opengl alpha fading

Graphics and GPU Programming Programming OpenGL

Started by Julio July 18, 2000 06:22 PM

6 comments, last by Julio 23 years, 9 months ago

116

Author

July 18, 2000 06:22 PM

howdy, for my title screen sequence I want it to fade for the effect. I thought I could use the alpha channel to get the fading effect, but for some reason it''s not working. here''s some source:

    
float alevel; //alpha level

ts_time=GetTickCount();
glEnable(GL_TEXTURE_2D);
while(GetTickCount()-ts_time<2000)
{
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);	
	glLoadIdentity();						glPushMatrix();
	glTranslatef(0.0f,0.0f,-4.0f);
	glBindTexture(GL_TEXTURE_2D, t_texture[0]);
	glColor4f(1.0f,1.0f,1.0f,alevel);
	glBegin(GL_QUADS);
		glTexCoord2f(0.0f,1.0f); 
		glVertex3f(-2.0f,1.0f,0.0f);
		glTexCoord2f(0.0f,0.0f); 
		glVertex3f(-2.0f,0.0f,0.0f);
		glTexCoord2f(1.0f,0.0f); 
		glVertex3f(2.0f,0.0f,0.0f);
		glTexCoord2f(1.0f,1.0f); 
		glVertex3f(2.0f,1.0f,0.0f);
	glEnd();
		
	alevel+=0.001f;
	SwapBuffers(hDC);
}

basically, after every frame it increments the alpha level by .001. it doesn''t appear to fade at all. what am I missing? JoeMont001@aol.com

www.polarisoft.n3.net

My HomepageSome shoot to kill, others shoot to mame. I say clear the chamber and let the lord decide. - Reno 911

Gorky

122

July 18, 2000 07:16 PM

I see two things wrong.
1) You don''t initialize the alpha to zero!
2) It''s a local variable. It will always be the same. If you want to keep the variable local to this function, declare it as static.

Julio

116

Author

July 18, 2000 10:39 PM

the alpha level 1.0f by default? that makes sense. I''m not sure I understand the second statement. why would it not increment?
thanks again,
Joe

JoeMont001@aol.com

www.polarisoft.n3.net

My HomepageSome shoot to kill, others shoot to mame. I say clear the chamber and let the lord decide. - Reno 911

Xtreme

122

July 18, 2000 11:01 PM

it would increment but only local to that function. so when that
function exits, you lose your incremented value. so, that''s why you declare it as static (or a global variable).

Secondly, i don''t know if this is incorrect but it seems like your while loop is not right. you might need extra brackets between the gettickcount and the other variable, coz < has higher priority than -.

So, is that how you get the fading effect? I have been trying to do the same but so far i have been unsuccessful

sig u say?

Yes! There are kangaroos in Australia but I haven't seen them...yet

Anonymous

July 18, 2000 11:06 PM

Here is a simple pseudo-code class to create an fade effect.

class fadeEffect
{
fadeEffect(screenWidth, screenHeight, fadeincrement);

Anonymous

July 18, 2000 11:10 PM

Here is a simple pseudo-code class to create an fade effect.

class fadeEffect
{
fadeEffect(screenWidth, screenHeight, fadeincrement, color)

fade()
//draw a quad screenwidth, screenheight of color with alpha value(internal to the class)
inc() // increment the fade

done() //alpha == 0
}

here is how you use it :

if (fade) then
fader = new fadeEffect(640,480, 0.01, red);
end if

while not(fader.done())
//draw stuff
fader.fade();
fader.incr();
wend

delete fader;

Julio

116

Author

July 19, 2000 11:25 AM

the while loop works fine. it waits 2 seconds (2000 miliseconds) then proceeds. I changed the float alevel variable to global. it didn''t change anything. I changed it to static, that didn''t work either. so I think I have it narrowed down to it not changing the alpha level. I''ll figure it out.

JoeMont001@aol.com