Thread: Blending troubles

Alright, I'm working on checkers in opengl. The user will use the arrow keys to move a cursor around the board to select and move the pieces. I want the cursor to be blue at the bottom, and slowly move up to be completely transparent

http://files.gtanet.com/gtasa/walkth...ssantos043.jpg

Here is a mediocre example of the effect i am aiming for, but I am having trouble with Blending.

my call to glBlendfunc:

Code:

glBlendFunc(GL_SRC_ALPHA,GL_ONE);

and when i draw the cursor:

Code:

          glEnable(GL_BLEND);
	  glBegin(GL_QUADS);
		glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex3f(-1.0f,  1.0f, -1.0f); 
		glColor4f(1.0f, 1.0f, 1.0f, 0.0f); glVertex3f(-1.0f,  1.0f,  1.0f); 
		glColor4f(1.0f, 1.0f, 1.0f, 0.0f); glVertex3f( 1.0f,  1.0f,  1.0f); 
		glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex3f( 1.0f,  1.0f, -1.0f);

		glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex3f(-1.0f, -1.0f, -1.0f);
		glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex3f( 1.0f, -1.0f, -1.0f);
		glColor4f(1.0f, 1.0f, 1.0f, 0.0f); glVertex3f( 1.0f, -1.0f,  1.0f); 
		glColor4f(1.0f, 1.0f, 1.0f, 0.0f);  glVertex3f(-1.0f, -1.0f,  1.0f);

		glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex3f( 1.0f, -1.0f, -1.0f);
		glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex3f( 1.0f,  1.0f, -1.0f);
		glColor4f(1.0f, 1.0f, 1.0f, 0.0f); glVertex3f( 1.0f,  1.0f,  1.0f);
		glColor4f(1.0f, 1.0f, 1.0f, 0.0f); glVertex3f( 1.0f, -1.0f,  1.0f);

		glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex3f(-1.0f, -1.0f, -1.0f);
		glColor4f(1.0f, 1.0f, 1.0f, 0.0f); glVertex3f(-1.0f, -1.0f,  1.0f); 
		glColor4f(1.0f, 1.0f, 1.0f, 0.0f); glVertex3f(-1.0f,  1.0f,  1.0f);
		glColor4f(0.0f, 0.0f, 1.0f, 1.0f); glVertex3f(-1.0f,  1.0f, -1.0f);
	  glEnd();
	  glDisable(GL_BLEND);

and the results:
http://img455.imageshack.us/img455/5669/ck1jy.png

hopefully you can see what is going on in that picture

How can i get the effect i want?

Draw the cursor last. If you draw it before the pieces (which seems to be what you're doing) they will fail the Z test because your cursor is already there. But since it has alpha, it shows through to the back buffer.

Also, you probably want to use ONE_MINUS_SRC_ALPHA, not ONE

Originally Posted by skorman00

Draw the cursor last. If you draw it before the pieces (which seems to be what you're doing) they will fail the Z test because your cursor is already there. But since it has alpha, it shows through to the back buffer.

Also, you probably want to use ONE_MINUS_SRC_ALPHA, not ONE

Yep, that was it.

Thanks alot man.

I get so confused with which blending method to use:

GL_ONE_MINUS_SRC_ALPHA
GL_ONE
GL_SRC_ALPHA
etc.

What's the difference between all of these? I've used them all before, but I can never see the diffference. OpenGL documentation just gives some weird math results:

(f, f, f, 1); f=min(As, 1-Ad)
(1, 1, 1, 1)-(Ad, Ad, Ad, Ad)

Maybe they expect people to know this, but it just confuses me further.

It's hard to say "this is how it's going to look" so the math is the best way to explain it. I don't remember what they each get me (despite the fact that the name implies that) so I always wind up looking them up, and experimenting with each. Just know that ONE_MINUS_SRC_ALPHA is most likely what you'll want.