基于VB/C#的文档扫描字节数组压缩方案咨询

当前项目未考虑扫描仪品牌/类型，单页原始图像转字节数组后大小超过3MB。不想使用付费的LEADTOOLS SDK（功能冗余、成本高，且不愿大幅修改现有代码），现有图像转字节数组的VB代码如下：

Private Shared Function ConvertImageToByteArray(ByVal imageToConvert As System.Drawing.Image, ByVal formatOfImage As Drawing.Imaging.ImageFormat) As Byte()
    Dim Ret As Byte()
    Using ms As New IO.MemoryStream()
        imageToConvert.Save(ms, formatOfImage)
        Ret = ms.ToArray()
    End Using
    Return Ret
End Function

编辑：2022/7/27 - 2:48 - 修正拼写

以下几种方法无需引入付费SDK，仅需对现有代码做小幅修改，即可显著降低字节数组体积：

1. 改用高压缩率格式并控制质量 #

现有代码直接使用传入的ImageFormat，若当前用的是BMP、PNG这类无压缩/低压缩格式，换成带质量参数的JPEG或黑白专用的TIFF压缩是最直接的优化方式：

VB代码示例（JPEG带质量控制） #

Private Shared Function ConvertImageToByteArray(ByVal imageToConvert As System.Drawing.Image, Optional ByVal quality As Integer = 70) As Byte()
    Dim Ret As Byte()
    Using ms As New IO.MemoryStream()
        ' 获取JPEG编码器
        Dim jpegCodec As Imaging.ImageCodecInfo = Imaging.ImageCodecInfo.GetImageEncoders().FirstOrDefault(Function(c) c.FormatID = Imaging.ImageFormat.Jpeg.Guid)
        If jpegCodec IsNot Nothing Then
            Dim encoderParams As New Imaging.EncoderParameters(1)
            ' 质量范围0-100，值越小压缩率越高（70是兼顾质量和体积的常用值）
            encoderParams.Param(0) = New Imaging.EncoderParameter(Imaging.Encoder.Quality, quality)
            imageToConvert.Save(ms, jpegCodec, encoderParams)
        Else
            ' 编码器获取失败时 fallback 默认保存
            imageToConvert.Save(ms, Imaging.ImageFormat.Jpeg)
        End If
        Ret = ms.ToArray()
    End Using
    Return Ret
End Function

TIFF黑白压缩（适合文字类扫描件） #

如果是黑白文档扫描件，使用TIFF的CCITT Group 4压缩能达到极高的压缩比，且几乎不损失文字可读性：

Private Shared Function ConvertImageToByteArrayTiff(ByVal imageToConvert As System.Drawing.Image) As Byte()
    Dim Ret As Byte()
    Using ms As New IO.MemoryStream()
        Dim tiffCodec As Imaging.ImageCodecInfo = Imaging.ImageCodecInfo.GetImageEncoders().FirstOrDefault(Function(c) c.FormatID = Imaging.ImageFormat.Tiff.Guid)
        If tiffCodec IsNot Nothing Then
            Dim encoderParams As New Imaging.EncoderParameters(2)
            ' 设置压缩方式为CCITT Group 4
            encoderParams.Param(0) = New Imaging.EncoderParameter(Imaging.Encoder.Compression, Imaging.EncoderValue.CompressionCCITT4)
            ' 转换为1位黑白图像
            encoderParams.Param(1) = New Imaging.EncoderParameter(Imaging.Encoder.ColorDepth, 1L)
            imageToConvert.Save(ms, tiffCodec, encoderParams)
        Else
            imageToConvert.Save(ms, Imaging.ImageFormat.Tiff)
        End If
        Ret = ms.ToArray()
    End Using
    Return Ret
End Function

2. 降低图像分辨率 #

扫描文档通常默认300DPI以上，若无需高精度打印/识别，可将分辨率降至150DPI，同时按比例缩小图像尺寸，进一步减小体积：

Private Shared Function ResizeImageAndConvertToByteArray(ByVal imageToConvert As System.Drawing.Image, ByVal targetDpi As Integer = 150, ByVal quality As Integer = 70) As Byte()
    ' 计算缩放比例
    Dim scaleRatio As Double = targetDpi / imageToConvert.HorizontalResolution
    Dim newWidth As Integer = CInt(imageToConvert.Width * scaleRatio)
    Dim newHeight As Integer = CInt(imageToConvert.Height * scaleRatio)
    
    ' 创建缩小后的图像并保持目标分辨率
    Using resizedImage As New Bitmap(newWidth, newHeight)
        resizedImage.SetResolution(targetDpi, targetDpi)
        Using g As Graphics = Graphics.FromImage(resizedImage)
            ' 保证缩放后图像质量
            g.InterpolationMode = Drawing2D.InterpolationMode.HighQualityBicubic
            g.DrawImage(imageToConvert, 0, 0, newWidth, newHeight)
        End Using
        ' 调用JPEG压缩逻辑
        Return ConvertImageToByteArray(resizedImage, quality)
    End Using
End Function

3. 转换为灰度图像 #

文档类图像以文字为主，转换为灰度图像可减少色彩通道数量，配合压缩格式能进一步降低体积：

Private Shared Function ConvertToGrayscaleAndSave(ByVal imageToConvert As System.Drawing.Image, ByVal quality As Integer = 70) As Byte()
    Using grayBitmap As New Bitmap(imageToConvert.Width, imageToConvert.Height)
        Using g As Graphics = Graphics.FromImage(grayBitmap)
            ' 灰度转换矩阵
            Dim grayMatrix As New Imaging.ColorMatrix(New Single()() {
                New Single() {0.299F, 0.299F, 0.299F, 0, 0},
                New Single() {0.587F, 0.587F, 0.587F, 0, 0},
                New Single() {0.114F, 0.114F, 0.114F, 0, 0},
                New Single() {0, 0, 0, 1, 0},
                New Single() {0, 0, 0, 0, 1}
            })
            Dim imageAttr As New Imaging.ImageAttributes()
            imageAttr.SetColorMatrix(grayMatrix)
            ' 绘制灰度图像
            g.DrawImage(imageToConvert, New Rectangle(0, 0, imageToConvert.Width, imageToConvert.Height),
                        0, 0, imageToConvert.Width, imageToConvert.Height, GraphicsUnit.Pixel, imageAttr)
        End Using
        ' 调用压缩保存逻辑
        Return ConvertImageToByteArray(grayBitmap, quality)
    End Using
End Function

内容的提问来源于stack exchange，提问作者Jesus J. Nieves-Padilla