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Allow DC coefficient differences beyond 16 bits and validate the reconstructed value #268

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Merged
cpncf merged 1 commit into from
Aug 13, 2026
+9 −4
Merged

Allow DC coefficient differences beyond 16 bits and validate the reconstructed value #268

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6 changes: 4 additions & 2 deletions src/oapv.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1569,8 +1569,10 @@ static int dec_block(oapvd_ctx_t *ctx, oapvd_core_t *core, int log2_w, int log2_
{
int bit_depth = ctx->bit_depth;

// DC prediction
core->coef[0] = core->dc_diff + core->prev_dc[c];
// DC prediction; the reconstructed value must fit the coefficient range
int dc = core->dc_diff + core->prev_dc[c];
oapv_assert_rv(dc >= MIN_TX_VAL && dc <= MAX_TX_VAL, OAPV_ERR_MALFORMED_BITSTREAM);
core->coef[0] = (s16)dc;
core->prev_dc[c] = core->coef[0];
// Inverse quantization
ctx->fn_dquant[0](core->coef, core->q_mat[c], log2_w, log2_h, core->dq_shift[c]);
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7 changes: 5 additions & 2 deletions src/oapv_vlc.c
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Expand Up @@ -866,12 +866,15 @@ int oapvd_vlc_dc_coef(oapv_bs_t *bs, int *dc_diff, int *kparam_dc)
abs_dc_diff = dec_vlc_read(bs, *kparam_dc);
if(abs_dc_diff < 0) // dec_vlc_read error sentinel
return OAPV_ERR_MALFORMED_BITSTREAM;
if(abs_dc_diff > 32767)
// DC coefficients are in [-32768, 32767], so the difference between two
// neighboring blocks can legally reach 65535; only the reconstructed DC
// value is bounded, which dec_block() checks after prediction
if(abs_dc_diff > 65535)
return OAPV_ERR_MALFORMED_BITSTREAM;
if(abs_dc_diff) {
if(bs->leftbits == 0) BSR_FLUSH_1BYTE(bs);
BSR_READ_1BIT(bs, sign);
*dc_diff = oapv_set_sign16(abs_dc_diff, sign);
*dc_diff = oapv_set_sign(abs_dc_diff, sign);
*kparam_dc = KPARAM_DC(abs_dc_diff);
}
else {
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