私密緣: ASP .NET - 如何線上產生高品質的縮圖

ASP .NET的線上縮圖程式，一般都是使用Image.GetThumbnailImage的方式來實作。使用Image.GetThumbnailImage雖然是很簡便，但其縮圖的品質有時真是慘不忍睹，尤其是對高畫質﹝High Quailty﹞的原圖來作縮圖。

因此，要產生高品質的縮圖，我們必須捨棄Image.GetThumbnailImage的方式，自行對圖片來作縮圖處理：



<%@ Page Language="VB" %>



<script runat="server">



Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs)


'----------------------------------------------------------------


Dim sFileName As String = Server.MapPath("SANY0722.jpg")


Dim tFileName As String = Server.MapPath("SANY0722_thumb.jpg")


'----------------------------------------------------------------


' Delete the file if it exists.


'----------------------------------------------------------------


If File.Exists(tFileName) Then


File.Delete(tFileName)


End If


'----------------------------------------------------------------


Dim sFileStream As FileStream = File.OpenRead(sFileName)


Dim tFileStream As FileStream = File.Create(tFileName)


'----------------------------------------------------------------


ResizeImage(0.1, sFileStream, tFileStream)


'----------------------------------------------------------------


sFileStream.Close()


tFileStream.Close()


'----------------------------------------------------------------


End Sub



Public Sub ResizeImage(ByVal scaleFactor As Double, ByVal fromStream As Stream, ByVal toStream As Stream)


'----------------------------------------------------------------


Dim image As System.Drawing.Image = System.Drawing.Image.FromStream(fromStream)


Dim newWidth As Integer = CType((image.Width * scaleFactor),Integer)


Dim newHeight As Integer = CType((image.Height * scaleFactor),Integer)


'----------------------------------------------------------------


Dim thumbnailBitmap As Bitmap = New Bitmap(newWidth, newHeight)


Dim thumbnailGraph As Graphics = Graphics.FromImage(thumbnailBitmap)


'----------------------------------------------------------------


thumbnailGraph.CompositingQuality = CompositingQuality.HighQuality


thumbnailGraph.SmoothingMode = SmoothingMode.HighQuality


thumbnailGraph.InterpolationMode = InterpolationMode.HighQualityBicubic


'----------------------------------------------------------------


Dim imageRectangle As Rectangle = New Rectangle(0, 0, newWidth, newHeight)


'----------------------------------------------------------------


thumbnailGraph.DrawImage(image, imageRectangle)


thumbnailBitmap.Save(toStream, image.RawFormat)


'----------------------------------------------------------------


thumbnailGraph.Dispose


thumbnailBitmap.Dispose


image.Dispose


'----------------------------------------------------------------


End Sub



</script>

以下是利用Image.GetThumbnailImage和自行縮圖處理的範例比較。左圖是Image.GetThumbnailImage所產生的縮圖；右圖是自行處理的縮圖﹝原圖尺寸為2910x4370；縮圖比例為10%﹞：

網路上流傳：使用兩次Image.GetThumbnailImage來縮圖，取其第二次的縮圖其品質會比較好，以下我們也就二次縮圖的品質來作一下測試：



<%@ Page Language="VB" %>



<script runat="server">



Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs)


'----------------------------------------------------------------


Dim sFileName As String = Server.MapPath("SANY0722.jpg")


Dim tFileName_1st As String = Server.MapPath("SANY0722_thumb_1st.jpg")


Dim tFileName_2nd As String = Server.MapPath("SANY0722_thumb_2nd.jpg")


'----------------------------------------------------------------


' Delete the file if it exists.


'----------------------------------------------------------------


If File.Exists(tFileName_1st) Then


File.Delete(tFileName_1st)


End If


'----------------------------------------------------------------


If File.Exists(tFileName_2nd) Then


File.Delete(tFileName_2nd)


End If


'----------------------------------------------------------------


Dim sFileStream As FileStream = File.OpenRead(sFileName)


Dim tFileStream_1st As FileStream = File.Create(tFileName_1st)


Dim tFileStream_2nd As FileStream = File.Create(tFileName_2nd)


'----------------------------------------------------------------


ResizeImage(0.1, sFileStream, tFileStream_1st, tFileStream_2nd)


'----------------------------------------------------------------


sFileStream.Close()


tFileStream_1st.Close()


tFileStream_2nd.Close()


'----------------------------------------------------------------


End Sub



Public Sub ResizeImage(ByVal scaleFactor As Double, ByVal fromStream As Stream, ByVal toStream_1st As Stream, ByVal toStream_2nd As Stream)


'----------------------------------------------------------------


Dim image As System.Drawing.Image = System.Drawing.Image.FromStream(fromStream)


Dim newWidth As Integer = CType((image.Width * scaleFactor),Integer)


Dim newHeight As Integer = CType((image.Height * scaleFactor),Integer)


'----------------------------------------------------------------


Dim thumbnail_1st As System.Drawing.Image = image.GetThumbnailImage(newWidth, newHeight, Nothing, IntPtr.Zero)


Dim thumbnail_2nd As System.Drawing.Image = image.GetThumbnailImage(newWidth, newHeight, Nothing, IntPtr.Zero)


'----------------------------------------------------------------


thumbnail_1st.Save(toStream_1st, image.RawFormat)


thumbnail_2nd.Save(toStream_2nd, image.RawFormat)


'----------------------------------------------------------------


thumbnail_1st.Dispose


thumbnail_2nd.Dispose


image.Dispose


'----------------------------------------------------------------


End Sub



</script>