Bash 脚本检查图像是否为动画 png (apng)

Question

试图在我的 bash 脚本中找出一种方法来检查文件是否是动画 PNG (apng) 文件。就我而言，如果是的话，我想忽略它。有任何想法吗 ？

更新：下面使用 pngcheck 的答案允许我检查图像是否是动画。另外，我检查文件的大小，如果它“大”，我也会忽略它。最后，截至 2017 年 11 月，“识别”不起作用，这主要是我最初的问题。感谢马克的帮助。

Answer 1

动画 PNG 的特点是存在acTL（动画控制块）和fcTL（帧控制块） - 请参阅维基百科文章。

因此，我认为合适的测试是pngcheck使用详细选项运行，并至少查找acTL块：

pngcheck -v animated.png | grep -E "acTL|fcTL"

样本输出

  chunk acTL at offset 0x00025, length 8
  chunk fcTL at offset 0x00039, length 26
  chunk fcTL at offset 0x02f27, length 26
  chunk fcTL at offset 0x05901, length 26
  chunk fcTL at offset 0x083a2, length 26
  chunk fcTL at offset 0x0aea8, length 26
  chunk fcTL at offset 0x0d98c, length 26
  chunk fcTL at offset 0x10406, length 26
  chunk fcTL at offset 0x12e19, length 26
  chunk fcTL at offset 0x15985, length 26
  chunk fcTL at offset 0x185e2, length 26
  chunk fcTL at offset 0x1b2b0, length 26
  chunk fcTL at offset 0x1dfe1, length 26
  chunk fcTL at offset 0x20d24, length 26
  chunk fcTL at offset 0x23a03, length 26
  chunk fcTL at offset 0x26663, length 26
  chunk fcTL at offset 0x29218, length 26
  chunk fcTL at offset 0x2bcdf, length 26
  chunk fcTL at offset 0x2e7e0, length 26
  chunk fcTL at offset 0x312b0, length 26
  chunk fcTL at offset 0x33c51, length 26
  chunk fcTL at offset 0x36598, length 26
  chunk fcTL at offset 0x38f49, length 26
  chunk fcTL at offset 0x3b9bd, length 26
  chunk fcTL at offset 0x3e45e, length 26
  chunk fcTL at offset 0x40ed9, length 26
  chunk fcTL at offset 0x4393c, length 26
  chunk fcTL at offset 0x46521, length 26
  chunk fcTL at offset 0x4919b, length 26
  chunk fcTL at offset 0x4bde2, length 26
  chunk fcTL at offset 0x4eabd, length 26
  chunk fcTL at offset 0x51827, length 26
  chunk fcTL at offset 0x5453a, length 26
  chunk fcTL at offset 0x571c7, length 26
  chunk fcTL at offset 0x59d94, length 26

因此，这将建议在脚本中进行此测试：

# Test an animated image, `grep` exit status is zero meaning `acTL` was found
pngcheck -v animated.png | grep -q "acTL"
echo $?
0

# Test a still image, `grep` exit status is 1 meaning `acTL` was not found
pngcheck -v still.png | grep -q "acTL"
echo $?
1

如果你没有，或者不想pngcheck随你的项目一起发布，我制作了一个小 Perl 脚本，它只是对 PNG 文件进行分块，并告诉你分块和偏移量，并且它应该在任何地方运行，因为它是 Perl。欢迎您使用它。

样品运行

./pngchunks ball.png

13 IHDR
8 acTL                   <--- This one means it is animated
26 fcTL
4634 IDAT
26 fcTL
4344 fdAT
26 fcTL
4042 fdAT
26 fcTL
3828 fdAT
26 fcTL
3521 fdAT
26 fcTL
3168 fdAT
26 fcTL
2777 fdAT
26 fcTL
2588 fdAT
26 fcTL
2720 fdAT
26 fcTL
2792 fdAT
26 fcTL
2665 fdAT
26 fcTL
2581 fdAT
26 fcTL
2652 fdAT
26 fcTL
2774 fdAT
26 fcTL
2844 fdAT
26 fcTL
2886 fdAT
26 fcTL
2966 fdAT
26 fcTL
3197 fdAT
26 fcTL
3518 fdAT
26 fcTL
3995 fdAT
0 IEND

#!/usr/bin/perl -w
################################################################################
# pngchunks
# Mark Setchell
#
# Simple Perl tool to read the chunks in a PNG image file
# See https://en.wikipedia.org/wiki/Portable_Network_Graphics
#
# Usage: pngchunks image.png
################################################################################
use strict;
use Fcntl qw( SEEK_CUR );

my $f=shift or die "Usage: pngchunks image.png\n";

my ($handle,$offset,$buffer,$type,$length);

# Open file
open($handle,'<',$f) || die("Error opening file\n");

# Check 8 byte PNG signature
read($handle,$buffer,8);
if(substr($buffer,0,8) ne "\x89\x50\x4e\x47\x0d\x0a\x1a\x0a"){
   die("ERROR: Invalid PNG signature\n");
}

# Loop till IEND chunk
for(;;){

   # Read 4 bytes of length, Network (big-endian)
   read($handle,$buffer,4);
   $length=unpack("N",$buffer);

   # Read 4 bytes of chunk type
   read($handle,$buffer,4);
   $type=substr($buffer,0,4);

   printf("%d %s\n",$length,$type);

   # Break out of loop if IEND chunk
   last if $type eq "IEND";

   # Seek past this chunk and its 4 byte CRC
   $offset=4+$length;
   seek($handle,$offset,SEEK_CUR);
}