Screen Capture Configuration

Configure Windows Graphics Capture (WGC), copy pools, frame queuing, and MMCSS settings

Overview

The host.capture section configures Windows Graphics Capture (WGC) for screen capture, including GPU texture copying, frame queuing, adaptive backoff, and thread priority optimization.

Texture Copy Pool

GPU texture copy operations require pre-allocated D3D11 resources.

capture.copyPoolSize · integer · default: "4"

Number of D3D11 texture copy resources in the pool.

Default: 4

Higher values allow more parallel copy operations but use more GPU memory.

Recommended:

• 4 - Standard (default)
• 6-8 - High frame rate (120+ FPS)
• 2-3 - Memory constrained systems

Increase this if you see frame drops at high frame rates.

capture.framePoolBuffers · integer · default: "2"

Number of frame buffers for WGC capture.

Default: 2

Minimum of 2 required for double-buffering. Higher values add latency.

Frame Queue Management

capture.maxQueueDepth · integer · default: "2"

Maximum frames queued for encoding.

Default: 2

Lower values reduce latency but may cause frame drops if encoder is slow.

Recommended:

• 1 - Absolute minimum latency (may drop frames)
• 2 - Low latency (default)
• 3-4 - More stable, slightly higher latency

Values above 3 significantly increase latency. Only use for debugging.

capture.dropWindowMs · integer · default: "500"

Time window in milliseconds for frame drop detection.

Default: 500 ms

Used to detect sustained frame dropping vs. occasional drops.

capture.dropMinEvents · integer · default: "12"

Minimum frame drops within window to trigger warnings.

Default: 12

Prevents spurious warnings from occasional frame drops.

Cursor and Border

capture.cursor · boolean · default: "false"

Render cursor into captured frames.

Default: false

When disabled, the client renders its own cursor for lower latency.

Keep disabled for better cursor responsiveness. Enable only if client-side cursor has issues.

capture.borderRequired · boolean · default: "false"

Require window border for capture.

Default: false

When enabled, WGC only captures windows with borders (excludes fullscreen exclusive).

Frame Change Detection

capture.skipUnchanged · boolean · default: "true"

Skip encoding frames identical to previous frame.

Default: true

Saves bandwidth and GPU cycles when screen content is static.

Keep enabled unless you need constant frame output for testing.

capture.minUpdateInterval100ns · integer · default: "166666"

Minimum frame capture interval in 100-nanosecond units.

Default: 166666 (16.67 ms = 60 FPS)

Controls the maximum capture rate. For 60 FPS: 1000000 / 60 * 10 = 166666

Common values:

• 83333 - 120 FPS
• 166666 - 60 FPS (default)
• 333333 - 30 FPS

Adaptive Backoff

Dynamically reduce capture rate when encoder falls behind.

capture.adaptiveBackoff · boolean · default: "false"

Enable adaptive capture rate backoff.

Default: false

When enabled, automatically reduces capture rate if encoder cannot keep up.

This adds complexity and may cause stuttering. Only enable if experiencing consistent encoder saturation.

capture.adaptiveWindowMs · integer · default: "2000"

Time window for adaptive backoff measurements.

Default: 2000 ms

Longer windows provide more stable adaptation but slower response.

capture.adaptiveEagainThreshold · integer · default: "8"

Number of EAGAIN (encoder busy) errors to trigger backoff.

Default: 8

Higher values tolerate more encoder saturation before reducing capture rate.

Thread Priority (MMCSS)

Multimedia Class Scheduler Service (MMCSS) provides real-time thread priority for capture.

capture.mmcss.enable · boolean · default: "true"

Enable MMCSS for capture thread.

Default: true

Boosts capture thread priority to reduce jitter and frame timing variance.

Keep enabled for consistent frame timing and lowest latency.

capture.mmcss.priority · integer · default: "4"

MMCSS task priority level.

Default: 4

Range: 1 (lowest) to 8 (highest)

Higher values give more CPU priority but may impact other system tasks.

