[FFmpeg-devel] [PATCH v3 0/2] PNG sBIT chunk support

[FFmpeg-devel] [PATCH v3 0/2] PNG sBIT chunk support

[Leo Izen]

Add support to the PNG decoder and encoder to support sBIT chunks,
by using AVCodecContext->bits_per_raw_sample.

Changes from v2:
- Added chunk size check to make sure there's enough bytes left

Leo Izen (2):
  avcodec/pngdec: support sBIT chunks
  avcodec/pngenc: write sBIT chunks

 libavcodec/pngdec.c | 48 +++++++++++++++++++++++++++++++++++++++++++++
 libavcodec/pngenc.c |  6 ++++++
 2 files changed, 54 insertions(+)

-- 
2.40.0

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[FFmpeg-devel] [PATCH v3 1/2] avcodec/pngdec: support sBIT chunks

[Leo Izen]

Add support for reading sBIT chunks, which mark the significant
bit depth of the PNG file. This passes the metadata using the field
bits_per_raw_sample of AVCodecContext.

Signed-off-by: Leo Izen <leo.izen@gmail.com>
---
 libavcodec/pngdec.c | 48 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
index 9403d72f2f..8fbb71f60f 100644
--- a/libavcodec/pngdec.c
+++ b/libavcodec/pngdec.c
@@ -98,6 +98,7 @@ typedef struct PNGDecContext {
     int bpp;
     int has_trns;
     uint8_t transparent_color_be[6];
+    int significant_bits;
 
     uint32_t palette[256];
     uint8_t *crow_buf;
@@ -716,6 +717,14 @@ static int populate_avctx_color_fields(AVCodecContext *avctx, AVFrame *frame)
     avctx->colorspace = frame->colorspace = AVCOL_SPC_RGB;
     avctx->color_range = frame->color_range = AVCOL_RANGE_JPEG;
 
+    /*
+     * tRNS sets alpha depth to full, so we ignore sBIT if set.
+     * As a result we must wait until now to set
+     * avctx->bits_per_raw_sample in case tRNS appears after sBIT
+     */
+    if (!s->has_trns && s->significant_bits > 0)
+        avctx->bits_per_raw_sample = s->significant_bits;
+
     return 0;
 }
 
@@ -996,6 +1005,41 @@ fail:
     return ret;
 }
 
+static int decode_sbit_chunk(AVCodecContext *avctx, PNGDecContext *s,
+                             GetByteContext *gb)
+{
+    int bits = 0;
+    int channels;
+
+    if (!(s->hdr_state & PNG_IHDR)) {
+        av_log(avctx, AV_LOG_ERROR, "sBIT before IHDR\n");
+        return AVERROR_INVALIDDATA;
+    }
+
+    if (s->pic_state & PNG_IDAT) {
+        av_log(avctx, AV_LOG_ERROR, "sBIT after IDAT\n");
+        return AVERROR_INVALIDDATA;
+    }
+
+    channels = ff_png_get_nb_channels(s->color_type);
+
+    if (bytestream2_get_bytes_left(gb) != channels)
+        return AVERROR_INVALIDDATA;
+
+    for (int i = 0; i < channels; i++) {
+        int b = bytestream2_get_byteu(gb);
+        bits = FFMAX(b, bits);
+    }
+
+    if (bits < 0 || bits > s->bit_depth) {
+        av_log(avctx, AV_LOG_ERROR, "Invalid significant bits: %d\n", bits);
+        return AVERROR_INVALIDDATA;
+    }
+    s->significant_bits = bits;
+
+    return 0;
+}
+
 static void handle_small_bpp(PNGDecContext *s, AVFrame *p)
 {
     if (s->bits_per_pixel == 1 && s->color_type == PNG_COLOR_TYPE_PALETTE) {
@@ -1440,6 +1484,10 @@ static int decode_frame_common(AVCodecContext *avctx, PNGDecContext *s,
 
             break;
         }
+        case MKTAG('s', 'B', 'I', 'T'):
+            if ((ret = decode_sbit_chunk(avctx, s, &gb_chunk)) < 0)
+                goto fail;
+            break;
         case MKTAG('g', 'A', 'M', 'A'): {
             AVBPrint bp;
             char *gamma_str;
-- 
2.40.0

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[FFmpeg-devel] [PATCH v3 2/2] avcodec/pngenc: write sBIT chunks

[Leo Izen]

Add support for writing sBIT chunks, which mark the significant
bit depth of the PNG file. This obtains the metadata using the field
bits_per_raw_sample of AVCodecContext.

