[Libav-user] [help][ffmpeg]Compress png picture failed:Invalid PNG signature 0x00000000

[baijundong]

Hello every one

I user ffmpeg API for Compressing png picture failed: [Invalid PNG signature
0x00000000.]
There is no output file.please help me~Thank you very much!

1、
Soft environment：
Ubuntu: 14.04
FFmpeg:3.0.2

2、Problem Description：
1) The base code is Version 3.0.2 FFmpeg's source:
doc/example/transcoding.c. I modify the coding for compress the src picture
to 320X240:
transcoding.c :: static int open_output_file(const char *filename):127~128
step:
-----
enc_ctx->height = 240;
enc_ctx->width = 320;
-----

2) transcoding.c :: static int init_filters(void):354 step。Set scale is
320:240
-----
 filter_spec = "scale=320:240";
-----

3) Compile and run:'/transcoding /home/neusoft/baijd/scale_test/6.png
/home/neusoft/baijd/scale_test/62-output.png'
   No output file be created.
   Note that：compressing jpeg file or jpg file no problem.

4) the attachment of email is the source code.
-----
[file @ 0x7e3670] Setting default whitelist 'file,crypto'
Probing image2 score:50 size:2048
Probing png_pipe score:99 size:2048
[png_pipe @ 0x7e3010] Format png_pipe probed with size=2048 and score=99
[png_pipe @ 0x7e3010] Before avformat_find_stream_info() pos: 0 bytes
read:32768 seeks:0
[png_pipe @ 0x7e3010] 0: start_time: -9223372036854.775 duration:
-9223372036854.775
[png_pipe @ 0x7e3010] stream: start_time: -9223372036854.775 duration:
-9223372036854.775 bitrate=0 kb/s
[png_pipe @ 0x7e3010] After avformat_find_stream_info() pos: 382026 bytes
read:382026 seeks:0 frames:2
detected 4 logical cores
Input #0, png_pipe, from '/home/neusoft/baijd/scale_test/6.png':
  Duration: N/A, bitrate: N/A
    Stream #0:0, 2, 1/25: Video: png, 1 reference frame, rgba(pc), 580x820
[SAR 2835:2835 DAR 29:41], 1/25, 25 tbr, 25 tbn, 25 tbc
Output #0, image2, to '/home/neusoft/baijd/scale_test/62-output.png':
    Stream #0:0, 0, 0/0: Video: png, 1 reference frame, rgb24, 320x240 [SAR
2835:2835 DAR 4:3], 1/25, q=2-31, 128 kb/s, 25 tbc
[image2 @ 0x7f3e20] Using AVStream.codec.time_base as a timebase hint to the
muxer is deprecated. Set AVStream.time_base instead.
[in @ 0x83a3a0] Setting 'video_size' to value '580x820'
[in @ 0x83a3a0] Setting 'pix_fmt' to value '28'
[in @ 0x83a3a0] Setting 'time_base' to value '1/25'
[in @ 0x83a3a0] Setting 'pixel_aspect' to value '2835/2835'
[in @ 0x83a3a0] w:580 h:820 pixfmt:rgba tb:1/25 fr:0/1 sar:2835/2835
sws_param:
[Parsed_scale_0 @ 0x83aa30] Setting 'w' to value '320'
[Parsed_scale_0 @ 0x83aa30] Setting 'h' to value '240'
[Parsed_scale_0 @ 0x83aa30] w:320 h:240 flags:'bilinear' interl:0
[AVFilterGraph @ 0x839c60] query_formats: 3 queried, 2 merged, 0 already
done, 0 delayed
[swscaler @ 0x83afb0] Forcing full internal H chroma due to input having non
subsampled chroma
[Parsed_scale_0 @ 0x83aa30] w:580 h:820 fmt:rgba sar:2835/2835 -> w:320
h:240 fmt:rgb24 sar:87/164 flags:0x2
Demuxer gave frame of stream_index 0
Going to reencode&filter the frame
Demuxer gave frame of stream_index 0
Going to reencode&filter the frame
Pushing decoded frame to filters
Pulling filtered frame from filters
[png @ 0x7ea4e0] Invalid PNG signature 0x00000000.
[AVIOContext @ 0x7eb770] Statistics: 382026 bytes read, 0 seeks
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