param(
  [Parameter(Mandatory=$true)][string]$InputPath,
  [Parameter(Mandatory=$true)][string]$OutputPath,
  [string]$BackupPath = "",
  [int]$AlphaThreshold = 1,
  [int]$Padding = 0
)

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 stream to avoid locking
$fs = [System.IO.File]::Open($InputPath, [System.IO.FileMode]::Open, [System.IO.FileAccess]::Read, [System.IO.FileShare]::ReadWrite)
try {
  $bmp = New-Object System.Drawing.Bitmap($fs)
} finally {
  $fs.Dispose()
}

$minX = $bmp.Width
$minY = $bmp.Height
$maxX = -1
$maxY = -1

for ($y = 0; $y -lt $bmp.Height; $y++) {
  for ($x = 0; $x -lt $bmp.Width; $x++) {
    $c = $bmp.GetPixel($x, $y)
    if ($c.A -ge $AlphaThreshold) {
      if ($x -lt $minX) { $minX = $x }
      if ($y -lt $minY) { $minY = $y }
      if ($x -gt $maxX) { $maxX = $x }
      if ($y -gt $maxY) { $maxY = $y }
    }
  }
}

if ($maxX -lt 0 -or $maxY -lt 0) {
  # No non-transparent pixels, just copy input
  $bmp.Dispose()
  Copy-Item -LiteralPath $InputPath -Destination $OutputPath -Force
  exit 0
}

$minX = [math]::Max(0, $minX - $Padding)
$minY = [math]::Max(0, $minY - $Padding)
$maxX = [math]::Min($bmp.Width - 1, $maxX + $Padding)
$maxY = [math]::Min($bmp.Height - 1, $maxY + $Padding)

$newW = $maxX - $minX + 1
$newH = $maxY - $minY + 1

$rect = New-Object System.Drawing.Rectangle($minX, $minY, $newW, $newH)
$cropped = $bmp.Clone($rect, [System.Drawing.Imaging.PixelFormat]::Format32bppArgb)

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

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