Signed-off-by: Leo Izen <leo.izen@gmail.com>
---
 libavcodec/pngenc.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/libavcodec/pngenc.c b/libavcodec/pngenc.c
index 1489256d00..21b033ea16 100644
--- a/libavcodec/pngenc.c
+++ b/libavcodec/pngenc.c
@@ -442,6 +442,12 @@ static int encode_headers(AVCodecContext *avctx, const AVFrame *pict)
     if (png_get_gama(pict->color_trc, s->buf))
         png_write_chunk(&s->bytestream, MKTAG('g', 'A', 'M', 'A'), s->buf, 4);
 
+    if (avctx->bits_per_raw_sample > 0 && avctx->bits_per_raw_sample < s->bit_depth) {
+        int len = ff_png_get_nb_channels(s->color_type);
+        memset(s->buf, avctx->bits_per_raw_sample, len);
+        png_write_chunk(&s->bytestream, MKTAG('s', 'B', 'I', 'T'), s->buf, len);
+    }
+
     /* put the palette if needed, must be after colorspace information */
     if (s->color_type == PNG_COLOR_TYPE_PALETTE) {
         int has_alpha, alpha, i;
-- 
2.40.0

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Re: [FFmpeg-devel] [PATCH v3 0/2] PNG sBIT chunk support

[Leo Izen]

On 3/21/23 23:26, Leo Izen wrote:
> Add support to the PNG decoder and encoder to support sBIT chunks,
> by using AVCodecContext->bits_per_raw_sample.
> 
> Changes from v2:
> - Added chunk size check to make sure there's enough bytes left
> 
> Leo Izen (2):
>    avcodec/pngdec: support sBIT chunks
>    avcodec/pngenc: write sBIT chunks
> 
>   libavcodec/pngdec.c | 48 +++++++++++++++++++++++++++++++++++++++++++++
>   libavcodec/pngenc.c |  6 ++++++
>   2 files changed, 54 insertions(+)
> 

If there's no further objections I'll push this in ~3 days or so.

- Leo Izen (thebombzen)
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Re: [FFmpeg-devel] [PATCH v3 0/2] PNG sBIT chunk support

[Leo Izen]

On 3/28/23 12:24, Leo Izen wrote:
> On 3/21/23 23:26, Leo Izen wrote:
>> Add support to the PNG decoder and encoder to support sBIT chunks,
>> by using AVCodecContext->bits_per_raw_sample.
>>
>> Changes from v2:
>> - Added chunk size check to make sure there's enough bytes left
>>
>> Leo Izen (2):
>>    avcodec/pngdec: support sBIT chunks
>>    avcodec/pngenc: write sBIT chunks
>>
>>   libavcodec/pngdec.c | 48 +++++++++++++++++++++++++++++++++++++++++++++
>>   libavcodec/pngenc.c |  6 ++++++
>>   2 files changed, 54 insertions(+)
>>
> 
> If there's no further objections I'll push this in ~3 days or so.
> 
> - Leo Izen (thebombzen)

Pushed as 84b454935fae and c125860892e9.

- Leo Izen (thebombzen)
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Re: [FFmpeg-devel] [PATCH v3 0/2] PNG sBIT chunk support

[Anton Khirnov]

Quoting Leo Izen (2023-04-01 06:39:12)
> On 3/28/23 12:24, Leo Izen wrote:
> > On 3/21/23 23:26, Leo Izen wrote:
> >> Add support to the PNG decoder and encoder to support sBIT chunks,
> >> by using AVCodecContext->bits_per_raw_sample.
> >>
> >> Changes from v2:
> >> - Added chunk size check to make sure there's enough bytes left
> >>
> >> Leo Izen (2):
> >>    avcodec/pngdec: support sBIT chunks
> >>    avcodec/pngenc: write sBIT chunks
> >>
> >>   libavcodec/pngdec.c | 48 +++++++++++++++++++++++++++++++++++++++++++++
> >>   libavcodec/pngenc.c |  6 ++++++
> >>   2 files changed, 54 insertions(+)
> >>
> > 
> > If there's no further objections I'll push this in ~3 days or so.
> > 
> > - Leo Izen (thebombzen)
> 
> Pushed as 84b454935fae and c125860892e9.

Would be nice to have a test.

-- 
Anton Khirnov
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