如何使用System.Drawing绘制透明图像？

I'm trying to return a transparent GIF from an .aspx page for display within a web page. I am trying to get the image to have transparency, but I just keep getting Black being where the image should be Transparent.

我正在尝试从.aspx页面返回透明GIF，以便在网页中显示。我试图让图像具有透明度，但我只是让黑色成为图像应该透明的地方。

Does anyone know what I'm doing wrong?

有谁知道我做错了什么？

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) _
  Handles Me.Load
    '' Change the response headers to output a GIF image.
    Response.Clear()
    Response.ContentType = "image/gif"

    Dim width = 110
    Dim height = width

    '' Create a new 32-bit bitmap image
    Dim b = New Bitmap(width, height)

    '' Create Grahpics object for drawing
    Dim g = Graphics.FromImage(b)

    Dim rect = New Rectangle(0, 0, width - 1, height - 1)

    '' Fill in with Transparent
    Dim tbrush = New System.Drawing.SolidBrush(Color.Transparent)
    g.FillRectangle(tbrush, rect)

    '' Draw Circle Border
    Dim bPen = Pens.Red
    g.DrawPie(bPen, rect, 0, 365)

    '' Fill in Circle
    Dim cbrush = New SolidBrush(Color.LightBlue)
    g.FillPie(cbrush, rect, 0, 365)


    '' Clean up
    g.Flush()
    g.Dispose()

    '' Make Transparent
    b.MakeTransparent()

    b.Save(Response.OutputStream, Imaging.ImageFormat.Gif)
    Response.Flush()
    Response.End()
End Sub

4 个解决方案

#1

Unfortunately, there is no easy way to create a transparent Gif using a Bitmap object. (See this KB article)

不幸的是，没有简单的方法可以使用Bitmap对象创建透明的Gif。（见这篇知识库文章）

You can alternatively use the PNG format that supports transparency with the code you are using.

您也可以使用支持所用代码透明度的PNG格式。

#2

Yes, as Jerome stated, there isn't anyway to create transparent GIF's using a Bitmap object. Crap!

是的，正如Jerome所说，无论如何都不能使用Bitmap对象创建透明的GIF。扯淡！

Well, anyway, I changed my code to generate a PNG and all works as expected.

好吧，无论如何，我改变了我的代码以生成一个PNG，并且所有工作都按预期工作。

There is one small work around I did need to do since you cannot write PNG's directly to the OutputStream. I needed to write the PNG to a MemoryStream, and then write that out to the OutputStream.

我确实需要做一件小工作，因为你不能直接将PNG写入OutputStream。我需要将PNG写入MemoryStream，然后将其写入OutputStream。

Here's the final code for my implementation:

这是我实现的最终代码：

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) _
  Handles Me.Load
    '' Change the response headers to output a JPEG image.
    Response.Clear()
    Response.ContentType = "image/png"

    Dim width = 11
    Dim height = width

    '' Create a new 32-bit bitmap image
    Dim b = New Bitmap(width, height)

    '' Create Grahpics object for drawing
    Dim g = Graphics.FromImage(b)

    '' Fill the image with a color to be made Transparent after drawing is finished.
    g.Clear(Color.Gray)

    '' Get rectangle where the Circle will be drawn
    Dim rect = New Rectangle(0, 0, width - 1, height - 1)

    '' Draw Circle Border
    Dim bPen = Pens.Black
    g.DrawPie(bPen, rect, 0, 365)

    '' Fill in Circle
    Dim cbrush = New SolidBrush(Color.Red)
    g.FillPie(cbrush, rect, 0, 365)

    '' Clean up
    g.Flush()
    g.Dispose()

    '' Make Transparent
    b.MakeTransparent(Color.Gray)

    '' Write PNG to Memory Stream then write to OutputStream
    Dim ms = New MemoryStream()
    b.Save(ms, Imaging.ImageFormat.Png)
    ms.WriteTo(Response.OutputStream)

    Response.Flush()
    Response.End()
End Sub

#3

It is possible, but not easy
If you are able to use unsafe code in your project, there are a few methods to use pointers to rip through the colour table and make the transparency work.

这是可能的，但并不容易如果您能够在项目中使用不安全的代码，有一些方法可以使用指针来翻阅颜色表并使透明度工作。

A sample forms app by Bob Powell is available here http://www.bobpowell.net/giftransparency.htm. I used a variation on this method on a web handler a few years ago that was getting hit about 10 million times a month, and it seemed to work fine.

Bob Powell的样本表单应用程序可在此处获取http://www.bobpowell.net/giftransparency.htm。几年前我在网络处理程序上使用了这种方法的一种变体，每个月都会受到大约1000万次的攻击，而且似乎工作得很好。

If you are only using a limited colour pallete you can reduce the colour table processing to just the colours you need (can't remember exactly how I did that...).

如果你只使用有限颜色的托盘，你可以将颜色表处理减少到你需要的颜色（不记得我是怎么做的......）。

That being said, png is a metric crapload easier.

话虽这么说，png是一个更容易的度量标准。

#4

here is some code to have a gif (that already have transparency in it) transformed (supposed you want to resize it) in bitmap and then can be showed properly with it's transparency.

这里有一些代码在位图中有一个gif（已经有透明度）转换（假设你想要调整它），然后可以用它的透明度正确显示。

imagePath = System.Web.HttpContext.Current.Request.MapPath(libraryPath + reqImageFile);
System.Drawing.Image image = null;
Bitmap resizedImage = null;

if (reqWidth == 0) { reqWidth = image.Width; }
if (reqHeight == 0) { reqHeight = image.Height; }
image = System.Drawing.Image.FromFile(imagePath);
reqWidth = image.Width;
reqHeight = image.Height;

//here is the transparency 'special' treatment
resizedImage = new Bitmap(reqWidth, reqHeight, PixelFormat.Format8bppIndexed);
ColorPalette pal = resizedImage.Palette;
for (int i = 0; i < pal.Entries.Length; i++)
{
       Color col = pal.Entries[i];
       pal.Entries[i] = Color.FromArgb(0, col.R, col.G, col.B);
}
resizedImage.Palette = pal;
BitmapData src = ((Bitmap)image).LockBits(new Rectangle(0, 0, reqWidth, reqHeight),  ImageLockMode.ReadOnly, image.PixelFormat);
BitmapData dst = resizedImage.LockBits(new Rectangle(0, 0, resizedImage.Width, resizedImage.Height),
ImageLockMode.WriteOnly, resizedImage.PixelFormat);
((Bitmap)image).UnlockBits(src);
resizedImage.UnlockBits(dst);

Good luck !

祝你好运！

Grégoire Lafortune

GrégoireLafortune

#1