我可以检测 PNG 文件是否从多页 PDF 文件转换而来，并使用 ImageMagick 将图像拆分为单独的文件吗？

Question

I'm having a problem where a PNG file that was created using the PDF export feature in OS X's Preview will display like an animated GIF in some browsers (Safari,Firefox) or display only the first page (Chrome) if the converted PDF file has more than one page (By animated GIF I mean it flickers between the different pages). Is there a way using PHP to detect if an uploaded PNG file was converted this way, and then split it into separate images and stitch them together using ImageMagick?

Answer 1

It is an animated PNG, or Mozilla APNG format file. You can detect it by running pngcheck like this:

pngcheck -v npma33-png-convert-test.png

Output

File: npma33-png-convert-test.png (981954 bytes)
  chunk IHDR at offset 0x0000c, length 13
    1275 x 1650 image, 32-bit RGB+alpha, non-interlaced
  chunk acTL at offset 0x00025, length 8                   <--- APNG
    unknown private, ancillary, unsafe-to-copy chunk
  chunk iCCP at offset 0x00039, length 281
    profile name = ICC Profile, compression method = 0 (deflate)
    compressed profile = 268 bytes
  chunk pHYs at offset 0x0015e, length 9: 5906x5906 pixels/meter (150 dpi)
  chunk fcTL at offset 0x00173, length 26                  <--- APNG
    unknown private, ancillary, unsafe-to-copy chunk
  chunk IDAT at offset 0x00199, length 16384
    zlib: deflated, 32K window, superfast compression
  chunk IDAT at offset 0x041a5, length 16384
  chunk IDAT at offset 0x081b1, length 16384
  chunk IDAT at offset 0x0c1bd, length 16384
  chunk IDAT at offset 0x101c9, length 16384
  chunk IDAT at offset 0x141d5, length 16384
  chunk IDAT at offset 0x181e1, length 16384
  chunk IDAT at offset 0x1c1ed, length 16384
  chunk IDAT at offset 0x201f9, length 16384
  chunk IDAT at offset 0x24205, length 16384
  chunk IDAT at offset 0x28211, length 16384
  chunk IDAT at offset 0x2c21d, length 16384
  chunk IDAT at offset 0x30229, length 16384
  chunk IDAT at offset 0x34235, length 16384
  chunk IDAT at offset 0x38241, length 16384
  chunk IDAT at offset 0x3c24d, length 16384
  chunk IDAT at offset 0x40259, length 16384
  chunk IDAT at offset 0x44265, length 16384
  chunk IDAT at offset 0x48271, length 16384
  chunk IDAT at offset 0x4c27d, length 16384
  chunk IDAT at offset 0x50289, length 16384
  chunk IDAT at offset 0x54295, length 16384
  chunk IDAT at offset 0x582a1, length 16384
  chunk IDAT at offset 0x5c2ad, length 16384
  chunk IDAT at offset 0x602b9, length 16384
  chunk IDAT at offset 0x642c5, length 15969
  chunk fcTL at offset 0x68132, length 26                 <--- APNG
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x68158, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x6c168, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x70178, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x74188, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x78198, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x7c1a8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x801b8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x841c8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x881d8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x8c1e8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x901f8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x94208, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x98218, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0x9c228, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xa0238, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xa4248, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xa8258, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xac268, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xb0278, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xb4288, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xb8298, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xbc2a8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xc02b8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xc42c8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xc82d8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xcc2e8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xd02f8, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xd4308, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xd8318, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xdc328, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xe0338, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xe4348, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xe8358, length 16388
    unknown private, ancillary, unsafe-to-copy chunk
  chunk fdAT at offset 0xec368, length 14406
    unknown private, ancillary, unsafe-to-copy chunk
  chunk IEND at offset 0xefbba, length 0

The giveaway that it is APNG is the presence of the acTL and fcTL chunks which are animation control and frame control .

You can suppress output of those chunks (which will stop the flickering) using the following command:

convert npma33-png-convert-test.png -define png:exclude-chunk=acTL result.png

but I have not done any testing on whether that will affect other things such as the compression achieved or how any other meta-data is affected - so please do your own checks.

So, if you use bash you will end up with something like this:

file="npma33-png-convert-test.png"
if pngcheck -v "$file" | grep -q acTL; then 
   convert "$file" -define png:exclude-chunk=acTL "$file"
fi

The expected way to separate and rejoin the frames side-by-side would probably be

convert npma.... -separate +append result.png

or

convert npma... -coalesce +append result.png

but ImageMagick seems reluctant to do either - I can only guess APNG is not really supported.

I also tried using Photoshop-style layer addressing, but that doesn't work either:

convert npma...[0] page1.png
convert npma...[1] page2.png