Hey there! I've run into similar DirectShow performance quirks on low-spec hardware before, so let's break down what's happening and how timed frame requests can fix it.

What's Causing the FPS & CPU Shift? #

Your initial 40% CPU usage makes sense—DirectShow is pushing frames at full speed to your preview. After 30 minutes, though, two common culprits are at play:

• Windows power-saving features throttle the CPU to save energy (super common on budget PCs), or
• DirectShow's default push-mode filters automatically reduce frame rate to lower sustained CPU load.

Since other DirectShowLib examples have the same issue, this is definitely an environment/behavior problem, not a bug in your code.

Can Timed Frame Requests Fix This? #

Absolutely! Switching from DirectShow's default push mode (filters send frames to your app automatically) to pull mode (your app actively requests frames on a schedule) lets you take control. This prevents the system from throttling performance because you're explicitly telling it how often to process frames.

Step-by-Step Implementation #

Here's how to set this up in your WinForms app:

1. Use ISampleGrabber for Frame Caching #

First, create a callback class to cache incoming frames so your timer can retrieve them later:

public class FrameCacheCallback : ISampleGrabberCB
{
    private byte[] _latestFrame;
    private readonly object _lockObj = new object();

    // Called when a new frame is pushed to the grabber
    public int BufferCB(double sampleTime, IntPtr pBuffer, int bufferLen)
    {
        lock (_lockObj)
        {
            if (_latestFrame == null || _latestFrame.Length != bufferLen)
                _latestFrame = new byte[bufferLen];
            
            Marshal.Copy(pBuffer, _latestFrame, 0, bufferLen);
        }
        return 0;
    }

    // Unused for our pull-mode setup
    public int SampleCB(double sampleTime, IMediaSample pSample) => 0;

    // Retrieve the latest cached frame safely
    public byte[] GetLatestFrame()
    {
        lock (_lockObj)
        {
            return _latestFrame?.Clone() as byte[];
        }
    }
}

2. Initialize the DirectShow Graph #

Set up your capture device, sample grabber, and connect them in the graph:

// Initialize core DirectShow objects
var graphBuilder = (IGraphBuilder)new FilterGraph();
var mediaControl = (IMediaControl)graphBuilder;
var sampleGrabber = new SampleGrabber();
var frameCallback = new FrameCacheCallback();

// Configure sample grabber for RGB24 video capture
var mediaType = new AMMediaType
{
    majorType = MediaType.Video,
    subType = MediaSubType.RGB24,
    formatType = FormatType.VideoInfo
};
sampleGrabber.SetMediaType(mediaType);
sampleGrabber.SetCallback(frameCallback, 1); // Enable buffer callbacks

// Add your capture device filter (replace with your device setup logic)
var captureFilter = GetYourCaptureDeviceFilter(); 
graphBuilder.AddFilter(captureFilter, "Video Capture");
graphBuilder.AddFilter(sampleGrabber, "Frame Grabber");

// Connect capture device output to sample grabber input
var captureOutputPin = GetPin(captureFilter, PinDirection.Output);
var grabberInputPin = GetPin(sampleGrabber, PinDirection.Input);
graphBuilder.Connect(captureOutputPin, grabberInputPin);

// Helper to get a filter's pin by direction
IPin GetPin(IBaseFilter filter, PinDirection direction)
{
    IEnumPins enumPins;
    filter.EnumPins(out enumPins);
    IPin[] pins = new IPin[1];
    while (enumPins.Next(1, pins, IntPtr.Zero) == 0)
    {
        PinInfo pinInfo;
        pins[0].QueryPinInfo(out pinInfo);
        if (pinInfo.dir == direction)
            return pins[0];
    }
    return null;
}

3. Add a Timer to Request Frames #

Use a WinForms Timer (adjust interval for your desired FPS) to pull frames and update your preview:

// Set timer for ~30 FPS (adjust interval as needed)
var frameTimer = new System.Windows.Forms.Timer
{
    Interval = 33 // 1000ms / 30 ≈ 33ms
};

frameTimer.Tick += (sender, e) =>
{
    var frameData = frameCallback.GetLatestFrame();
    if (frameData == null) return;

    // Get video resolution from the grabber's media type
    var mediaType = sampleGrabber.GetConnectedMediaType();
    var videoInfo = (VideoInfoHeader)Marshal.PtrToStructure(mediaType.pbFormat, typeof(VideoInfoHeader));
    
    // Convert frame data to Bitmap and update PictureBox
    using (var bitmap = new Bitmap(
        videoInfo.BmiHeader.biWidth,
        videoInfo.BmiHeader.biHeight,
        videoInfo.BmiHeader.biWidth * 3, // RGB24 = 3 bytes per pixel
        PixelFormat.Format24bppRgb,
        Marshal.UnsafeAddrOfPinnedArrayElement(frameData, 0)))
    {
        // Update UI safely (WinForms requires thread access)
        if (pictureBoxPreview.InvokeRequired)
        {
            pictureBoxPreview.Invoke(new Action(() => 
                pictureBoxPreview.Image = new Bitmap(bitmap)));
        }
        else
        {
            pictureBoxPreview.Image = new Bitmap(bitmap);
        }
    }
};

// Start the timer and media capture
frameTimer.Start();
mediaControl.Run();

4. Critical Cleanup #

Don't forget to release resources when closing your app to avoid leaks:

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    frameTimer.Stop();
    mediaControl.Stop();
    
    // Release all COM objects
    Marshal.ReleaseComObject(sampleGrabber);
    Marshal.ReleaseComObject(captureFilter);
    Marshal.ReleaseComObject(graphBuilder);
}

Extra Tips for Low-End Hardware #

• Adjust Power Plan: Switch to "High Performance" mode to prevent CPU throttling.
• Use Hardware Renderers: Replace DirectShow's default renderer with VMR-9 or EVR to offload work to the GPU.
• Lower Resolution: Reduce capture resolution (e.g., 640x480) if FPS is still an issue to cut CPU load.

内容的提问来源于stack exchange，提问作者dp2050