API: Expose minimum quantizer option

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

src/api.rs

@@ -81,6 +81,8 @@ pub struct EncoderConfig {
   pub reservoir_frame_delay: Option<i32>,
   pub low_latency: bool,
   pub quantizer: usize,
+  /// The minimum allowed base quantizer to use in bitrate mode.
+  pub min_quantizer: u8,
   pub bitrate: i32,
   pub tune: Tune,
   pub tile_cols_log2: usize,
@@ -127,6 +129,7 @@ impl EncoderConfig {
       time_base: Rational { num: 1, den: 30 },
       min_key_frame_interval: 12,
       max_key_frame_interval: 240,
+      min_quantizer: 0,
       reservoir_frame_delay: None,
       low_latency: false,
       quantizer: 100,
@@ -816,6 +819,7 @@ impl<T: Pixel> ContextInner<T> {
           enc.time_base.num as i64,
           enc.bitrate,
           maybe_ac_qi_max,
+          enc.min_quantizer,
           enc.max_key_frame_interval as i32,
           enc.reservoir_frame_delay
         ),

src/bin/common.rs

@@ -109,6 +109,12 @@ pub fn parse_cli() -> CliOptions {
         .long("quantizer")
         .takes_value(true)
     )
+    .arg(
+      Arg::with_name("MINQP")
+        .help("Minimum quantizer (0-255) to use in bitrate mode [default: 0]")
+        .long("min_quantizer")
+        .takes_value(true)
+    )
     .arg(
       Arg::with_name("BITRATE")
         .help("Bitrate (kbps)")
@@ -445,6 +451,7 @@ fn parse_config(matches: &ArgMatches<'_>) -> EncoderConfig {
   };
 
   cfg.quantizer = quantizer;
+  cfg.min_quantizer = matches.value_of("MINQP").unwrap().parse().unwrap_or(0);
   cfg.bitrate = bitrate.checked_mul(1000).expect("Bitrate too high");
   cfg.reservoir_frame_delay = matches.value_of("RESERVOIR_FRAME_DELAY").map(|reservior_frame_delay| reservior_frame_delay.parse().unwrap());
   cfg.show_psnr = matches.is_present("PSNR");

src/rate.rs

@@ -379,6 +379,8 @@ pub struct RCState {
   // The maximum quantizer index to allow (for the luma AC coefficients, other
   //  quantizers will still be adjusted to match).
   maybe_ac_qi_max: Option<u8>,
+  // The minimum quantizer index to allow (for the luma AC coefficients).
+  ac_qi_min: u8,
   // Will we drop frames to meet bitrate requirements?
   drop_frames: bool,
   // Do we respect the maximum reservoir fullness?
@@ -489,7 +491,8 @@ impl RCState {
   pub fn new(
     frame_width: i32, frame_height: i32, framerate_num: i64,
     framerate_den: i64, target_bitrate: i32, maybe_ac_qi_max: Option<u8>,
-    max_key_frame_interval: i32, maybe_reservoir_frame_delay: Option<i32>
+    ac_qi_min: u8, max_key_frame_interval: i32,
+    maybe_reservoir_frame_delay: Option<i32>
   ) -> RCState {
     // The default buffer size is set equal to 1.5x the keyframe interval, or 240
     //  frames; whichsever is smaller.
@@ -553,6 +556,7 @@ impl RCState {
       target_bitrate,
       reservoir_frame_delay,
       maybe_ac_qi_max,
+      ac_qi_min,
       drop_frames: false,
       cap_overflow: true,
       cap_underflow: false,
@@ -643,7 +647,7 @@ impl RCState {
           // There's no easy closed form solution, so we bisection searh for it.
           let bit_depth = ctx.config.bit_depth;
           // TODO: Proper handling of lossless.
-          let mut log_qlo = blog64(dc_q(0, 0, bit_depth) as i64)
+          let mut log_qlo = blog64(ac_q(self.ac_qi_min, 0, bit_depth) as i64)
             - q57(QSCALE + bit_depth as i32 - 8);
           // The AC quantizer tables map to values larger than the DC quantizer
           //  tables, so we use that as the upper bound to make sure we can use