program Project1;

{$APPTYPE CONSOLE}

uses
  System.SysUtils,
  System.Math,
  Vcl.Imaging.PNGImage;

type
  TRGB = record
    r,g,b: single;
  end;
  TIMG = record
    xmax, ymax: integer;
    data: array of array of TRGB;
  end;

function LoadPNG48(var img: TIMG; const fname: string): boolean;
var
  png: TPNGImage;
  x,y: integer;
  scan,extra: pRGBLine;
begin
  png:=TPNGImage.Create;
  png.LoadFromFile(fname);
  result:=png.Header.BitDepth=16;
  if result then begin
    img.xmax:=png.Width;
    img.ymax:=png.Height;
    SetLength(img.data, png.Height, png.Width);
    for y:=0 to img.ymax-1 do begin
      scan:=png.Scanline[y];
      extra:=png.ExtraScanline[y];
      for x:=0 to img.xmax-1 do begin
        img.data[y,x].r:=(scan[x].rgbtRed shl 8 + extra[x].rgbtRed) / 65535;
        img.data[y,x].g:=(scan[x].rgbtGreen shl 8 + extra[x].rgbtGreen) / 65535;
        img.data[y,x].b:=(scan[x].rgbtBlue shl 8 + extra[x].rgbtBlue) / 65535;
      end;
    end;
  end;
  png.Free;
end;

procedure SavePNG24(var img: TIMG; const fname: string);
var
  png: TPNGImage;
  x,y: integer;
  scan: pRGBLine;
begin
  png:=TPNGImage.CreateBlank(COLOR_RGB, 8, img.xmax, img.ymax);
  for y:=0 to img.ymax-1 do begin
    scan:=png.Scanline[y];
    for x:=0 to img.xmax-1 do begin
      scan[x].rgbtRed:=round(img.data[y,x].r*255);
      scan[x].rgbtGreen:=round(img.data[y,x].g*255);
      scan[x].rgbtBlue:=round(img.data[y,x].b*255);
    end;
  end;
  png.SaveToFile(fname);
end;

var
  img: TIMG;
  x,y: integer;
begin
  if LoadPNG48(img, 'test48.png') then begin
    for y:=0 to img.ymax-1 do begin
      for x:=0 to img.xmax-1 do begin
        img.data[y,x].r:=power(img.data[y,x].r, 1 / 2.2);
        img.data[y,x].g:=power(img.data[y,x].g, 1 / 2.2);
        img.data[y,x].b:=power(img.data[y,x].b, 1 / 2.2);
      end;
    end;
    SavePNG24(img, 'test24.png');
  end;
end.