UltimateSlice

Program Monitor

The Program Monitor shows the assembled timeline played back in real time, clip by clip.

Canvas Aspect Ratio

The program monitor constrains its video display area to the project canvas ratio (e.g. 16:9 for a 1920×1080 project). This means:

  • If a source clip has a different aspect ratio than the canvas (e.g. a 21:9 wide-screen clip on a 16:9 canvas), the program monitor will show black letterbox bars above and below the clip — exactly matching what the exported video will look like.
  • If the canvas is wider than the clip (e.g. a 4:3 clip on a 16:9 canvas), black pillarbox bars appear on the sides.
  • The canvas ratio updates automatically when you change the project resolution in Project Settings.

This makes it much easier to judge clip placement, scale, and whether content is inside or outside the export frame.

Controls

Element Description
Video display Renders the assembled sequence at the playhead position
Timecode label Current timeline position
VU meter Peak audio levels (L/R) with color zones (Green < −18 dBFS, Yellow < −6 dBFS, Red ≥ −6 dBFS)
Volume slider Adjust master preview volume for the assembled timeline
Play / Pause button Toggle playback
Stop button Stop and return to position 0

Colour Scopes

Click the ▾ Scopes button below the monitor to reveal professional analysis tools:

  • Waveform — luminance levels across the frame.
  • Histogram — distribution of pixels by brightness.
  • RGB Parade — separate luminance channels for Red, Green, and Blue.
  • Vectorscope — color saturation and hue distribution.

Docked Resize

  • When the Program Monitor is docked, you can drag the splitter between the preview and scopes area to resize how much space each gets.
  • If scopes are hidden, the scopes pane is fully collapsed (the splitter/pane disappears).
  • The docked splitter position is saved and restored on next launch.

Transform Overlay

Select a video clip in the timeline to see its transform handles overlaid directly on the monitor.

  • Move: Drag the center to move the clip.
  • Precision Move: Use arrow keys (←/→/↑/↓) for ±0.01 offset, or Shift+Arrow for ±0.1.
  • Scale: Drag corner handles to scale proportionally. Hold Shift for non-proportional scaling.
  • Scale Shortcut: Press + or - to scale the selected clip.
  • Crop: Drag side handles (left/right/top/bottom) to crop the clip.
  • Audio clips do not display a transform overlay in the Program Monitor.

Keyboard Shortcuts

Shortcut Action
Space Play / Pause (when timeline has focus)
L Shuttle forward — press repeatedly to increase speed (1×, 2×, 4×, 8×)
J Shuttle reverse — press repeatedly to increase speed (−1×, −2×, −4×, −8×)
K Pause / stop shuttle
← / → Move selected clip position (with transform overlay active)
+ / - Scale selected clip (with transform overlay active)

Playback Behaviour

  • The program monitor uses a GStreamer compositor pipeline that layers all active video tracks simultaneously at the playhead position.
  • Each active clip gets its own decoder branch with per-clip effects, connected to the compositor with correct z-ordering (higher tracks render on top).
  • Audio from all active video clips is mixed through an audiomixer element; audio-only tracks use a separate playbin.
  • Timeline position is tracked via wall-clock timing for reliable playhead movement — no seek-anchor heuristics needed.
  • Audio boundaries are enforced via GStreamer seek stop positions, so audio stops precisely at the clip’s source out-point.
  • When clip boundaries are crossed during playback (a clip starts or ends), the pipeline is briefly rebuilt with the new set of active clips.
  • All per-clip effects (color, denoise, sharpness, crop, rotate, flip, scale, position, title overlay, speed) are applied per-slot during playback.
  • Scale/Position edits from the Inspector and transform overlay are applied to the active preview clip immediately in both paused and playing states.
  • If optional denoise filters are unavailable in your GStreamer runtime, Program Monitor still applies crop/scale/position transforms.
  • Program Monitor normalizes preview output to square pixels (PAR 1:1) so 21:9/ultra-wide sources don’t keep aspect-ratio bars after zoom scaling.
  • Playback priority can be set in Preferences → Playback (Smooth, Balanced, Accurate) to control smoothness vs seek precision.
  • Proxy preview mode can be enabled in Preferences → Playback to generate lightweight proxy files for smoother playback with large media. Export always uses original full-resolution media.
  • Preview quality (Full / Half / Quarter) downscales the composed monitor output while preserving full-frame fit/framing in the Program Monitor.
  • Preview quality Auto dynamically adjusts effective monitor output quality from the current Program Monitor canvas size (including resize/zoom changes) to balance clarity and performance.

Seeking

  • Click on the ruler in the timeline to seek the program monitor to that position.
  • The program monitor seeks to the correct source position within the appropriate clip, accounting for clip speed.
  • When scrubbing within the same clip, the existing decoder is seeked in-place (no pipeline rebuild) so the monitor shows the frame at the exact playhead position without a black-screen or first-frame flash.
  • When the playhead crosses a clip boundary (different clips become active), the pipeline is briefly rebuilt for the new set of active clips.
  • During paused scrubbing, UltimateSlice waits for a fresh post-seek preroll frame so the Program Monitor and transform overlay update to the new playhead frame instead of showing black.
  • During paused scrubbing, active clip decoder branches are created before preroll/seek settle so the monitor does not remain stuck on a black frame after moving the playhead.
  • Manual timeline seeks use the paused accurate-seek path and then resume playback if it was active, so the frame shown at the playhead is updated before playback continues.
  • While paused, the monitor is repainted continuously so delayed post-seek frame updates still appear without requiring playback to resume.

Playhead Accuracy

  • When you seek and then press Play, UltimateSlice rebuilds the compositor pipeline for the active clips at the playhead position and waits for post-seek preroll (up to ~2 seconds in paused accurate mode for long-GOP media) before transitioning back to Playing. This ensures playback starts from the correct frame rather than jumping to position 0.

Speed Change Preview

When a clip has a speed multiplier set (see Inspector), the program monitor plays it at that rate using GStreamer’s rate-seek mechanism. Audio pitch is not corrected in the preview (it sounds higher/lower pitched). The exported file uses atempo for proper pitch correction.

MCP Automation

  • play: starts program monitor playback.
  • pause: pauses program monitor playback.
  • stop: stops playback and returns to position 0.
  • seek_playhead: seeks the timeline/program-monitor playhead to an absolute nanosecond position.
  • export_displayed_frame: exports the current displayed frame to a binary PPM (P6) image file.
  • take_screenshot: captures a PNG screenshot of the full application window. Returns the path to the saved file.