Simulates artifacts of a certain lossy image compression

png.lua

local png = {}

local ffi = require('ffi')

local libpng = ffi.load('libpng16.dll')

ffi.cdef[[
typedef struct png_control *png_controlp;
typedef struct
{
   png_controlp opaque;    /* Initialize to NULL, free with png_image_free */
   uint32_t  version;   /* Set to PNG_IMAGE_VERSION */
   uint32_t  width;     /* Image width in pixels (columns) */
   uint32_t  height;    /* Image height in pixels (rows) */
   uint32_t  format;    /* Image format as defined below */
   uint32_t  flags;     /* A bit mask containing informational flags */
   uint32_t  colormap_entries;
                           /* Number of entries in the color-map */
   uint32_t  warning_or_error;

   char       message[64];
} png_image, *png_imagep;

typedef struct png_color_struct
{
   uint8_t* red;
   uint8_t* green;
   uint8_t* blue;
} png_color;
typedef png_color * png_colorp;
typedef const png_color * png_const_colorp;

int png_image_begin_read_from_file(
   png_imagep image, const char *file_name);

int png_image_finish_read(
   png_imagep image,
   png_const_colorp background,
   void *buffer, int32_t row_stride,
   void *colormap);

void png_image_free(png_imagep image);

int png_image_write_to_file(
   png_imagep image,
   const char *file, int convert_to_8bit, const void *buffer,
   int32_t row_stride, const void *colormap);
]]

local PNG_IMAGE_VERSION = 1

local PNG_IMAGE_WARNING = 1
local PNG_IMAGE_ERROR = 2

local PNG_FORMAT_RGBA = 3

local function newimage()
    local i = ffi.gc(ffi.new('png_image'), libpng.png_image_free)
    i.version = PNG_IMAGE_VERSION
    return i
end

local function checkerror(i)
    local woe = bit.band(i.warning_or_error, 3) ~= 0
    if woe == 2 or woe == 3 then
        error(ffi.string(i.message))
    elseif woe == 1 then
        printf('png warning: ' .. ffi.string(i.message))
    end
end

function png.readfile(path)
    local i = newimage()
    libpng.png_image_begin_read_from_file(i, path)
    checkerror(i)
    local bm = ffi.new(ffi.typeof('uint8_t[$][$][4]', i.height, i.width))
    i.format = PNG_FORMAT_RGBA
    i.flags = 0
    libpng.png_image_finish_read(i, nil, bm, i.width * 4, nil)
    checkerror(i)
    libpng.png_image_free(i)
    return i.width, i.height, bm
end

function png.writefile(path, width, height, bm)
    local i = newimage()
    i.width = width
    i.height = height
    i.format = PNG_FORMAT_RGBA
    i.flags = 0
    libpng.png_image_write_to_file(i, path, 0, bm, width * 4, nil)
    checkerror(i)
end



local ka = 7
local blocksize = 64

local function quantize(q, ra, rd)
    if rd <= 0 then
        return q
    else
        local u = (q - ra) / rd
        local v = math.floor(u * ka + 0.5) / ka
        return math.floor(v * rd + ra + 0.5)
    end
end

local function compress(p, xa, xb, ya, yb)
    local samples = {[0] = {}, {}, {}, {}}
    for y = ya, yb - 1 do
        for x = xa, xb - 1 do
            for c = 0, 3 do
                local v = p[y][x][c]
                samples[c][#samples[c] + 1] = v
            end
        end
    end
    local loa = {}
    local lod = {}
    local hia = {}
    local hid = {}
    for c = 0, 3 do
        table.sort(samples[c])
        local maxdensv = 0
        local maxdensi = 2
        for i = 2, #samples[c] - 2 do
            local lospan = (samples[c][i] - samples[c][1])
            local hispan = (samples[c][#samples[c]] - samples[c][i + 1])
            if lospan > 0 and hispan > 0 then
                local lodens = i*(i - 2) / lospan
                local hidens = (#samples[c] - i)*(#samples[c] - i - 2) / hispan
                local dens = lodens + hidens
                if dens > maxdensv then
                    maxdensv = dens
                    maxdensi = i
                end
            end
        end
        loa[c] = samples[c][1]
        lod[c] = samples[c][maxdensi] - samples[c][1]
        hia[c] = samples[c][maxdensi + 1]
        hid[c] = samples[c][#samples[c]] - samples[c][maxdensi + 1]
    end
    for y = ya, yb - 1 do
        for x = xa, xb - 1 do
            for c = 0, 3 do
                local v = p[y][x][c]
                if v > hia[c] then
                    p[y][x][c] = quantize(v, hia[c], hid[c])
                else
                    p[y][x][c] = quantize(v, loa[c], lod[c])
                end
            end
        end
    end
end



local width, height, p = png.readfile('C:\\Users\\delfi\\Desktop\\i3.png')

for ya = 0, height - 1, blocksize do
    local yb = ya + blocksize
    if yb > height then
        yb = height
    end
    for xa = 0, width - 1, blocksize do
        local xb = xa + blocksize
        if xb > width then
            xb = width
        end
        compress(p, xa, xb, ya, yb)
    end
end

png.writefile('C:\\Users\\delfi\\Desktop\\r3.png', width, height, p)