Re: [PyOpenGL-Users] opaque polygons

[Astan C. <st...@al...>]

Hi,
Thanks for these tips. It worked. I didnt know I had to do a blend.
Thanks again.
Cheers
Astan

Gary Herron wrote:
> Astan Chee wrote:
>> Hi,
>> Im trying to make this:
>>
>>     green = (( 0.0, 1.0, 0.0, 1 ))
>>     glColor4f( 0, 1, 0, .1 )
>>     glBegin( GL_POLYGON )
>>     glMaterialfv( GL_FRONT, GL_EMISSION, green )
>>     rad = 0.5
>>     for i in xrange(0,360,5):
>>         glVertex3f(x1+math.sin(i*math.pi/180.0)*rad , 
>> y1+math.cos(i*math.pi/180.0)*rad,z1)          glEnd( )
>>
>>  to be opaque. I read that the last part of glColor4f() allows alpha 
>> channels for transparency levels. I changed it and it still remains 
>> solid. What am I doing wrong?
>> Thanks again for any help.
>> Cheers
>> Astan
>>
>>   
> Several things wrong there:
>
> First, you must enable blending with
>  glEnable(GL_BLEND)
> before the glBegin and you should disable it with
>  glDisable(GL_BLEND)
> after the glEnd.
>
> Second, you are mixing two ways of specifying color.  Choose one of 
> these:
>
> Choice 1: Using glColor
> Choose this method with glDisable(GL_LIGHTING)
> Specify color per vertex with any of the glColorXX routines.
>
> Choice 2: Using Phong lighting
> Choose this method with glEnable(GL_LIGHTING)
> For lighting to work, you must also specify the light's position/color 
> and other properties with glLight,
> as well as the surface properties (diffuse/specula/shininess/ ...) per 
> vertex with glMaterial.
>
> (There is a third method where values specified with glColor affect 
> lighting values normally specified via glMaterial.  It uses the 
> glColorMaterial calls.  Call this mode advance, and ignore it for now.)
>
> Either method of specifying color can specify  RGBA 
> (red,green,blue,alpha) values.  If the alpha value is not equal to 1 
> and GL_BLEND is enabled, you will get blending.  In order for that 
> blending to look like transparency, you must make sure to draw the 
> background first, without blending enabled, then draw the transparent 
> object with blending enabled.
>
> The last piece of the puzzle is to specify how you want the 
> transparent color to be combined the the existing background color.  
> This is done with
>  glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
> when you enable blending (or earlier).
>
> Gary Herron
>
>
>
>

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