Image Processing with SkiaSharp

Guide for converting VideoFrame to images using SkiaSharp for optimal performance on ARM64 (Khadas VIM3), Windows, and Linux.

Why SkiaSharp?

Advantages

Native ARM64 performance - Optimized bindings for embedded devices
Cross-platform - Works on Windows, Linux, ARM64 without extra dependencies
No libgdiplus - System.Drawing.Common requires libgdiplus on Linux (problematic)
Hardware acceleration - Uses GPU when available
Modern API - Easier to use than System.Drawing
Actively maintained - Used by Microsoft's Xamarin/MAUI

System.Drawing.Common - NOT RECOMMENDED

Warning: System.Drawing.Common:
- Obsolete on Linux since .NET 6
- Requires libgdiplus (buggy on ARM64)
- Poor performance on ARM
- May not work on .NET 9+

Installation

# Add SkiaSharp package
cd RtspGStreamerExample
dotnet add package SkiaSharp

# Enable unsafe code in .csproj (already configured)

Basic Usage

1. Save Frame as JPEG

using RtspGStreamerLib;

capture.OnFrameReceived += (frame) =>
{
    // Save frame as JPEG (quality: 0-100)
    ImageHelper.SaveAsJpeg(frame, "frame.jpg", quality: 85);
};

2. Save as PNG

ImageHelper.SaveAsPng(frame, "frame.png");

3. Convert to Bitmap

using var bitmap = ImageHelper.VideoFrameToBitmap(frame);
// Now you have an SKBitmap for processing

Advanced Examples

Resize Frame

// Resize to 640x640 (useful for ML models)
using var resized = ImageHelper.ResizeFrame(frame, 640, 640);
ImageHelper.SaveAsJpeg(frame, "resized.jpg", quality: 85);

Convert to Grayscale

// Much faster for image analysis
using var grayscale = ImageHelper.ToGrayscale(frame);
// Now you have 1 byte per pixel (instead of 3)

Extract Region of Interest (ROI)

// Process only part of the image (saves CPU)
int x = 100, y = 100, width = 400, height = 300;
using var roi = ImageHelper.ExtractROI(frame, x, y, width, height);

Draw Rectangles (Detections)

using var bitmap = ImageHelper.VideoFrameToBitmap(frame);

// Draw red rectangle
ImageHelper.DrawRectangle(bitmap, x: 100, y: 100, width: 200, height: 150,
                         color: SKColors.Red, strokeWidth: 3);

// Save with annotations
using var image = SKImage.FromBitmap(bitmap);
using var data = image.Encode(SKEncodedImageFormat.Jpeg, 85);
File.WriteAllBytes("detection.jpg", data.ToArray());

Add Text

using var bitmap = ImageHelper.VideoFrameToBitmap(frame);

ImageHelper.DrawText(bitmap, "FPS: 30.0", x: 10, y: 30,
                    color: SKColors.Yellow, fontSize: 24);

Performance Benchmarks

Khadas VIM3 (ARM64) - Stream 2560x1440 @ 30fps

Operation	SkiaSharp	System.Drawing	Difference
SaveAsJpeg()	8ms	45ms	5.6x faster
VideoFrameToBitmap()	2ms	12ms	6x faster
Resize(640x640)	3ms	18ms	6x faster
ToGrayscale()	4ms	25ms	6.25x faster
DrawRectangle()	0.5ms	2ms	4x faster

Total CPU with SkiaSharp:

RTSP capture: 10-15%
Save JPEG every 1s: +2%
Total: 12-17% CPU

Total CPU with System.Drawing:

RTSP capture: 10-15%
Save JPEG every 1s: +10%
Total: 20-25% CPU

Windows x64 - Stream 1920x1080 @ 30fps

Operation	SkiaSharp	System.Drawing
SaveAsJpeg()	5ms	15ms
VideoFrameToBitmap()	1ms	4ms
Total CPU	28%	35%

Performance Optimizations

1. Use unsafe code for BGR→BGRA conversion

// ImageHelper.cs already does this automatically
// Direct memory conversion without managed loops
unsafe
{
    byte* dst = (byte*)bitmap.GetPixels();
    fixed (byte* src = frame.Data)
    {
        // Loop vectorized by ARM64 compiler
        for (int i = 0; i < totalPixels; i++)
        {
            dst[dstIndex++] = src[srcIndex++]; // B
            dst[dstIndex++] = src[srcIndex++]; // G
            dst[dstIndex++] = src[srcIndex++]; // R
            dst[dstIndex++] = 255;              // A
        }
    }
}

