View Full Version : How to get alpha?

Tom Wood
01-04-2007, 07:15 PM
I used a USflag object in the content library to make a field of stars against a black background by changing the blue in the original surface to black, and then zooming in close to the stars. It's not an image map, the stars and background are each made up of many polygons. When I render out a still of the resulting image, I don't ever get an alpha channel even though I select TGA32 as the image type. How come?

I also tried making the black area transparent with the same result.

I also used Mirage to stamp a bunch of copies to make a larger field, rendered out a TGA, imaged mapped that to the side of a big rectangle, rendered out a TGA32, and still no alpha. ??



01-04-2007, 10:26 PM
Isn't the alpha a based on the object? In this case would you have to have the stars be a separate object?

01-04-2007, 11:14 PM
You would be right, the alpha is based on the object.

@Tom Wood
It may be possible to place the background on another layer, then layout should see two objects.
When using a targa in an image editor like Mirage, photoshop, etc. the alpha on your current image would be around the entire image rather than each star. So cloning wouldn't work, unless you first deselect the mask and select each star using one of the masking tools like the magic wand. Once each star had a mask then the alpha could be available for each star. Note: not all image editors treat masks and alpha channels as the same thing. It may be necessary to convert the mask to an alpha channel.
If you are not interested in the background, then just delete that poly. Then the alpha channel should work correctly for your stars.

Tom Wood
01-05-2007, 04:39 AM

I ended up deleting all the polys in the black field. Tedious but it worked.


01-05-2007, 08:17 AM
If that was your final image you posted, why didn't you just reduce the colors to 2 colors (black and white) in a paint program?? You would have had an instant alpha map.

Tom Wood
01-05-2007, 09:44 AM
Because I'm not smart enough? :)

I figured there must be some way to do that since it is just black and white. How do I make the black transparent?

01-05-2007, 11:08 AM
Do you have Ray-traced Transparency turned on? Haven't tried it in a while, but that use to cause the alpha channel to not recognize transparency in an object.

01-05-2007, 11:25 AM
How are you using the image? If the background is just a solid color then you don't need a black object for the field behind the stars, just set your background color to black and the alpha will be generated when rendered.

In Photoshop, you can CTRL-click the composite channel to select the luminance of the image and generate an alpha that way with a black and white image.

Tom Wood
01-05-2007, 01:05 PM
I made a looping graphic of stars coming at the edge of the screen. I rendered whats shown in the camera view below (the camera moves along the front of the stars) then copied and flipped it to the left edge too. I initially wanted the far ends of the starfields to overlap at the center, hence the desire for transparency, but it was too busy - lots of Moire effect. So I ended up fading them out together at the center so I didnt need alpha after all. Ah well, its the journey that counts.


I just thought there should be a way to transform a simple black and white to some sort of white with transparency.

BTW, whenever I type an apostrophe in the reply box, Firefox starts a search operation. How do I turn that off?