Fix -Frames negative range calculation to extract last N frames correctly (off-by-one error), add validation to reject invalid '-%' and '-0' formats with exit code 1, change '%' alone to extract single last frame instead of range, remove duplicate last frame from PNG sequence concat list, update Get-LastFrame.ps1 and Get-Frame.ps1 to clamp out-of-range frame indices instead of erroring and improve error messages with inline stderr output, add -y flag and -- separator to ffprobe calls, replace deprecated

Get-LastFrame.ps1

@@ -12,7 +12,8 @@
 .PARAMETER Frame
     0-based frame index to extract. Negative values count from the end:
     -1 extracts the last frame (default), -2 extracts second-to-last, etc.
-    The resulting frame index is clamped to the valid range (0 to total frames - 1).
+    The resulting frame index is clamped to the valid range (0 to total frames - 1): a positive value past the last frame
+    gives the last frame, and a negative value before the first frame gives frame 0.
 .DESCRIPTION
     Extracts a specific frame of a video file to a PNG image using ffmpeg.
     Supports negative frame indices to count from the end of the video.
@@ -36,14 +37,14 @@ $ErrorActionPreference = 'Stop'
 
 $VideoPath = $Video
 
-if (-not (Test-Path $VideoPath)) {
+if (-not (Test-Path -LiteralPath $VideoPath)) {
     Write-Error ('Video file not found: {0}' -f $VideoPath)
     exit 1
 }
 
 $PSNativeCommandUseErrorActionPreference = $false
 
-$totalFrames = & ffprobe -v error -select_streams v:0 -count_frames -show_entries stream=nb_read_frames -of csv=p=0 $VideoPath 2>&1 | Select-Object -First 1
+$totalFrames = & ffprobe -v error -select_streams v:0 -count_frames -show_entries stream=nb_read_frames -of csv=p=0 -- $VideoPath 2>&1 | Select-Object -First 1
 
 if (-not $totalFrames -or $totalFrames -notmatch '^\d+$') {
     Write-Error ('Could not determine frame count for: {0}' -f $VideoPath)
@@ -51,21 +52,15 @@ if (-not $totalFrames -or $totalFrames -notmatch '^\d+$') {
 }
 
 $totalFramesInt = [int]$totalFrames
+if ($totalFramesInt -lt 1) {
+    Write-Error ('Video has no frames: {0}' -f $VideoPath)
+    exit 1
+}
 $maxFrameIndex = $totalFramesInt - 1
 
-$frameToExtract = if ($Frame -lt 0) {
-    # Negative values count from the end
-    $calculatedFrame = $totalFramesInt + $Frame
-    # Clamp to valid range [0, maxFrameIndex]
-    [Math]::Max(0, [Math]::Min($calculatedFrame, $maxFrameIndex))
-} else {
-    # Positive values are direct indices
-    if ($Frame -gt $maxFrameIndex) {
-        Write-Error ('Frame index {0} is out of range. Valid range: 0 to {1}' -f $Frame, $maxFrameIndex)
-        exit 1
-    }
-    $Frame
-}
+# Negative values count from the end; the result is clamped to [0, maxFrameIndex]
+$requestedFrame = if ($Frame -lt 0) { $totalFramesInt + $Frame } else { $Frame }
+$frameToExtract = [Math]::Max(0, [Math]::Min($requestedFrame, $maxFrameIndex))
 
 Write-Host ('Total frames: {0} - extracting frame index {1}' -f $totalFramesInt, $frameToExtract)
 
@@ -98,7 +93,7 @@ if ($PSCmdlet.ShouldProcess($ImagePath, ('Extract frame {0} to PNG' -f $frameToE
         '-y'
         '-i', $VideoPath
         '-vf', $videoFilter
-        '-vsync', 'vfr'
+        '-fps_mode', 'vfr'
         '-frames:v', '1'
         '-q:v', '1'
         '--', $ImagePath
@@ -107,8 +102,12 @@ if ($PSCmdlet.ShouldProcess($ImagePath, ('Extract frame {0} to PNG' -f $frameToE
     $ffmpegStderr = & ffmpeg @ffmpegArgs 2>&1
 
     if ($LASTEXITCODE -ne 0) {
-        $ffmpegStderr | ForEach-Object { Write-Error $_ }
-        Write-Error 'ffmpeg failed to extract the last frame.'
+        Write-Error ('ffmpeg failed to extract frame {0}: {1}' -f $frameToExtract, (($ffmpegStderr | Out-String).Trim()))
+        exit 1
+    }
+
+    if (-not (Test-Path -LiteralPath $ImagePath)) {
+        Write-Error ('ffmpeg reported success but no image was written for frame {0}: {1}' -f $frameToExtract, $ImagePath)
         exit 1
     }