Question

32-bit textures and alpha transparency (OpenGL)

Asked by: rossryan

I think I'm going insane...

I'm loading bitmaps from a .Theme file. They are 32-bit. But, for some reason, OpenGL draws the transparent portions as black instead of clear. I'm using the BGRA extension, if that helps.


public void LoadTexture(Bitmap name, ref int target)
            {
                  //Debug.WriteLine(file.PixelFormat);                              
                  using(Bitmap bitmap = new Bitmap(name))
                  {
                        bitmap.RotateFlip(RotateFlipType.RotateNoneFlipY);
                        
                        Rectangle rectangle = new Rectangle(0, 0, bitmap.Width,
                              bitmap.Height);
                        BitmapData bitmapData = bitmap.LockBits(rectangle,
                              ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);

                        
                              
                        Gl.glGenTextures(1, out target);
                        Gl.glBindTexture(Gl.GL_TEXTURE_2D, target);
                        Gl.glTexParameteri(Gl.GL_TEXTURE_2D,
                              Gl.GL_TEXTURE_MAG_FILTER, Gl.GL_LINEAR);
                        Gl.glTexParameteri(Gl.GL_TEXTURE_2D,
                              Gl.GL_TEXTURE_MIN_FILTER, Gl.GL_LINEAR);
                        Glu.gluBuild2DMipmaps(Gl.GL_TEXTURE_2D, 3,
                              bitmapData.Width, bitmapData.Height,
                              Gl.GL_BGRA_EXT, Gl.GL_UNSIGNED_BYTE,
                              bitmapData.Scan0);
                        bitmap.UnlockBits(bitmapData);
                        
                              

                  }
            }

Any ideas why it displays as black?

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Asked On
2004-05-09 at 14:00:50ID20983278
Tags

opengl

,

transparency

,

alpha

,

texture

Topic

3D Game Programming

Participating Experts
2
Points
500
Comments
12

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Answers

 

by: davebytesPosted on 2004-05-09 at 23:09:40ID: 11028735

You need to enable alpha-blending. Plus set the blending modes to something useful, like SRC = ALPHA + DEST = INVSRCALPHA.  I think you want something like:

glEnable(GL_BLEND);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

If you don't have blending enabled, or have a bad blend mode, alpha will end up blending against black (or white, depending on setup).

d

 

by: rossryanPosted on 2004-05-09 at 23:28:48ID: 11028824

Yes, but doesn't alpha blending affect the entire texture? I just want to blend out the black portions...

 

by: davebytesPosted on 2004-05-09 at 23:32:00ID: 11028837

the blend function controls how things blend.

SRC + INVSRC is the standard alpha-blending mode.  It will use the alpha values of the src pixels to blend, assume other normal texturing settings are in place.

That's separate from the pixel pipeline that grabs the colors in the first place.  If you wanted an entire polygon transparent, you'd set the same mode, but then use vertex alphas to apply uniformly to a polygon.

d

 

by: enkimutePosted on 2004-05-16 at 03:00:07ID: 11080426

you could leave blending for what it is and turn on the alphatest for this .. it will draw the entire texture, except for where the alpha value is lower than a given treshold.

glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0);

Will draw all texture samples where the alpha value is greater than 0

This technique is comonly called 'superblack keying'

grtz

e.

 

by: davebytesPosted on 2004-05-16 at 09:20:37ID: 11082064

Hmmm, I've never heard of 'superblack keying' before.  That's an interesting name for it!

Yeah, the problem with alpha-test is that you get hard edges, assuming you have a soft-edged alpha.  I'm also nearly sure you still need to enable the blending stuff, as without it alpha is likely ignored (and never gets into the pixel pipeline for evaluation).

However, enki is good to point this out, as for performance reasons, once you are drawing a LOT of alpha'd objects, you'll want alpha-test-zero on, as the alpha test exits pixel processing earlier.  Saves rendering time.

d

 

by: davebytesPosted on 2004-05-17 at 22:31:43ID: 11095486

ross, have you gotten enough to close this out?

 

by: rossryanPosted on 2004-05-17 at 23:23:54ID: 11095643

There must be something seriously messed up about the textures I'm using. Enabling AlphaTest with the above code completeing wipes the texture. Hmm.

 

by: enkimutePosted on 2004-05-17 at 23:34:39ID: 11095690

that means all your alpha values are zero ..

 

by: davebytesPosted on 2004-05-17 at 23:35:47ID: 11095693

That generally means the alpha value is full-on.  either missing, bad data, bad filler code... something.

Try setting the blend mode to something like one minus src alpha (can't remember the str offhand) or inverse alpha, whatever the terminology is, and see if that works.

d

 

by: enkimutePosted on 2004-05-17 at 23:37:09ID: 11095697

try looping the pixeldata and do something like

if (r+g+b = 0)
{
  alpha = 0;
}
else
{
  alpha = 255;
}

assuming gl_unsigned_byte here off course ..

grtz

20120131-EE-VQP-002

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