param(
  [Parameter(Mandatory=$true)][string]$InputPath,
  [Parameter(Mandatory=$true)][string]$OutputPath,
  [string]$BackupPath = ""
)

if (!(Test-Path -LiteralPath $InputPath)) {
  throw "Input file not found: $InputPath"
}

if ($BackupPath -ne "") {
  Copy-Item -LiteralPath $InputPath -Destination $BackupPath -Force
}

Add-Type -AssemblyName System.Drawing

# Load from a stream to avoid locking the input file (required if output overwrites input)
$fs = [System.IO.File]::Open($InputPath, [System.IO.FileMode]::Open, [System.IO.FileAccess]::Read, [System.IO.FileShare]::ReadWrite)
try {
  $bmp = New-Object System.Drawing.Bitmap($fs)
} finally {
  $fs.Dispose()
}

$out = New-Object System.Drawing.Bitmap($bmp.Width, $bmp.Height, [System.Drawing.Imaging.PixelFormat]::Format32bppArgb)

function IsBgPixel($c) {
  $r = [int]$c.R
  $g = [int]$c.G
  $b = [int]$c.B

  $nearGray = ([math]::Abs($r - $g) -le 10) -and ([math]::Abs($g - $b) -le 10)
  if (-not $nearGray) { return $false }

  # Typical checkerboard: very light gray and mid light gray
  $isLight = ($r -ge 235 -and $g -ge 235 -and $b -ge 235)
  $isMid = ($r -ge 205 -and $g -ge 205 -and $b -ge 205 -and $r -le 235)
  return ($isLight -or $isMid)
}

for ($y = 0; $y -lt $bmp.Height; $y++) {
  for ($x = 0; $x -lt $bmp.Width; $x++) {
    $c = $bmp.GetPixel($x, $y)

    if (IsBgPixel $c) {
      $out.SetPixel($x, $y, [System.Drawing.Color]::FromArgb(0, $c.R, $c.G, $c.B))
    } else {
      $out.SetPixel($x, $y, [System.Drawing.Color]::FromArgb(255, $c.R, $c.G, $c.B))
    }
  }
}

$tmp = "$OutputPath.tmp"

try {
  $out.Save($tmp, [System.Drawing.Imaging.ImageFormat]::Png)
} finally {
  $bmp.Dispose()
  $out.Dispose()
}

Move-Item -LiteralPath $tmp -Destination $OutputPath -Force