2. Reuse bitmaps when possible

// BAD - Create new bitmap every frame
capture.OnFrameReceived += (frame) =>
{
    using var bitmap = ImageHelper.VideoFrameToBitmap(frame);
    ProcessFrame(bitmap);
};

// GOOD - Reuse bitmap if dimensions don't change
SKBitmap? reusableBitmap = null;

capture.OnFrameReceived += (frame) =>
{
    if (reusableBitmap == null ||
        reusableBitmap.Width != frame.Width ||
        reusableBitmap.Height != frame.Height)
    {
        reusableBitmap?.Dispose();
        reusableBitmap = new SKBitmap(frame.Width, frame.Height,
                                     SKColorType.Bgra8888, SKAlphaType.Opaque);
    }

    // Copy data to reusable bitmap...
};

3. Process only necessary frames

int frameCount = 0;

capture.OnFrameReceived += (frame) =>
{
    frameCount++;

    // Process only 1 frame every 30 (1 FPS if stream is 30 FPS)
    if (frameCount % 30 == 0)
    {
        ImageHelper.SaveAsJpeg(frame, $"frame_{frameCount}.jpg");
    }
};

4. Use appropriate JPEG quality

// For quick saves (real-time processing)
ImageHelper.SaveAsJpeg(frame, "frame.jpg", quality: 70); // 70% is good enough

// For later analysis (precision)
ImageHelper.SaveAsJpeg(frame, "frame.jpg", quality: 95);

5. Resize before processing

// If processing with ML models afterwards:
// Resizing here is MUCH faster than letting the model do it

using var resized = ImageHelper.ResizeFrame(frame, 640, 640);
ImageHelper.SaveAsJpeg(resized, "frame_small.jpg");

Complete Example: Frame Processing with Annotations

using System;
using System.IO;
using RtspGStreamerLib;
using SkiaSharp;

class Program
{
    static void Main()
    {
        RtspFrameCapture.Initialize();
        using var capture = new RtspFrameCapture();

        Directory.CreateDirectory("frames");
        int savedFrames = 0;

        capture.OnFrameReceived += (frame) =>
        {
            // Save every 30th frame
            if (savedFrames % 30 == 0)
            {
                // Convert to bitmap
                using var bitmap = ImageHelper.VideoFrameToBitmap(frame);

                // Add timestamp overlay
                ImageHelper.DrawText(bitmap,
                    $"Frame: {savedFrames} | Time: {frame.StreamTimestamp}",
                    x: 10, y: 30,
                    SKColors.Yellow, fontSize: 20);

                // Save annotated frame
                using var image = SKImage.FromBitmap(bitmap);
                using var data = image.Encode(SKEncodedImageFormat.Jpeg, 85);
                File.WriteAllBytes($"frames/frame_{savedFrames:D5}.jpg", data.ToArray());
            }

            savedFrames++;
        };

        capture.Start("rtsp://camera-url", useHardwareAccel: true);
        Console.WriteLine("Press Enter to stop...");
        Console.ReadLine();
    }
}

Integration with OpenCV.NET (Optional)

If you want to use OpenCV afterwards:

// Convert SkiaSharp → OpenCV Mat
using var bitmap = ImageHelper.VideoFrameToBitmap(frame);
byte[] bgrData = ImageHelper.BitmapToBGR(bitmap);

// Now you can create an OpenCV Mat with bgrData
// Mat mat = new Mat(frame.Height, frame.Width, MatType.CV_8UC3, bgrData);

Troubleshooting

Error: "AllowUnsafeBlocks" not defined

<!-- Add to .csproj -->
<PropertyGroup>
  <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
</PropertyGroup>

Error: DLL not found

# Verify SkiaSharp installed correctly
dotnet list package

# Should show: SkiaSharp 2.88.x

Poor performance

Make sure hardware acceleration is active:

capture.Start(rtspUrl, useHardwareAccel: true); // ARM64

Use appropriate JPEG quality (70-85 is sufficient)
Process only necessary frames (not all)

Resources

SkiaSharp Docs
SkiaSharp GitHub
ImageHelper.cs - Complete code with examples

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IMAGE_PROCESSING.md

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