Scaling transparent GIF / Transparency gets black

[Hrungdak]

Hi all

I am new here and i hope i do not make any mistakes with this question.

I try to change a GIF-File that contains transparency. The problem lies within the following code:

' Open a gif with transparency
Dim nBm As Bitmap = New Bitmap("C:\Temp\TestTransparent.gif")
 
' Create a new bitmap with the same size
Dim nResult As Bitmap = New Bitmap(nBm.Width, nBm.Height)
 
' Get a graphics object
Dim nGr As Graphics = Graphics.FromImage(nResult)
 
' Draw the old bitmap to the new one
nGr.DrawImage(nBm, New Point(0, 0))
 
' Make black transparent
nResult.MakeTransparent(Color.Black)
 
' save the whole thing as gif
nResult.Save("C:\Temp\TestTransparent_Neu.gif", Imaging.ImageFormat.Gif)

Problem after that: all transparency has gone black in the resulting gif.

The purpose of the code at the end should be to reduce the gif in size. So i have to build a new bitmap out of the old.

I searched for "gif transparent", "transparent black", DrawImage transparent" and found nothing to help me on. Also google is not my friend in this case. Many tips, most for java or browsers.

I post this in the .net-forums. I dont think that this is a VB- or C#-issue. Correct me if i am wrong.

Greetings
Alex

[ram09]

so your problem is that, when you reduce the size of your gif file, the transparent portion became black? is this a web proj?

[Hrungdak]

@ram09
Hi ram09

should there be a difference? I need the (smaller) GIF for a web-project. But it should work independent of web or not web.

Alex

[ram09]

i think theres no difference..i just want to clarify your problem... so you want to manipulate a .gif image file dynamically?? and you had this problem when you resize the image?

[Hrungdak]

@ram09
Yes. I have to create a thumb of a GIF-file and the transparency has to be transformed also (some gifs overlap each other, so the transparency is important).

Alex

[ram09]

I think you also need to consider the image file that you manipulated, if you edit the image itself properly, without any background if you want it to be transparent.... have you tried using other image files?? .png is also a good image file when you have some transparency...

[Hrungdak]

@ram09
Sorry, ram09, i have no choice here. I have to use the gif-files.

Alex

[vbwebprofi]

Hi,

the following steps should help:

a) Save image to memory stream as GIF with 89a version
b) Load image from memory stream
c) Detect transparent color from the image palette (Hint: component A is zero)
d) Take a GIF file description
e) Walk through the bytes on the stream and get color the index of your transparent color (Hint: MS often stores more than one equal entry in the palette, use the highest color index)
f) Ensure that transparency flag in "Flags" of the "Graphic Control Extension Bock" is set
g) Set the "Transparent Color Index" in the same block
h) Load the image again from the stream

Now you have a transparent GIF in your image instance and on the stream.


Regards

Holger

[Hrungdak]

Hi Holger

a very complex solution. I will try that and report the result. Thanks for your help.

Alex

PS: und Grüße aus Bayern!