if not modules then modules = { } end modules ['grph-png'] = {
    version   = 1.001,
    comment   = "companion to grph-inc.mkiv",
    author    = "Hans Hagen, PRAGMA-ADE, Hasselt NL",
    copyright = "PRAGMA ADE / ConTeXt Development Team",
    license   = "see context related readme files"
}

-- This is a rather specialized module and only used for the more artistic
-- \LUAMETAFUN\ applicaztions.

local tocardinal4 = sio.tocardinal4
local tocardinal2 = sio.tocardinal2
local tocardinal1 = string.char
local substring   = string.sub
local concat      = table.concat
local rep         = string.rep

local checksum    = xzip.crc32
local compress    = xzip.compress

-- the checksum is calculated over the chunk type (tag) and data but the
-- length excludes the 4 character tag

local function blob(data)
    local pre  = tocardinal4(#data - 4)
    local post = tocardinal4(checksum(data))
    return pre .. data .. post
end

local function IHDR(width,height,bitdepth,colortype)
    return blob("IHDR"
      .. tocardinal4(width)
      .. tocardinal4(height)
      .. tocardinal1(bitdepth)  -- 8, 16, etc. (often just 8)
      .. tocardinal1(colortype) -- 0 = gray, 2 = rgb, 6 = rgba (often 0 or 2)
      .. tocardinal1(0)         -- compression
      .. tocardinal1(0)         -- filter
      .. tocardinal1(0)         -- interlace
    )
end

local function IDAT(bytes)
    return blob("IDAT" .. bytes)
end

local function acTL(frames, plays)
    return blob("acTL"
     .. tocardinal4(frames)
     .. tocardinal4(plays)
    )
end

local function fcTL(sequence,width,height,numerator,denominator,dispose,blend)
    return blob("fcTL"
     .. tocardinal4(sequence)
     .. tocardinal4(width)
     .. tocardinal4(height)
     .. tocardinal4(0)           -- xoffset
     .. tocardinal4(0)           -- yoffset
     .. tocardinal2(numerator)   -- delay
     .. tocardinal2(denominator) -- delay
     .. tocardinal1(dispose)     -- dispose op (1 = clear background)
     .. tocardinal1(blend)       -- blend op (0 = source)
    )
end

local function fdAT(sequence, bytes)
    return blob("fdAT"
     .. tocardinal4(sequence)
     .. bytes
    )
end

local function SIGN()
    return "\137\080\078\071\013\010\026\010"
end

local function IEND()
    return blob("IEND")
end

local function filtered(bytes,width,height,bpp)
    local rowsize = width * bpp
    local result  = { }
    local start   = 1
    local r       = 0
    for i=1,height do
        r = r + 1 ; result[r] = tocardinal1(0) -- filter 0 : none
        r = r + 1 ; result[r] = substring(bytes,start,start+rowsize-1)
        start = start + rowsize
    end
    return concat(result)
end

local report = logs.reporter("export [a]png")

local function openpng(specification)
    --
    local nofframes = specification.nofframes or 0
    local width     = specification.width     or 0
    local height    = specification.height    or 0
    local bitdepth  = specification.bitdepth  or 8
    local colortype = specification.colortype or 0 -- gray
    local bpp       = specification.bpp       or 1
    local loop      = specification.loop      or 0 -- infinity
    local nofbytes  = width * height * bpp
    local filename  = specification.filename
    --
    if nofframes == 0 then
        report("no frames")
        return
    end
    --
    local f = filename and io.open(filename, "wb")
    if not f then
        report("unable to open %a",filename)
        return
    end
    --
    local sequence = 0
    --
    local numerator   = specification.numerator    or 4   -- delay
    local denominator = specification.denominator  or 100 -- delay
    local dispose     = specification.dispose      or 1
    local blend       = specification.blend        or 0
    --
    statistics.starttiming(specification,true)
    f:write(SIGN())
    f:write(IHDR(width,height,bitdepth,colortype))
    f:write(acTL(nofframes,loop))
    statistics.stoptiming(specification) -- still zero
    --
    local done = 0
    --
    local function close()
        if f then
            f:write(IEND())
            f:close()
            report("png sequence of size %i saved in %a, %s seconds spent",nofframes,filename,statistics.elapsedtime(specification))
            f = nil
        end
    end
    --
    local function add(frame)
        if f then
            statistics.starttiming(specification)
            if #frame ~= nofbytes then
                report("inconsistent frame %i, expected %i bytes instead of %i",done+1,nofbytes,#frame)
                frame = rep("\0x7F",nofbytes)
            end
            local bytes  = filtered(frame,width,height,bpp)
            local stream = compress(bytes)
            f:write(fcTL(sequence,width,height,numerator,denominator,dispose,blend))
            sequence = sequence + 1
            if sequence == 1 then
                -- the first frame uses standard IDAT and does not increment sequence
                f:write(IDAT(stream))
            else
                -- following frames use fdAT and includes its own sequence number
                f:write(fdAT(sequence, stream))
                sequence = sequence + 1
            end
            done = done + 1
            if done >= nofframes then
                close()
            end
            statistics.stoptiming(specification)
            return done
        end
    end
    --
    return add, close
end

local function savepng(specification)
    local frames    = specification.frames
    local nofframes = frames and #frames or 0
    if nofframes == 0 then
        report("no frames given")
        return
    end
    specification.nofframes = nofframes
    local add, close = openpng(specification)
    if add and close then
        for i=1,nofframes do
            add(frames[i])
        end
        close()
    end
end

bytemap.png = {
    open = openpng,
    save = savepng,
}

-- if not context then
--
--     local width           = 500
--     local height          = 500
--     local bitdepth        = 8
--     local colortype       = 0
--     local bpp             = 1 -- bytes per pixel
--     local frames          = { }
--
--     for i=1,100 do
--         frames[i] = string.rep(
--             tocardinal1(i*2),
--             width * height
--         )
--     end
--
--     savepng {
--         filename    = "crap.png",
--         width       = width,
--         height      = height,
--         bitdepth    = bitdepth,
--         colortype   = colortype,
--         bpp         = bpp,
--         frames      = frames,
--         loop        = 0, -- 1
--         loop        = 1, -- 1
--         numerator   = 8,
--         demominator = 100,
--     }
--
-- end