从JPEG解析图像大小

Question

我想知道在加载一个字节数组后是否有一种廉价的方法来获得JPEG的宽度和高度.

我知道JpegBitmapDecoder可以得到JPEG的像素宽度和高度,但它也加载了很多信息,我认为这将是一个昂贵的操作.

还有另一种方法可以从字节数组中获取宽度和高度而无需解码吗？

谢谢

Answer 1

由于某种未知的原因,我没有去睡觉,而是去做这件事.

这里有一些代码以最小的存储要求解决了这个问题.

void Main()
{
    var filePath=@"path\to\my.jpg";
    var bytes=File.ReadAllBytes(filePath);
    var dimensions=GetJpegDimensions(bytes);
    //or
    //var dimensions=GetJpegDimensions(filePath);
    Console.WriteLine(dimensions);
}
public static Dimensions GetJpegDimensions(byte[] bytes)
{
    using(var ms=new MemoryStream(bytes))
    {
        return GetJpegDimensions(ms);
    }
}
public static Dimensions GetJpegDimensions(string filePath)
{
    using(var fs=File.OpenRead(filePath))
    {
        return GetJpegDimensions(fs);
    }
}
public static Dimensions GetJpegDimensions(Stream fs)
{
    if(!fs.CanSeek) throw new ArgumentException("Stream must be seekable");
    long blockStart;
    var buf = new byte[4];
    fs.Read(buf, 0, 4);
    if(buf.SequenceEqual(new byte[]{0xff, 0xd8, 0xff, 0xe0}))
    {
        blockStart = fs.Position;
        fs.Read(buf, 0, 2);
        var blockLength = ((buf[0] << 8) + buf[1]);
        fs.Read(buf, 0, 4);
        if(Encoding.ASCII.GetString(buf, 0, 4) == "JFIF" 
            && fs.ReadByte() == 0)
        {
            blockStart += blockLength;
            while(blockStart < fs.Length)
            {
                fs.Position = blockStart;
                fs.Read(buf, 0, 4);
                blockLength = ((buf[2] << 8) + buf[3]);
                if(blockLength >= 7 && buf[0] == 0xff && buf[1] == 0xc0)
                {
                    fs.Position += 1;
                    fs.Read(buf, 0, 4);
                    var height = (buf[0] << 8) + buf[1];
                    var width = (buf[2] << 8) + buf[3];
                    return new Dimensions(width, height);
                }
                blockStart += blockLength + 2;
            }
        }
    }
    return null;
}

public class Dimensions
{
    private readonly int width;
    private readonly int height;
    public Dimensions(int width, int height)
    {
        this.width = width;
        this.height = height;
    }
    public int Width
    {
        get{return width;}
    }
    public int Height
    {
        get{return height;}
    }
    public override string ToString()
    {
        return string.Format("width:{0}, height:{1}", Width, Height);
    }
}