[DayLateDon]

Hello ...


I'm trying to port some graphics aspects of an app to OpenGL for (MUCH) greater performance, but I'm getting hung up on the methodology of drawing alpha-masked .tiffs (to take the place of using NSImage "compositeToPoint" commands in Cocoa).

Thanks to the Cocoa ports of sample code from the NeHe tutorials, I can load a .tiff properly; and with these settings

glEnable( GL_BLEND );
glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );
glEnable( GL_ALPHA_TEST );
glAlphaFunc( GL_GREATER, 0.0 );

I can render the images _almost_ the way I want: the effects of the alpha mask are apparent, creating black or darkened areas within the image; but I can't see _through_ those areas to what's "under" the image.

NeHe has a tutorial that covers OpenGL masking via separate black-and-white bitmaps, but I need levels of transparency. Another demo discusses loading TGA files, which support an alpha channel, but I don't see what it's doing that I'm not.

Searching the OpenGL.org discussion groups hasn't helped much, since I'm still new to a lot of the jargon and command options.

Can someone explain what I'm missing, or point me to some simple sample Cocoa/Obj-C code?


Thanks and regards,

DayLateDon

[00101001]

try JUST this:

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA,GL_ONE);

enabling alpha-test i don't think does what you want... blending alone should work.

[DayLateDon]

Hello ...

Well, now I seem to have the opposite problem: I can see through the holes and semi-transparent areas of the .tiff texture just fine, but the should-be-opaque areas are combining with the background color.

Perhaps relatedly, my scene also involves a figure created as a collection of white lines. No matter where I draw the .tiff-textured quad in relation to the line figure ("above" or "below", z-axis-wise), the quad never obscures the line figure. (Using glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA ), I get obscuration ... but then I can't see through the "holes".)


Maybe it's not just what I'm doing wrong, but _where_ I'm doing it wrong. Here's the essence of my code (in an NSOpenGLView subclass), excluding only the details of the figure drawing and texture loading:

initGL
{
( load .tiff texture --works fine)

glClearColor( .5f, .5f, .5f, 0.50f );
glClearDepth( 1.0f );
glEnable( GL_BLEND );
glBlendFunc(GL_SRC_ALPHA, GL_ONE);
}
end initGL

drawRect
{
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

( draw white line figure )

glEnable( GL_TEXTURE_2D );
glBindTexture( GL_TEXTURE_2D, ( my texture ) );

( draw .tiff-textured quad --works fine, apart from the alpha problems )

glDisable( GL_TEXTURE_2D );

[ [ self openGLContext ] flushBuffer ];
}
end drawRect


Any more suggestions?

Thanks and regards,

DayLateDon

[00101001]

another shot in the dark:

did you:
glEnable(GL_DEPTH_TEST); ??

[DayLateDon]

Hello ...


glEnable( GL_DEPTH_TEST ) ?

I did at one point, but it wasn't in my current build. Doesn't seem to help: With it enabled, only the top-most item (the textured quad or my line figure) appears.

If I enable it before drawing the textured quad (which is on top) then disable it after, I get the same thing as if I hadn't had it at all. (Enabling/disabling around the line drawing doesn't do anything.)

How does Apple draw the icons in the zooming icons screen saver?


I appreciate the help,

DayLateDon

[00101001]

in that case, i have no idea -- sorry... perhaps it's something wrong with your tiff?

[DayLateDon]

Hello ...


Well, it turns out (I think) that something was wrong with the .tiff ... not the file itself, but the way it got loaded. The texture-loading routine's call to glTexImage2d() had a "color components" value of only 3 (R, G, B); changing that to 4 (R, G, B, A) eliminated my problem with holes and semi-transparent areas of the rendered texture. (The sample code from which I took the texture-loading routine distinguishes between alphad and non-alphad images when setting some properties, and I thought it had covered all of the bases.) Note to self: The problem is always the part you _know_ isn't the problem.

I don't have alpha-testing or depth-testing enabled. Just blending, with glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA ).


Now to go figure out how to make the NSOpenGLView and the OpenGL scene's background transparent ...


Thanks for the help!

DayLateDon