How to create a semi-transparent image?

Hi there,
I’m loading a TGA file and i want to use from its alpha value. As you know, in photoshop, we can select the appropriate parts of the image and then create a qucik mask.In this case, our image has two alpha values: 0 and 1. If we use from OpenGL with the following function, we can clip part of the image:

Now i need to use from an alpha value of 0.5 for part of my image. If i create a quick mask, i have two alpha values: 0 and 1. So i can not add an alpha value of 0.5. So how can i add an alpha value other than 0 and 1 to my targa(*.TGA) image?

I found the response. In photoshop, we can select many colors for the alpha channel. So if we don’t use from the black and white colors, we can specify other alpha values .