Save image to .PNG with transparent background?

[Martin]

I'm using the code show below (in VB.Net 2003) to create an image. I
want to save this image to a .PNG file such that when I subsequently
display it as part of a web page, the background of the image will be
transparent.

This test/learning code simply draws a diagonal green line across a
square box. When I display the resulting file on a web page, I want
only the line itself to be visible but what is happening is that the
entire "box" is shown (in a light grey color) with the line drawn
across it.

Any suggestions as to how I can do this?

Thanks.

-----------------------------------------------------------
Dim myPen As Pen = New Pen(Color.Green, 9))
Dim bmp As Bitmap = New Bitmap(100, 100)
Dim g As Graphics = Graphics.FromImage(bmp)

g.Clear(Color.Plum)
g.DrawLine(myPen, 0, 0, 100, 100)
bmp.MakeTransparent(Color.Plum)
bmp.Save("TestFile.png", System.Drawing.Imaging.ImageFormat.Png)

 

[Ken Tucker [MVP]]

Hi,

Try this instead.

Dim myPen As Pen = New Pen(Color.Green, 9)

Dim bmp As Bitmap = New Bitmap(100, 100)

Dim g As Graphics = Graphics.FromImage(bmp)

g.Clear(Color.Transparent)

g.DrawLine(myPen, 0, 0, 100, 100)

bmp.Save("TestFile.png", System.Drawing.Imaging.ImageFormat.Png)



Ken

------------------------

I'm using the code show below (in VB.Net 2003) to create an image. I
want to save this image to a .PNG file such that when I subsequently
display it as part of a web page, the background of the image will be
transparent.

This test/learning code simply draws a diagonal green line across a
square box. When I display the resulting file on a web page, I want
only the line itself to be visible but what is happening is that the
entire "box" is shown (in a light grey color) with the line drawn
across it.

Any suggestions as to how I can do this?

Thanks.

-----------------------------------------------------------
Dim myPen As Pen = New Pen(Color.Green, 9))
Dim bmp As Bitmap = New Bitmap(100, 100)
Dim g As Graphics = Graphics.FromImage(bmp)

g.Clear(Color.Plum)
g.DrawLine(myPen, 0, 0, 100, 100)
bmp.MakeTransparent(Color.Plum)
bmp.Save("TestFile.png", System.Drawing.Imaging.ImageFormat.Png)

 

[Martin]

Thanks, Ken, but that doesn't seem to do anything different. The
resulting .PNG image still appears on the web page as a light-grey
box.

 

[Herfried K. Wagner [MVP]]

Thanks, Ken, but that doesn't seem to do anything different. The
resulting .PNG image still appears on the web page as a light-grey
box.

Notice that MSIE doesn't fully support transparent PNGs! Did you try to
open the file in explorer?

 

[Dominic Marks]

Thanks, Ken, but that doesn't seem to do anything different. The
resulting .PNG image still appears on the web page as a light-grey
box.

Internet Explorer doesn't support transparent PNGs, as far as I
can tell from the Web. So if your testing it using IE, you won't be
seeing what you should be.

 

[Martin]

Yes, I am using Internet Explorer. And, I wasn't aware that it had a
problem with transparent PNGs.

This is a big problem for me. I need to display these graphics (with
transparent backgrounds) in Internet Explorer. Do you have any
suggestions as to how I might accomplish that?

How about .GIF files - do you know if they would display correctly?

Any suggestions?

A bit of background: I currently have a series of web pages (that are
used only on an Intranet) that display some simple, created-on-the-fly
images. I'm doing this now by using VML (Vector Markup Language). I
have server-side scripts that create the VML statements as part of the
web page itself. The big problem is that VML is supported ONLY in
Internet Explorer (and it's a dead technology).

I'm experimenting with using some individual images and sending them
out as part of the web pages. My thinking is that such would be
viewable in ANY browser.

 

[Martin]

Well, I just tried it as a .gif file - that doesn't work either. The
image has a black background.

 

[Herfried K. Wagner [MVP]]

How about .GIF files - do you know if they would display correctly?

GIF (and maybe transparent PNG8) support of MSIE is IMO better.