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Blending problem.

Started by As-12 May 17, 2006 at 9:10 AM 2 replies 700+ views

Original Post

May 17, 2006 at 9:10 AM

I have a problem with blending in opengl.. This is my code. THe problem is the object blend nothing and result with 1 rectangle and another rectangle behind but it cant see both of it. I do enable blend and put blendfunc(); and stuff. But Still not working. Anyideas to fix it? My code void glRect(GLfloat width,GLfloat height) { glBegin(GL_QUADS); glVertex3f((width) / 2,height / 2,0); glVertex3f(-(width) / 2,(height / 2),0); glVertex3f(-(width) / 2,-(height / 2),0); glVertex3f((width) / 2,-(height / 2),0); glEnd(); } void Display() { glClearColor (0.0f, 0.0f, 0.0f, 0.0f); glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glClearDepth(1.0); glEnable(GL_DEPTH_TEST); glShadeModel(GL_FLAT); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(52.0f,256/256,1,1000); glViewport(0,0,256,256); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glTranslatef(0,0,-5); glPushMatrix (); glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA); glEnable(GL_BLEND); glColor4f(0,0,1,0.5); glTranslatef(0,0,0.3); glRect(2,2); glTranslatef(0,0,-0.3); glColor4f(1,0,0,1); glRect(1,3); glPopMatrix (); }

levjs

May 17, 2006 at 9:34 AM

When you post that much code, try to use tags

.

But as to your problem, I think you want to disable depth testing before you use blending.
Levi

Kalidor

May 17, 2006 at 9:56 AM

Quote:
Original post by levjs
...
I think you want to disable depth testing before you use blending.
...

In this case that should work. In general you would want to render all opaque objects, disable depth writing only and leave depth testing enabled (so that transparent objects are still occluded by opaque objects that are in front of them), and then render transparent objects from back to front with blending enabled. Use glDepthMask to enable/disable depth writing.

There are a couple other things I notice that are slightly odd to me.

Quote:
glColor4f(0,0,1,0.5);
glTranslatef(0,0,0.3);
glRect(2,2);
glTranslatef(0,0,-0.3);

glColor4f(1,0,0,1);
glRect(1,3);

That looks like you are rendering one quad and then another quad behind it. This will not blend correctly; you need to render objects further away first. You will still get blending, but it will be the further object blending with the closer object instead of the other way around. For some blend modes this will have the same result, but not for regular alpha blending (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA).

And there is already a glRect* function. You shouldn't be making your own. You shouldn't use the gl prefix on your own functions either because it makes it confusing for other people when they read your code, or for yourself if you're reading some of your old code.

Hope that helped.

As-12

Author

As-12

May 17, 2006 at 11:52 PM

Thx alot for the advice. Well from the book it didn't say about the order. So I thought that I have to draw transparent object 1st then draw the solid object after. But I was wrong. And Sry I shouldn't use gl prefix for my function. So I will improved . Thanks a lot ..

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