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.NET Forum / .NET Framework / New Users / August 2005Transparency, ASP.NET, and GDI+
I am trying to create graphics with GDI+ that include transparency. However, the transparency never seems to show up, even though my colors have an alpha value of 0. How can I generate a graphic that is completely transparent in certain areas (so that the web page background shows through)? I save my graphics as gif files. I would appreciate, if possible, a simple example so that I can see the code. Thanks.  SignatureNathan Sokalski njsokalski@hotmail.com http://www.nathansokalski.com/ The only graphics formats that have transparency in HTML are GIF and PNG. What format are you using? SignatureHTH, Kevin Spencer Microsoft MVP .Net Developer Everybody picks their nose, But some people are better at hiding it. >I am trying to create graphics with GDI+ that include transparency. >However, the transparency never seems to show up, even though my colors >have an alpha value of 0. How can I generate a graphic that is completely >transparent in certain areas (so that the web page background shows >through)? I save my graphics as gif files. I would appreciate, if possible, >a simple example so that I can see the code. Thanks. Like I said in my original message, I am using GIF. The line that I use to save my image is as follows (testbitmap is the name of my System.Drawing.Bitmap object): testbitmap.Save(Server.MapPath("GDItest.gif"), Imaging.ImageFormat.Gif) SignatureNathan Sokalski njsokalski@hotmail.com http://www.nathansokalski.com/ > The only graphics formats that have transparency in HTML are GIF and PNG. > What format are you using? [quoted text clipped - 5 lines] >>through)? I save my graphics as gif files. I would appreciate, if >>possible, a simple example so that I can see the code. Thanks. I think IE don't support transparency. At least I can say for sure it doesn't support PNG transparency. It is an issue that should be fixed with IE 7 or could be fixd with Firefox! > Like I said in my original message, I am using GIF. The line that I use to > save my image is as follows (testbitmap is the name of my [quoted text clipped - 11 lines] >>>through)? I save my graphics as gif files. I would appreciate, if >>>possible, a simple example so that I can see the code. Thanks. I know for sure that IE correctly supports transparency in GIF files because I have made transparent GIF files using Adobe Photoshop in the past that worked correctly in IE. What the problem appears to be to me, although I could be wrong, is that when a new System.Drawing.Bitmap is created in ASP.NET it automatically gives it a background with an Alpha value of 255 (completely opaque). Because of this, whenever anything is drawn on the bitmap, it is drawn on top of the opaque background. This means that anything with an Alpha value of 0 (completely transparent) is not visible. If this is the case, I think the only solution would be to "erase" previously drawn pixels (which I do not know of a way to do), including the background or to have the initial background color have an Alpha value of 0. This is only a theory, so I could be wrong. A workaround that I have come up with (but it requires the developer to create an extra file) is to use an existing image file that is completely transparent in the System.Drawing.Bitmap constructor. Let me know if you come up with any ideas. Thanks. SignatureNathan Sokalski njsokalski@hotmail.com http://www.nathansokalski.com/ >I think IE don't support transparency. > At least I can say for sure it doesn't support PNG transparency. [quoted text clipped - 16 lines] >>>>shows through)? I save my graphics as gif files. I would appreciate, if >>>>possible, a simple example so that I can see the code. Thanks. mmhh... never really investigate this matter, but I would have guess that maybe you could do: Bitmap bmp = new Bitmap(width, height); bmp.MakeTransparent(); or with bmp.LockBit() you could directly access the bitmap's color pointer as explained here: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dncscol/html/cs harp11152001.asp The MakeTransparent() didn't seem to have any affect, and I'm not sure why (maybe the example wasn't quite clear enough) but the link you gave didn't seem very clear to me. I think what I really need is a just a basic example that just draws a simple shape or two and then adds a transparent area. Thanks. SignatureNathan Sokalski njsokalski@hotmail.com http://www.nathansokalski.com/ > mmhh... > never really investigate this matter, but I would have guess that maybe [quoted text clipped - 7 lines] > as explained here: > http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dncscol/html/cs harp11152001.asp Not correct. While IE doesn't support PNG transparency, it has lawys supported GIF transparency. SignatureHTH, Kevin Spencer Microsoft MVP .Net Developer Everybody picks their nose, But some people are better at hiding it. >I think IE don't support transparency. > At least I can say for sure it doesn't support PNG transparency. [quoted text clipped - 16 lines] >>>>shows through)? I save my graphics as gif files. I would appreciate, if >>>>possible, a simple example so that I can see the code. Thanks. Hi Nathan, Sorry, I missed the "gif" reference in your OP. A GIF image palette can define one color as transparent. I'm not sure how you're