Supported PNG types

   (0) Gray scale of depth 1, 2, 4, 8, or 16 bits per pixel (2, 4, 16, 256,
       or 65536 gray levels). Full transparency (of one 16-bit gray value) 
       via the tRNS chunk is allowed, unless the notrans option specifies 
       that it be ignored.
   (2) RGB truecolor with 8 or 16 bits per sample (3 samples: 16.7 million 
       or 281.5 trillion colors). Full transparency (of one 3x16-bit RGB 
       color value) via the tRNS chunk is allowed, unless the notrans 
       option specifies that it be ignored.
   (3) Palette color with 1, 2, 4, or 8 bits per pixel (2, 4, 16, or 256
       color table/palette entries). 16 bpp is not currently supported by
       PNG or PDF. Partial transparency (8-bit Alpha) for each palette
       entry via the tRNS chunk is allowed, unless the notrans option 
       specifies that it be ignored (all entries fully opaque).
   (4) Gray scale of depth 8 or 16 bits per pixel plus equal-sized Alpha
       channel (256 or 65536 gray levels and 256 or 65536 levels of
       transparency). The Alpha channel is ignored if the notrans 
       option is given. The tRNS chunk is not permitted.
   (5) B<RESERVED> for grayscale via palette + Alpha channel
   (6) RGB truecolor with 8 or 16 bits per sample, with equal-sized 
       Alpha channel (256 or 65536 levels of transparency). The Alpha 
       channel is ignored if the notrans option is given. The tRNS 
       chunk is not permitted.
   (7) B<RESERVED> for truecolor via palette + Alpha channel

In all cases, 16 bits per sample forces PDF 1.5 (or higher) output, unless you give the "force8bps" option, to "strip" 16 bit samples to 8 bits, and permit PDF 1.4-compatible output. The libpng.a library is assuming standard "network" bit and byte ordering (Big Endian), although flags might be added to change this.

The transparency chunk (tRNS) will specify one gray level entry or one RGB entry to be treated as transparent (Alpha = 0). For palette color, up to 256 palette entry 8-bit Alpha values are specified (256 levels of transparency, from 0 = transparent to 255 = opaque).

Only a limited number of chunks are handled: IHDR, IDAT (internally), PLTE, tRNS, and IEND (internally). All other chunks are ignored at this time. Filters and compression applied to data is handled internally by libpng.a -- there may be unsupported methods.

$mode = $png->usesLib()

Returns 1 if Image::PNG::Libpng installed and used, 0 if not installed, or -1 if installed but not used (nouseIPL option given to "image_png").

Caution: this method can only be used after the image object has been created. It can't tell you whether Image::PNG::Libpng is available in advance of actually using it, in case you want to use some functionality available only in PNG_IPL. See the PDF::Builder LA_IPL() call if you need to know in advance.