Transparency Question?

I wish to draw an image to the screen, but I also want to adjusts its alpha value’s with a slider control.

I am using OpenIL so the Texture is loaded and passed to Opengl without a pointer to the textures pixels.

Is the best and most efficient way to do this:

use glGetTeximage to get a pointer to the pixels of the texture.

then manually manipulate the pixels alpha value in a loop.

And finally use glTextSubImage to place the altered texture over the top of the old one?

does this sound right?

Why not change the material properties ?


There you can directly change the transparency of your polygon’s material with having to reload the texture

I will try this paddy.

and dont wink at me yer cheeky monkey:-)

Finally someone with humor on these boards

Ooops sorry …

Hey paddy do a search on “The Ironduke” on these forums.

We use to have a laugh

Paddy I dont think material will make the texture transparent thus showing other objects beneath it?

or will it?

please explain.

Yes it will !
First set the blending mode :


Then set the material properties :

GLfloat glparms[4];


glMaterialfv(GL_FRONT_AND_BACK, GL_AMBIENT, glparms);
glMaterialfv(GL_FRONT_AND_BACK, GL_DIFFUSE, glparms);
glMaterialfv(GL_FRONT_AND_BACK, GL_SPECULAR, glparms);

You can of course use different colors for ambient, diffuse and specular !
If you use an alpha value smaller than 1.0, the material will be rendered with transparency.

If you want the object to ‘glow’ in the dark :

glMaterialfv(GL_FRONT_AND_BACK, GL_EMISSION, glparms);

What you have to do is to render the polygons from farthest to nearest for correct transparency management.


PS : Damn english, have to edit the post again (i’m French )

[This message has been edited by paddy (edited 01-28-2001).]

A french man called paddy?

Maybe patrice? Cancelled french in school last year… But the girls are gerat

My real name is Patrick … but i love Ireland and Whiskey !
And yes, french girls are great… other things are cool in France too, it’d worth a visit

I’ve been three times to france. Next holidays I’ll make a trip there with my sister again. Will be great fun. Though I think that french people like their joints better than the girls…

Thanx paddy it worked although I had to remove the line.

glTexEnvi…etc as this reversed the colours of the original texture when alpha was = 1 in the material colour.

do you know why?