defining the transparent color in your palette, but the following Microsoft KB article explains how to do it. Note that the article is not specifically about making a transparent gif, but it covers creating a palette with a single transparent color, which is encoded as the transparent color for the GIF: http://support.microsoft.com/default.aspx?scid=kb;EN-US;Q319061 SignatureHTH, Kevin Spencer Microsoft MVP .Net Developer Everybody picks their nose, But some people are better at hiding it. > Like I said in my original message, I am using GIF. The line that I use to > save my image is as follows (testbitmap is the name of my [quoted text clipped - 11 lines] >>>through)? I save my graphics as gif files. I would appreciate, if >>>possible, a simple example so that I can see the code. Thanks. I looked at the article you mentioned, and it definitely cleared up for me why I was never getting any transparency, but I am still having trouble figuring out how to fix the problem. I think what I really need is just a basic example that just draws a simple shape or two and then adds a transparent area. (Preferably in VB.NET, but I can usually get enough from C# that it helps enough to solve my problem) Thanks. SignatureNathan Sokalski njsokalski@hotmail.com http://www.nathansokalski.com/ > Hi Nathan, > [quoted text clipped - 24 lines] >>>>shows through)? I save my graphics as gif files. I would appreciate, if >>>>possible, a simple example so that I can see the code. Thanks. Hi Nathan, Okay, basically you have a palette to work with. The article tells you how to define the colors in the palette. Also note that the palette can certainly have LESS than 256 colors if you don't need them all. So, assuming you're drawing with GDI+, you just clear the bitmap to the transparent color, and draw with the others. All of the pixels that have the transparent color will be transparent. It doesn't matter what the transparent color is; it will be defined as "the transparent color" in the palette. SignatureHTH, Kevin Spencer Microsoft MVP .Net Developer Expect the unaccepted. >I looked at the article you mentioned, and it definitely cleared up for me >why I was never getting any transparency, but I am still having trouble [quoted text clipped - 31 lines] >>>>>appreciate, if possible, a simple example so that I can see the code. >>>>>Thanks. oops.. GIF! what about Bitmap.MakeTransparent (Color) where Color is your background which might be white per default, I believe SignatureIf you're in a war, instead of throwing a hand grenade at the enemy, throw one of those small pumpkins. Maybe it'll make everyone think how stupid war is, and while they are thinking, you can throw a real grenade at them. Jack Handey. > Hi Nathan, > [quoted text clipped - 42 lines] >>>>>>appreciate, if possible, a simple example so that I can see the code. >>>>>>Thanks. I tried putting a color inside the MakeTransparent method, but it not only did not make it transparent, but prevented all of the other stuff from being drawn as well. Here is my code (note that the Clear method has the same color as the MakeTransparent method): Dim transbitmap As New Bitmap(400, 400) Dim transgraphics As Graphics = Graphics.FromImage(transbitmap) Dim transpen As New Pen(Color.FromArgb(255, 0, 255, 0), 20) Dim transbrush As New SolidBrush(Color.FromArgb(255, 255, 0, 0)) transgraphics.Clear(Color.FromArgb(0, 255, 255, 255)) transbitmap.MakeTransparent(Color.FromArgb(0, 255, 255, 255)) transgraphics.FillRectangle(transbrush, 100, 100, 200, 200) transgraphics.DrawRectangle(transpen, 50, 50, 150, 150) transbitmap.Save(Server.MapPath("GDItest.gif"), Imaging.ImageFormat.Gif) Is there something wrong with the way I am using the MakeTransparent method? What do I need to do to make areas of my image transparent? Thanks. SignatureNathan Sokalski njsokalski@hotmail.com http://www.nathansokalski.com/ > oops.. GIF! > what about [quoted text clipped - 48 lines] >>>>>>>appreciate, if possible, a simple example so that I can see the code. >>>>>>>Thanks. GWBYF (aka Google Would Be Your Friend != RTFM) http://www.bobpowell.net/giftransparency.htm SignatureIf you're in a war, instead of throwing a hand grenade at the enemy, throw one of those small pumpkins. Maybe it'll make everyone think how stupid war is, and while they are thinking, you can throw a real grenade at them. Jack Handey. >I tried putting a color inside the MakeTransparent method, but it not only >did not make it transparent, but prevented all of the other stuff from [quoted text clipped - 68 lines] >>>>>>>>appreciate, if possible, a simple example so that I can see the >>>>>>>>code. Thanks. I have tried searching (I've spent almost 2 months trying to find an answer), but none of the stuff I found seemed to help. SignatureNathan Sokalski njsokalski@hotmail.com http://www.nathansokalski.com/ > GWBYF (aka Google Would Be Your Friend != RTFM) > http://www.bobpowell.net/giftransparency.htm [quoted text clipped - 71 lines] >>>>>>>>>appreciate, if possible, a simple example so that I can see the >>>>>>>>>code. Thanks. allright, allright, but have you checked the link I posted as well? (this one: http://www.bobpowell.net/giftransparency.htm) should fix it! "Nathan Sokalski" <njsokalski@hotmail.com> wrote in message >I have tried searching (I've spent almost 2 months trying to find an > answer), but none of the stuff I found seemed