SkiaSharp与MAUI坐标系统理解及绘图异常问题求助

已知SkiaSharp矩形原点位于左上角，但实际使用中出现理解偏差，尝试官方教程后仍未解决。

问题场景 #

• 目标Bitmap尺寸为1080×1080：直接创建SKRect(0,0,1080,1080)作为目标区域时，画布绘图空白；但通过Maui.Rect(0,0,1080,1080)转换为SKRect却能正常工作。
• 调整源区域坐标测试：
  • 目标区域设为SKRect(100,100,980,980)（避免超出Bitmap范围），仍空白；
  • 源区域设为SKRect(100,100,150,150)（确认在源Bitmap范围内），同样空白。

补充：有评论提到坐标基于Image控件，但当前仅上传图片未显示，无法获取该控件。项目目标是掌握通过设置源矩形坐标提取图像任意指定区域的方法。

示例代码对比 #

正常显示的代码 #

SKBitmap destination = new SKBitmap(1080, 1080);
Rect destRec = new Rect(0,0,1080,1080);
SKRect dest = SkiaSharp.Views.Maui.Extensions.ToSKRect(destRec);
var height = sourceMap.Height;
var width = sourceMap.Width;
SKRect source = new SKRect(0,0,width,height);

异常的代码（源矩形宽高各加100，源图宽2000、高1332） #

SKBitmap destination = new SKBitmap(1080, 1080);
Rect destRec = new Rect(0,0,1080,1080);
SKRect dest = SkiaSharp.Views.Maui.Extensions.ToSKRect(destRec);
var height = sourceMap.Height;
var width = sourceMap.Width;
SKRect source = new SKRect(0,0,width+100,height+100);

附完整代码 #

View代码（XAML） #

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:toolkit="http://schemas.microsoft.com/dotnet/2022/maui/toolkit"
         xmlns:ImageCroppingTry2="clr-namespace:ImageCroppingTry2"
         x:DataType="ImageCroppingTry2:MainPageViewModel"
         x:Class="ImageCroppingTry2.MainPage">

<ContentPage.Resources>
    <ResourceDictionary>
        <toolkit:ByteArrayToImageSourceConverter x:Key="ByteArrayToImageSourceConverter" />
    </ResourceDictionary>
</ContentPage.Resources>

<VerticalStackLayout>
    <Image x:Name="displayImage" Source="{Binding Image, Converter={StaticResource ByteArrayToImageSourceConverter}}"/>
    <Button Command="{Binding UploadImageCommand}"></Button>
</VerticalStackLayout>

ViewModel代码 #

[ObservableProperty]
public byte[] image;

[RelayCommand]
public async void UploadImage()
{
    // 选择照片
    FileResult im = await MediaPicker.Default.PickPhotoAsync();

    SKBitmap sourceMap;
    using (Stream s = await im.OpenReadAsync())
    {
        using (MemoryStream ms = new MemoryStream())
        {
            await s.CopyToAsync(ms);
            ms.Seek(0, SeekOrigin.Begin);
            sourceMap = SKBitmap.Decode(ms);
        }
    }

    SKBitmap destination = new SKBitmap(1080, 1080);
    Rect destRec = new Rect(0,0,1080,1080);
    //SKRect dest = new SKRect(0,0 , 1080,1080);
    SKRect dest = SkiaSharp.Views.Maui.Extensions.ToSKRect(destRec);
    //Rect sourceRect = new Rect(0,0,destination.Width,destination.Height);
    SKRect source = new SKRect(0,0,150,150);
    //SKRect source = SkiaSharp.Views.Maui.Extensions.ToSKRect(sourceRect);
    using (SKCanvas canvas = new SKCanvas(destination))
    {
        canvas.Clear();
        canvas.DrawBitmap(sourceMap, source, dest);
    }

    SKImage skImage = SKImage.FromBitmap(destination);
    SKData encoded = skImage.Encode();
    using (MemoryStream memory = new MemoryStream())
    {
        encoded.AsStream().CopyTo(memory);
        Image = memory.ToArray();
    }
}

内容的提问来源于stack exchange，提问作者WoistdasNiveau