I want my GUI to have a transparent background. Not fully transparent, just like turning down the opacity.

PNG's support transparency, however I'm not sure what alpha values Minecraft accepts for GUI's. Just try making the GUI texture transparent in paint.net/gimp/photoshop/whatever. MSPaint doesn't support transparency as far as I'm aware (it might support it these days but it never used to).

 

EDIT: The doesn't seem to work, neither does modifying:

 

GL11.glColor4f(1.0F, 1.0F, 1.0F, 1.0F);

 

And lowering the alpha values.

You can do it ether by adjusting the image or using GL11.glColor4f(1.0F, 1.0F, 1.0F, 1.0F); but you also need to enable glBlend for it to work.

GL11.glPushMatrix();
GL11.glEnable(GL11.GL_BLEND);
OpenGlHelper.glBlendFunc(770, 771, 1, 0);
GL11.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
GL11.glColor4f(1.0F, 1.0F, 1.0F, 0.5F);

drawTexturedModalRect();

GL11.glPopMatrix();

 

Im not sure exactly what the glBlend Functions do because there arnt really any tutorials on GL11 that i have been able to find.

Im not sure exactly what the glBlend Functions do because there arnt really any tutorials on GL11 that i have been able to find.

 

Unity has one of the better explanations.  Its talking about shader code, but GL11 uses the same parameters.

http://docs.unity3d.com/Manual/SL-Blend.html

 

It's still bare-bones, but it does have the four most commonly used blends listed out.