使用LibTiff.net保存JPG压缩TIF时图像倾斜问题求助

问题场景 #

我使用以下VB.NET代码将图像保存为JPG压缩的TIF格式：

主代码 #

Using memStream As New IO.MemoryStream()
    Using output As BitMiracle.LibTiff.Classic.Tiff = BitMiracle.LibTiff.Classic.Tiff.ClientOpen("in-memory", "w", memStream, New BitMiracle.LibTiff.Classic.TiffStream())
        Dim page As Integer = 0
        Dim pages As Integer = InputImages.Count

        For Each inImage As Drawing.Image In InputImages
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.IMAGEWIDTH, inImage.Width)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.IMAGELENGTH, inImage.Height)

            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.COMPRESSION, BitMiracle.LibTiff.Classic.Compression.JPEG)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.PHOTOMETRIC, BitMiracle.LibTiff.Classic.Photometric.YCBCR)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.SUBFILETYPE, BitMiracle.LibTiff.Classic.FileType.PAGE)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.ROWSPERSTRIP, inImage.Height)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.ORIENTATION, BitMiracle.LibTiff.Classic.Orientation.TOPLEFT)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.XRESOLUTION, inImage.HorizontalResolution)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.YRESOLUTION, inImage.VerticalResolution)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.RESOLUTIONUNIT, BitMiracle.LibTiff.Classic.ResUnit.INCH)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.BITSPERSAMPLE, 8)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.SAMPLESPERPIXEL, 3)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.PLANARCONFIG, BitMiracle.LibTiff.Classic.PlanarConfig.CONTIG)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.YCBCRPOSITIONING, 1)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.YCBCRSUBSAMPLING, 1, 1)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.JPEGTABLESMODE, BitMiracle.LibTiff.Classic.JpegTablesMode.NONE)
            output.SetField(BitMiracle.LibTiff.Classic.TiffTag.PAGENUMBER, page, pages)

            Dim bytes As Byte() = GetImageRasterBytes(CType(inImage, Bitmap), PixelFormat.Format24bppRgb)
            convertSamples(bytes, inImage.Width, inImage.Height)
            output.WriteEncodedStrip(0, bytes, bytes.Length)
            page += 1
            output.WriteDirectory()
        Next

        output.Close()
        Dim byteArray() As Byte = memStream.ToArray()
        Return byteArray
    End Using
End Using

辅助函数 #

Private Function GetImageRasterBytes(ByVal bmp As Bitmap, ByVal format As PixelFormat) As Byte()
    Dim rect = New Rectangle(0, 0, bmp.Width, bmp.Height)
    Dim bits As Byte()

    Try
        Dim bmpdata = bmp.LockBits(rect, ImageLockMode.ReadWrite, format)
        bits = New Byte(bmpdata.Stride * bmpdata.Height - 1) {}
        System.Runtime.InteropServices.Marshal.Copy(bmpdata.Scan0, bits, 0, bits.Length)
        bmp.UnlockBits(bmpdata)
    Catch
        Return Nothing
    End Try

    Return bits
End Function

Private Sub convertSamples(ByVal data As Byte(), ByVal width As Integer, ByVal height As Integer)
    Dim stride As Integer = data.Length \ height
    Const samplesPerPixel As Integer = 3

    For y As Integer = 0 To height - 1
        Dim offset As Integer = stride * y
        Dim strideEnd As Integer = offset + width * samplesPerPixel

        Dim i As Integer = offset
        While i < strideEnd
            Dim temp As Byte = data(i + 2)
            data(i + 2) = data(i)
            data(i) = temp
            i += samplesPerPixel
        End While
    Next
End Sub

运行后发现：1040x800的图像生成正常，但1035x800的图像会出现倾斜，同时控制台输出警告：in-memory: Warning, fractional scanline discarded。

问题原因 #

• Bitmap的Stride对齐机制：Windows GDI+的Bitmap会将每一行的字节数对齐到4的倍数（内存对齐优化）。对于24bpp格式（每个像素占3字节）：
  • 1040×3=3120，刚好是4的倍数，因此Stride等于实际像素行的字节数，数据无冗余。
  • 1035×3=3105，不是4的倍数，GDI+会自动填充3个字节，将Stride调整为3108（4的最小倍数），此时每行数据包含3105个有效像素字节+3个冗余填充字节。
• 代码中convertSamples函数仅处理了每行的有效像素部分，但保留了冗余填充字节。LibTiff按IMAGEWIDTH设置的1035来解析每行数据，导致冗余字节被误判为下一行的起始数据，最终出现图像倾斜。同时LibTiff检测到数据总长度与预期的扫描行长度不匹配，输出fractional scanline discarded警告。

修复方案 #

方案一：裁剪冗余字节，传递纯像素数据 #

修改辅助函数，处理后移除每行的冗余填充字节，生成仅包含有效像素的数组：

Private Sub convertSamplesAndTrimStride(ByRef data As Byte(), ByVal width As Integer, ByVal height As Integer)
    Dim originalStride As Integer = data.Length \ height
    Dim pixelRowLength As Integer = width * 3 ' 24bpp每个像素3字节
    Dim trimmedData As New List(Of Byte)()

    For y As Integer = 0 To height - 1
        Dim offset As Integer = originalStride * y
        ' 转换RGB到BGR
        Dim i As Integer = offset
        While i < offset + pixelRowLength
            Dim temp As Byte = data(i + 2)
            data(i + 2) = data(i)
            data(i) = temp
            i += 3
        End While
        ' 提取有效行数据，丢弃冗余字节
        trimmedData.AddRange(data.Skip(offset).Take(pixelRowLength))
    Next

    ' 替换原数据为裁剪后的纯像素数组
    data = trimmedData.ToArray()
End Sub

将主代码中的convertSamples(bytes, inImage.Width, inImage.Height)替换为convertSamplesAndTrimStride(bytes, inImage.Width, inImage.Height)即可。

方案二：配置LibTiff识别实际Stride #

在设置TIF标签时，添加STRIPBYTECOUNTS标签，告知LibTiff每个Strip的实际字节数（即Stride×Height）：

' 在获取bytes数组后、转换像素前添加
Dim stride As Integer = bytes.Length \ inImage.Height
output.SetField(BitMiracle.LibTiff.Classic.TiffTag.STRIPBYTECOUNTS, stride * inImage.Height)

这种方式不需要修改像素数据，让LibTiff按照实际的Stride读取每行内容。

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