UWP中如何从CapturedPhoto获取子图像并保存摄像头拍摄图片区域

Great progress so far! To save just a specific region of your captured photo, we’ll extend your existing code to crop the image right after capture before saving it. Here’s a complete, working implementation with clear steps:

Core Workflow: #

• Wrap up the low-lag capture setup
• Extract the captured frame as a SoftwareBitmap
• Define your target crop area (adjust coordinates/size to your needs)
• Create a cropped version of the bitmap
• Encode and save the cropped image to a file

Full Updated Code: #

using Windows.Media.Capture;
using Windows.Media.MediaProperties;
using Windows.Storage;
using Windows.UI.Xaml.Media.Imaging;

private MediaCapture _mediaCapture;
private bool _isPreviewing = false;

// Initialize camera and start preview
public async Task InitializeCameraAsync()
{
    _mediaCapture = new MediaCapture();
    await _mediaCapture.InitializeAsync();
    await _mediaCapture.StartPreviewAsync();
    _isPreviewing = true;
}

// Capture photo, crop desired region, and save
public async void TakePhoto()
{
    if (_mediaCapture == null || !_isPreviewing)
        return;

    try
    {
        // Prepare low-lag photo capture with uncompressed BGRA8 format
        var lowLagCapture = await _mediaCapture.PrepareLowLagPhotoCaptureAsync(
            ImageEncodingProperties.CreateUncompressed(MediaPixelFormat.Bgra8));
        
        // Capture the photo frame
        var capturedPhoto = await lowLagCapture.CaptureAsync();
        
        // Extract the original bitmap from the captured frame
        using (var originalBitmap = capturedPhoto.Frame.SoftwareBitmap)
        {
            // Define your crop region (example: 200x200 square starting at (100, 100))
            // Note: Coordinates are relative to the original bitmap's width/height
            var cropBounds = new Windows.Foundation.Rect(100, 100, 200, 200);
            
            // Create the cropped bitmap
            using (var croppedBitmap = SoftwareBitmap.Crop(originalBitmap, cropBounds))
            {
                // Convert to BGRA8 with ignored alpha (required for most encoders)
                if (croppedBitmap.BitmapPixelFormat != BitmapPixelFormat.Bgra8 || 
                    croppedBitmap.BitmapAlphaMode != BitmapAlphaMode.Ignore)
                {
                    croppedBitmap = SoftwareBitmap.Convert(croppedBitmap, BitmapPixelFormat.Bgra8, BitmapAlphaMode.Ignore);
                }
                
                // Save the cropped image to your Pictures library
                await SaveCroppedImage(croppedBitmap);
            }
        }
        
        // Clean up capture resources
        await lowLagCapture.FinishAsync();
    }
    catch (Exception ex)
    {
        // Handle common errors (e.g., invalid crop bounds, camera access issues)
        System.Diagnostics.Debug.WriteLine($"Error: {ex.Message}");
    }
}

// Helper method to save the cropped bitmap to a file
private async Task SaveCroppedImage(SoftwareBitmap bitmap)
{
    // Create a unique file name in the Pictures library
    var saveFile = await KnownFolders.PicturesLibrary.CreateFileAsync(
        $"CroppedShot_{DateTime.Now:yyyyMMddHHmmss}.png", 
        CreationCollisionOption.GenerateUniqueName);
    
    // Encode and write the bitmap to the file
    using (var fileStream = await saveFile.OpenAsync(FileAccessMode.ReadWrite))
    {
        var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, fileStream);
        encoder.SetSoftwareBitmap(bitmap);
        await encoder.FlushAsync();
    }
}

Key Tips: #

• Adjust the Crop Region: Modify the cropBounds values to match your desired area. Make sure the rectangle stays within the original bitmap’s dimensions to avoid runtime errors.
• Resource Cleanup: The using statements ensure bitmaps and streams are properly disposed, preventing memory leaks.
• Change File Format: To save as JPEG instead of PNG, replace BitmapEncoder.PngEncoderId with BitmapEncoder.JpegEncoderId and update the file extension to .jpg.
• Permissions: Ensure your app has the webcam and picturesLibrary capabilities enabled in your Package.appxmanifest file.

内容的提问来源于stack exchange，提问作者Erik Bong