to help. >> GWBYF (aka Google Would Be Your Friend != RTFM) >> http://www.bobpowell.net/giftransparency.htm Bitmap Transparency is not supported in browsers. Only GIFs and PNGs are transparent, and only GIFs are universally transparent in browsers. The article I sent you to doesn't use BitMap.MakeTransparent(). I am not sure why you keep bringing this up. It is a simple matter of following instructions. The functions you use in your app are not a goal, but should only be used if they are a means TO your goal. Let's say you're baking a cake. Why on earth would you need or want to use a meat grinder in the process? A meat grinder is for grinding meat, not for baking a cake. SignatureHTH, Kevin Spencer Microsoft MVP .Net Developer Expect the unaccepted. > oops.. GIF! > what about [quoted text clipped - 48 lines] >>>>>>>appreciate, if possible, a simple example so that I can see the code. >>>>>>>Thanks. I am having trouble defining the colors in the palette. The first method I tried gave me the error Object reference not set to an instance of an object. (Which didn't surprise me, but it would not let me use the keyword New when declaring the Imaging.ColorPalette). Here is that code: Dim transbitmap As New Bitmap(400, 400) Dim transgraphics As Graphics = Graphics.FromImage(transbitmap) Dim transpen As New Pen(Color.FromArgb(255, 0, 255, 0), 20) Dim transbrush As New SolidBrush(Color.FromArgb(255, 255, 0, 0)) Dim transpalette As Imaging.ColorPalette transpalette.Entries(0) = Color.FromArgb(0, 255, 255, 255) transpalette.Entries(1) = Color.FromArgb(255, 0, 255, 0) transpalette.Entries(2) = Color.FromArgb(255, 255, 0, 0) transgraphics.Clear(Color.FromArgb(0, 255, 255, 255)) transbitmap.Palette = transpalette transgraphics.FillRectangle(transbrush, 100, 100, 200, 200) transgraphics.DrawRectangle(transpen, 50, 50, 150, 150) transbitmap.Save(Server.MapPath("GDItest.gif"), Imaging.ImageFormat.Gif) The second method I tried gave me the error Index was outside the bounds of the array. Here is that code: Dim transbitmap As New Bitmap(400, 400) Dim transgraphics As Graphics = Graphics.FromImage(transbitmap) Dim transpen As New Pen(Color.FromArgb(255, 0, 255, 0), 20) Dim transbrush As New SolidBrush(Color.FromArgb(255, 255, 0, 0)) transbitmap.Palette.Entries(0) = Color.FromArgb(0, 255, 255, 255) transbitmap.Palette.Entries(1) = Color.FromArgb(255, 0, 255, 0) transbitmap.Palette.Entries(2) = Color.FromArgb(255, 255, 0, 0) transgraphics.Clear(Color.FromArgb(0, 255, 255, 255)) transgraphics.FillRectangle(transbrush, 100, 100, 200, 200) transgraphics.DrawRectangle(transpen, 50, 50, 150, 150) transbitmap.Save(Server.MapPath("GDItest.gif"), Imaging.ImageFormat.Gif) What I expected from the code above was to create a GIF file with a red filled rectangle, a green unfilled rectangle, and a transparent background. If I remove the lines where I attempt to define the palette, this is what I got except instead of a transparent background I ended up with a black background. What am I supposed to do to create/edit the palette? The article you mentioned took the palette from another Bitmap, which is not something I am planning to do. Thanks. SignatureNathan Sokalski njsokalski@hotmail.com http://www.nathansokalski.com/ > Hi Nathan, > [quoted text clipped - 42 lines] >>>>>>appreciate, if possible, a simple example so that I can see the code. >>>>>>Thanks. Check out the sample code from the article I referred to you: protected ColorPalette GetColorPalette( uint nColors ) { // Assume monochrome image. PixelFormat bitscolordepth = PixelFormat.Format1bppIndexed; ColorPalette palette; // The Palette we are stealing Bitmap bitmap; // The source of the stolen palette // Determine number of colors. if (nColors > 2) bitscolordepth = PixelFormat.Format4bppIndexed; if (nColors > 16) bitscolordepth = PixelFormat.Format8bppIndexed; // Make a new Bitmap object to get its Palette. bitmap = new Bitmap( 1, 1, bitscolordepth ); palette = bitmap.Palette; // Grab the palette bitmap.Dispose(); // cleanup the source Bitmap return palette; // Send the palette back } That doesn't look anything like what you wrote, regardless of the language. The palette is retrieved from a Bitmap of a specific paletted format. Are you having trouble translating from C# to VB? If so, I can do it for you. SignatureHTH, Kevin Spencer Microsoft MVP .Net Developer Expect the unaccepted. >I am having trouble defining the colors in the palette. The first method I >tried gave me the error Object reference not set to an instance of an [quoted text clipped - 86 lines] >>>>>>>appreciate, if possible, a simple example so that I can see the code. >>>>>>>Thanks. You should ckeck out this article. It helped me a lot in over coming problems with keeping transparency in GIF's. http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnaspp/html/col orquant.asp Martyn > I am trying to create graphics with GDI+ that include transparency. However, > the transparency never seems to show up, even though my colors have an alpha > value of 0. How can I generate a graphic that is completely transparent in > certain areas (so that the web page background shows through)? I save my > graphics as gif files. I would appreciate, if possible, a simple example so > that I can see the code. Thanks. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. 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