通过JPEG压缩将JPEG转换为8位TIFF

Question

以下代码通过JPEG DCT压缩成功将JPEG转换为TIFF，但输出为24位TIFF。 如何使用相同的压缩类型将其转换为 8位 TIFF？

Using bmp As New Bitmap(oImage.FilePath)
            Using tif As Tiff = Tiff.Open("BitmapTo24BitColorTiff.tif", "w")
                Dim raster As Byte() = getImageRasterBytes(bmp, PixelFormat.Format24bppRgb)
                tif.SetField(TiffTag.IMAGEWIDTH, bmp.Width)
                tif.SetField(TiffTag.IMAGELENGTH, bmp.Height)
                tif.SetField(TiffTag.COMPRESSION, Compression.JPEG)
                tif.SetField(TiffTag.PHOTOMETRIC, Photometric.RGB)

                tif.SetField(TiffTag.ROWSPERSTRIP, bmp.Height)

                tif.SetField(TiffTag.XRESOLUTION, bmp.HorizontalResolution)
                tif.SetField(TiffTag.YRESOLUTION, bmp.VerticalResolution)

                tif.SetField(TiffTag.BITSPERSAMPLE, 8)
                tif.SetField(TiffTag.SAMPLESPERPIXEL, 3)

                tif.SetField(TiffTag.PLANARCONFIG, PlanarConfig.CONTIG)

                Dim stride As Integer = raster.Length \ bmp.Height
                '    convertSamples(raster, bmp.Width, bmp.Height)

                Dim i As Integer = 0, offset As Integer = 0
                While i < bmp.Height
                    tif.WriteScanline(raster, offset, i, 0)
                    offset += stride
                    i += 1
                End While
            End Using

            System.Diagnostics.Process.Start("BitmapTo24BitColorTiff.tif")
 End Using