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PNG reader

local libpng = require'libpng'

A ffi binding of the ubiquitous libpng.

API

libpng.load(t) -> image

Read and decode a PNG image. t is a table specifying:

  • where to read the data from (one of the following):
    • path: read data from a file given its filename
    • string: read data from a string
    • cdata, size: read data from a buffer of specified size
    • stream: read data from an opened FILE * stream
    • read: read data from a reader function of form:
      • read(needed_size) -> cdata, size | string | nil
        • needed_size is informative, the function can return however many bytes it wants, as long as it returns at least 1 byte.
  • loading options:
    • accept: if present, it is a table specifying conversion options. libpng implements many of the pixel conversions itself, while other conversions are supported through [bmpconv bmpconv.convert_best()]. If no accept option is given, the image is returned in a normalize 8 bit per channel, top down, palette expanded, 'g', 'rgb', 'rgba' or 'ga' format.
    • [pixel_format] = true - specify one or more accepted pixel formats (they are all implicitly 8 bit per channel since that is the only supported bit depth):
      • 'rgb', 'bgr', 'rgba', 'bgra', 'argb', 'abgr', 'g', 'ga', 'ag'.
        • if no pixel format is specified, resulted bit depth will not necessarily be 8 since no conversion will take place.
    • [orientation] = true - specify one or more accepted orientations:
      • 'top_down', 'bottom_up' (default is 'top_down')
    • padded: true/false (default is false) - specify that the row stride should be a multiple of 4
    • header_only: do not decompress the image; return only the image header fields.
    • sparkle: true/false (default is false) - alternative render mode for interlaced images.
  • callbacks:
    • warning: a function to be called as warning(msg) on non-fatal errors.
    • render_scan: a function to be called as render_scan(image, is_last_scan, scan_number) for each pass of an interlaced PNG. It can be used to implement progressive display of images.
      • also called once for non-interlaced images.
      • also called on error, as render_scan(nil, true, scan_number, error), where scan_number is the scan number that was supposed to be rendering next and error the error message.

For more info on decoding process and options, read the libpng doc (have coffee/ibuprofen ready).

The returned image object is a table with the fields:

  • pixel, orientation, stride, data, size, w, h: image format and dimensions and pixel data.
  • file.pixel, file.paletted, file.bit_depth, file.interlaced, file.w, file.h: format of the original image before conversion.

Help needed

  • saving API
  • jit is turned off because we can't call error() from a ffi callback called from C; and yet we must not return control to C on errors. Is there a way around it?
  • the read callback cannot yield since it is called from C code. This means coroutine-based socket schedulers are out, so much for progressive loading. Is there a way around it?