[FFmpeg-devel] [PATCH] RFC: avcodec/vima: add IMA4 support (PR #20243)

[Manuel Lauss <code@ffmpeg.org>]

PR #20243 opened by Manuel Lauss (mlauss2)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20243
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20243.patch

Add support to the LucasArts VIMA decoder for the IMA4 variant.
This is a fix for Trac Ticket #5406 (https://trac.ffmpeg.org/ticket/5406)

RFC because:  the implementation I made here works for the first chunk, but all subsequent chunks have popping noises in them.  Maybe it's too hot here at the moment, but I cannot find the reason why, especially since I've derived this from an already flawlessly working implementation I made earlier.  Am I missing something in ffmpeg?
2 sample files are at https://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket5406/

Thanks!



From 62f3796e78cfde3ef0d1505b97e1b4528f8f2943 Mon Sep 17 00:00:00 2001
From: Manuel Lauss <manuel.lauss@gmail.com>
Date: Fri, 15 Aug 2025 11:40:55 +0200
Subject: [PATCH] avcodec/vima: add IMA4 support

ADPCM based on 4-bit input nibbles.
---
 libavcodec/vima.c | 100 ++++++++++++++++++++++++++++++----------------
 1 file changed, 66 insertions(+), 34 deletions(-)

diff --git a/libavcodec/vima.c b/libavcodec/vima.c
index 56cc1b7a85..63b5690a89 100644
--- a/libavcodec/vima.c
+++ b/libavcodec/vima.c
@@ -85,6 +85,10 @@ static const int8_t *const step_index_tables[] = {
     index_table4, index_table5, index_table6
 };
 
+static const int8_t ima4_table[] = {
+    -1, -1, -1, -1,  2,  4,  6,  8, -1, -1, -1, -1,  2,  4,  6,  8
+};
+
 static av_cold void predict_table_init(void)
 {
     for (int start_pos = 0; start_pos < 64; start_pos++) {
@@ -159,49 +163,77 @@ static int decode_frame(AVCodecContext *avctx, AVFrame *frame,
     if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
         return ret;
 
-    for (chan = 0; chan < channels; chan++) {
-        uint16_t *dest = (uint16_t *)frame->data[0] + chan;
-        int step_index = channel_hint[chan];
-        int output = pcm_data[chan];
-        int sample;
+    if (show_bits_long(&gb, 32) == MKBETAG('I','M','A','4')) {
+        int16_t *dest = (int16_t *)frame->data[0];
+        int dat, nib, adpcm_step, diff, step_index;
 
-        for (sample = 0; sample < samples; sample++) {
-            int lookup_size, lookup, highbit, lowbits;
+        adpcm_step = 7;
+        skip_bits_long(&gb, 32);        /* skip the 'IMA4' tag */
+        dat = (int16_t)get_sbits(&gb, 16);
+        step_index = get_bits(&gb, 8);
+        for (int i = 0; i < samples; i++) {
+            nib = get_bits(&gb, 4);
+            diff = adpcm_step >> 3;
+            if (nib & 4)
+                diff += adpcm_step;
+            if (nib & 2)
+                diff += (adpcm_step >> 1);
+            if (nib & 1)
+                diff += (adpcm_step >> 2);
+            if (nib & 8)
+                diff = -diff;
+            dat = av_clip_int16(dat + diff);
+            for (int j = 0; j < channels; j++)
+                *dest++ = dat;
 
-            step_index  = av_clip(step_index, 0, 88);
-            lookup_size = size_table[step_index];
-            lookup      = get_bits(&gb, lookup_size);
-            highbit     = 1 << (lookup_size - 1);
-            lowbits     = highbit - 1;
+            step_index = av_clip(step_index + ima4_table[nib], 0, 88);
+            if (step_index > 0)
+                adpcm_step = ff_adpcm_step_table[step_index];
+        }
+    } else {
+        for (chan = 0; chan < channels; chan++) {
+            uint16_t *dest = (uint16_t *)frame->data[0] + chan;
+            int step_index = channel_hint[chan];
+            int output = pcm_data[chan];
+            int sample;
 
-            if (lookup & highbit)
-                lookup ^= highbit;
-            else
-                highbit = 0;
+            for (sample = 0; sample < samples; sample++) {
+                int lookup_size, lookup, highbit, lowbits;
 
-            if (lookup == lowbits) {
-                output = get_sbits(&gb, 16);
-            } else {
-                int predict_index, diff;
+                step_index  = av_clip(step_index, 0, 88);
+                lookup_size = size_table[step_index];
+                lookup      = get_bits(&gb, lookup_size);
+                highbit     = 1 << (lookup_size - 1);
+                lowbits     = highbit - 1;
 
-                predict_index = (lookup << (7 - lookup_size)) | (step_index << 6);
-                predict_index = av_clip(predict_index, 0, 5785);
-                diff          = predict_table[predict_index];
-                if (lookup)
-                    diff += ff_adpcm_step_table[step_index] >> (lookup_size - 1);
-                if (highbit)
-                    diff = -diff;
+                if (lookup & highbit)
+                    lookup ^= highbit;
+                else
+                    highbit = 0;
 
-                output = av_clip_int16(output + diff);
+                if (lookup == lowbits) {
+                    output = get_sbits(&gb, 16);
+                } else {
+                    int predict_index, diff;
+
+                    predict_index = (lookup << (7 - lookup_size)) | (step_index << 6);
+                    predict_index = av_clip(predict_index, 0, 5785);
+                    diff          = predict_table[predict_index];
+                    if (lookup)
+                        diff += ff_adpcm_step_table[step_index] >> (lookup_size - 1);
+                    if (highbit)
+                        diff = -diff;
+
+                    output = av_clip_int16(output + diff);
+                }
+
+                *dest = output;
+                dest += channels;
+
+                step_index += step_index_tables[lookup_size - 2][lookup];
             }
-
-            *dest = output;
-            dest += channels;
-
-            step_index += step_index_tables[lookup_size - 2][lookup];
         }
     }
-
     *got_frame_ptr = 1;
 
     return pkt->size;
-- 
2.49.1

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