, 0xBE, 0xEF]);
let cbdt = CbdtTable::parse(&bytes).expect("parse");
let entry = CblcEntry {
image_format: 19,
image_data_offset: 8,
data_len: 16 - 8,
ppem_x: 64,
ppem_y: 64,
bit_depth: 32,
fixed_metrics: Some(BigGlyphMetrics {
height: 7,
width: 9,
hori_bearing_x: 3,
hori_bearing_y: 11,
hori_advance: 13,
vert_bearing_x: 0,
vert_bearing_y: 0,
vert_advance: 0,
}),
};
let cb = cbdt.lookup(&entry).expect("lookup ok").expect("entry");
assert_eq!(cb.width, 9);
assert_eq!(cb.height, 7);
assert_eq!(cb.bearing_x, 3);
assert_eq!(cb.bearing_y, 11);
assert_eq!(cb.advance, 13);
assert_eq!(cb.png_bytes, &[0xDE, 0xAD, 0xBE, 0xEF]);
}
}