Recommended:

• 3-4 - Standard priority (default)
• 5-6 - High priority (gaming)
• 7-8 - Maximum priority (use with caution)

Windows Graphics Capture (WGC)

WGC is the modern Windows 10+ screen capture API. Configuration is primarily done through the parameters above.

How WGC Works

1. Capture: WGC provides GPU texture from compositor
2. Copy: Texture copied to staging resource from copyPoolSize pool
3. Queue: Frame queued for encoding (up to maxQueueDepth)
4. Encode: FFmpeg/NVENC encodes the frame
5. Stream: Encoded frame sent over WebRTC

WGC Requirements

• Windows 10 version 1903 or later
• DirectX 11 compatible GPU
• Target window must be visible (minimized windows cannot be captured)

WGC cannot capture UAC prompts, secure desktop, or some protected content (DRM).

Performance Optimization

Low Latency Configuration

Minimize every source of latency:

{
  "capture": {
    "copyPoolSize": 3,
    "maxQueueDepth": 1,
    "framePoolBuffers": 2,
    "cursor": false,
    "skipUnchanged": true,
    "minUpdateInterval100ns": 83333,
    "adaptiveBackoff": false,
    "mmcss": {
      "enable": true,
      "priority": 6
    }
  }
}

Setting maxQueueDepth to 1 provides minimum latency but may drop frames if encoding cannot maintain target frame rate.

High Frame Rate Configuration

For 120+ FPS capture:

{
  "capture": {
    "copyPoolSize": 8,
    "maxQueueDepth": 2,
    "framePoolBuffers": 3,
    "cursor": false,
    "skipUnchanged": true,
    "minUpdateInterval100ns": 83333,
    "mmcss": {
      "enable": true,
      "priority": 7
    }
  }
}

Stable Configuration

For consistent performance with minimal frame drops:

{
  "capture": {
    "copyPoolSize": 6,
    "maxQueueDepth": 3,
    "framePoolBuffers": 2,
    "cursor": false,
    "skipUnchanged": true,
    "adaptiveBackoff": true,
    "adaptiveWindowMs": 1000,
    "mmcss": {
      "enable": true,
      "priority": 4
    }
  }
}

Troubleshooting

Frame Drops

Symptoms: Frequent frame drops, stuttering

Solutions:

1. Increase copyPoolSize to 6-8
2. Increase maxQueueDepth to 3-4
3. Enable adaptiveBackoff
4. Reduce target fps in video configuration
5. Use faster NVENC preset (p1 or p2)

High Latency

Symptoms: Delayed response to inputs

Solutions:

1. Reduce maxQueueDepth to 1-2
2. Disable adaptiveBackoff
3. Set cursor to false
4. Ensure mmcss.enable is true
5. Increase mmcss.priority to 6-7

GPU Memory Issues

Symptoms: Crashes, D3D11 errors

Solutions:

1. Reduce copyPoolSize to 2-3
2. Reduce framePoolBuffers to 2
3. Lower capture resolution
4. Check GPU memory usage with Task Manager

CPU Usage

Symptoms: High CPU usage in capture thread

Solutions:

1. Enable skipUnchanged
2. Use hardware encoding (NVENC/QSV/AMF)
3. Increase minUpdateInterval100ns to reduce frame rate
4. Disable gpuTiming if enabled

Example: Gaming Streaming Profile

{
  "capture": {
    "copyPoolSize": 6,
    "maxQueueDepth": 2,
    "framePoolBuffers": 2,
    "dropWindowMs": 500,
    "dropMinEvents": 12,
    "mmcss": {
      "enable": true,
      "priority": 6
    },
    "cursor": false,
    "borderRequired": false,
    "minUpdateInterval100ns": 83333,
    "skipUnchanged": true,
    "adaptiveBackoff": false
  }
}

This configuration provides:

• 120 FPS capture capability
• Low latency (2 frame queue depth)
• High thread priority
• Efficient bandwidth usage (skip unchanged frames)
• Sufficient copy pool for smooth operation