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cdg: typefind: improve CDG packets detection heuristic

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We used to look at the first 10 seconds of data to compute the ratio of
CDG packets. This was not working great for files having a long intro
as no video was displayed during this time.

Split the single search window into multiple smaller ones and walk
through the whole file hoping to find a section containing CDG packets.

With this change we are now fetching at most 230400 bytes of data,
but in almost all matching cases we find the type way before that.

This heuristic is good enough to be able to properly detect all the cdg
files from my collection (77625 files).
This commit is contained in:
Guillaume Desmottes 2019-11-03 15:08:30 +01:00
parent e1b96960b2
commit 7ba1e6f60d
1 changed files with 32 additions and 7 deletions
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33
gst-plugin-cdg/src/lib.rs
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@ -21,18 +21,20 @@ mod constants;
use constants::{CDG_COMMAND, CDG_MASK, CDG_PACKET_PERIOD, CDG_PACKET_SIZE};
use gst::{Caps, TypeFind, TypeFindProbability};
use std::cmp;
const TYPEFIND_SEARCH_WINDOW_SEC: i64 = 13;
const NB_WINDOWS: u64 = 8;
const TYPEFIND_SEARCH_WINDOW_SEC: i64 = 4;
const TYPEFIND_SEARCH_WINDOW: i64 =
TYPEFIND_SEARCH_WINDOW_SEC * (CDG_PACKET_SIZE as i64 * CDG_PACKET_PERIOD as i64); /* in bytes */
/* Return the percentage of CDG packets in the first @len bytes of @typefind */
fn cdg_packets_ratio(typefind: &mut TypeFind, len: i64) -> i64 {
fn cdg_packets_ratio(typefind: &mut TypeFind, start: i64, len: i64) -> i64 {
let mut count = 0;
let total = len / CDG_PACKET_SIZE as i64;
for offset in (0..len).step_by(CDG_PACKET_SIZE as usize) {
match typefind.peek(offset, CDG_PACKET_SIZE as u32) {
match typefind.peek(start + offset, CDG_PACKET_SIZE as u32) {
Some(data) => {
if data[0] & CDG_MASK == CDG_COMMAND {
count += 1;
@ -44,12 +46,35 @@ fn cdg_packets_ratio(typefind: &mut TypeFind, len: i64) -> i64 {
(count * 100) / total
}
/* Some CDG files starts drawing right away and then pause for a while
* (typically because of the song intro) while other wait for a few
* seconds before starting to draw.
* In order to support all variants, scan through all the file per block
* of size TYPEFIND_SEARCH_WINDOW and keep the highest ratio of CDG packets
* detected. */
fn compute_probability(typefind: &mut TypeFind) -> TypeFindProbability {
match cdg_packets_ratio(typefind, TYPEFIND_SEARCH_WINDOW) {
let mut best = TypeFindProbability::None;
// Try looking at the start of the file if its length isn't available
let len = typefind
.get_length()
.unwrap_or(TYPEFIND_SEARCH_WINDOW as u64 * NB_WINDOWS);
let step = len / NB_WINDOWS;
for offset in (0..len).step_by(step as usize) {
let proba = match cdg_packets_ratio(typefind, offset as i64, TYPEFIND_SEARCH_WINDOW) {
0..=5 => TypeFindProbability::None,
6..=10 => TypeFindProbability::Possible,
_ => TypeFindProbability::Likely,
};
if proba == TypeFindProbability::Likely {
return proba;
}
best = cmp::max(best, proba);
}
best
}
fn typefind_register(plugin: &gst::Plugin) -> Result<(), glib::BoolError> {