OpenGL Texture transparency doesn't work

Question

I'm having an OpenGL texture that is binded to a simple quad.

My problem is: My texture is 128x128 pixels image. I'm only filling up about 100x60 pixels on that image, the other pixels are transparent. I saved it in a .png file. When I'm drawing, the transparent part of the binded texture is white.

Let's say I have a background. When I draw this new quad on this background I can't see the through the transparent part of my texture.

Any suggestions?

Code:

// Init code...
gl.glEnable(gl.GL_TEXTURE_2D);
gl.glDisable(gl.GL_DITHER);
gl.glDisable(gl.GL_LIGHTING);   
gl.glDisable(gl.GL_DEPTH_TEST);

gl.glTexEnvi(gl.GL_TEXTURE_ENV, gl.GL_TEXTURE_ENV_MODE, gl.GL_MODULATE); 

// Drawing code...
gl.glBegin(gl.GL_QUADS);
gl.glTexCoord2d(0.0, 0.0);
gl.glVertex3f(0.0f, 0.0f, 0.0f);
gl.glTexCoord2d(1.0, 0.0);
gl.glVertex3f(1.0f, 0.0f, 0.0f);
gl.glTexCoord2d(1.0, 1.0);
gl.glVertex3f(1.0f, 1.0f, 0.0f);
gl.glTexCoord2d(0.0, 1.0);
gl.glVertex3f(0.0f, 1.0f, 0.0f);
gl.glEnd();

I've tried almost everything, from enabling blending to change to GL_REPLACE, however I can't get it to work.

Edit:

// Texture. Have tested both gl.GL_RGBA and gl.GL_RGB8.
gl.glTexImage2D(gl.GL_TEXTURE_2D, 0, (int)gl.GL_RGBA, imgWidth, imgHeight,
            0, gl.GL_BGR_EXT, gl.GL_UNSIGNED_BYTE, bitmapdata.Scan0);

Answer 1

Check that your texture is of RGBA format, and enable blending and set the blending func:

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

And draw the texture. If your texture is not RGBA, then there is no alpha and blending won't do anything.

EDIT: Since you posted your code, i can a spot a serious error:

glTexImage2D(gl.GL_TEXTURE_2D, 0, (int)gl.GL_RGBA, imgWidth, imgHeight,  0, gl.GL_BGR_EXT, gl.GL_UNSIGNED_BYTE, bitmapdata.Scan0);

You're telling GL that the texture has internalFormat RGBA, but the bitmap data has BGR format, so, no alpha from your texture data. This assumes alpha = 1.0.

To correct it, load your PNG with RGBA format and use GL_RGBA as internalFormat and format parameters for glTexImage2D.

Answer 2

When I'm drawing, the transparent part of the binded texture is white.

That means your PNG-parser converted transparent regions to the value of white. If you want to render transparent layers with OpenGL you dont typically depend on texture-files to hold the transparency but instead use the GLBlendFunc(). More information here:

http://www.opengl.org/resources/faq/technical/transparency.htm

Answer 3

Also, should you render to a frame buffer and copy the result into a texture, check that the frame buffer has alpha turned on. For example when using osgViewer this can be achived by (do this before calling setUpViewInWindow):

osg::DisplaySettings *pSet = myviewer.getDisplaySettings();
if(pSet == NULL)
   pSet = new osg::DisplaySettings();
pSet->setMinimumNumAlphaBits(8);
myviewer.setDisplaySettings(pSet);

and under qt it should work with (from http://forum.openscenegraph.org/viewtopic.php?t=6411):

QGLFormat f;
f.setAlpha( true ); //enables alpha channel for this format
QGLFormat::setDefaultFormat( f ); //set it as default before instantiations
setupUi(this); //instantiates QGLWidget (ViewerQT)

Normally it is better to render directly into a frame buffer but I came alonge this while preparing some legacy code and in the beginning it was very hard to find this.