Transparent Polygon

[adrianboimvaser]

This is a more an OpenGL related question. I have a custom layer that renders polygons using OpenGL primitives. I´m trying to make this polygons semitransparent so terrain is visible. Anybody have a clue?

[remleduff]

// Enable transparency
gl.glEnable(GL.GL_BLEND);
gl.glBlendFunc(GL.GL_SRC_ALPHA, GL.GL_ONE_MINUS_SRC_ALPHA);


Make sure your polygon is rendered after whatever is obscured by it.

Its transparency is based on the alpha value of the polygon's color.

[patmurris]

Note that for OGL textures, you should use GL.GL_ONE instead of GL.GL_SRC_ALPHA. This has to do with pre multiplied alpha blending - which is a source of confusion... See Texture's javadoc (http://download.java.net/media/jogl/builds/nightly/javadoc_public/com/sun/opengl/util/texture/Texture.html). Not sure how it impacts polygons though.

[adrianboimvaser]

I´ve already tried that.

I set the color this way:

gl.glColor4ub((byte)Color.RED.getRed(), (byte)Color.RED.getGreen(), (byte)Color.RED.getBlue(), (byte)100);

All I get is a darker red polygon, but no transparency.

[remleduff]

Are you sure your polygon isn't being drawn too early?

If it's drawn first and updates the depth buffer, anything underneath it will get culled.

You can try gl.glDisable(GL.GL_DEPTH_TEST) while drawing your polygons, though I don't know if that is a good final solution for you or not.

[slade]

You can also try:

// Apply the depth buffer but don't change it.
gl.glEnable(GL.GL_DEPTH_TEST);
gl.glDepthMask(false);

[Shadre]

You may know this, so I apologize if you do.

To get nice transparency you have to use "lighter" shades of colors as well. So like a red value of 10-50 will do a _much_ nicer transparency than a red of 255. Each color set behaves the same way, using dimmer colors results in better transparency.

Using high alpha, like 255 and dim colors like 10-50 for each of r g and b results in the best effects. Maxing out any one channel of r,g or b seems to ruin transparency somewhat.

[Shadre]

Doesn't SurfaceQuad support alpha? I have screen shots during my development using a transparent SurfaceQuad...

-Nick

[bianconera]

Hi Nick, did the sufacequad shape turn out a little bit blurry on the outline, or was it with a perfectly sharp outline (border)?

Thanks

[patmurris]

Be aware that the initial question for this thread is about polygon transparency/blending using gl primitives. The actual surface shape implementation uses textures - which become somewhat blurry when you zoom too close, but do support transparency too. This is a different issue.