openCV: adding transparency to IplImage

Question

I have a 3-channel IplImage. I would like to create a 4-channel image and set the alpha channel for it to a value less than 1.0 to make it semi-transparent.

First I set the alpha channel (the 4-th channel) to 0.5:

cvSet(Image_c4, cvScalar(0,0,0,0.5);

Here is the command that I used to copy the 3-channel image into a 4-channel image.

cvCvtColor(Image_c3, Image_c4, CV_RGB2RGBA);

The problem: Image_c3 is in color. Image_c4 becomes a gray scale copy of Image_c3 (and with no transparency).

Update: It turned out that the code above is actually correct and works and is actually more concise than the suggested solutions in the answers below. I had an unrelated bug somewhere else.

Answer 1

Maybe there is another way but I add transparency like this:

// BGR split
cvSplit(im1_bgr, im1_b, im1_g, im1_r, NULL);

// Alpha channel creation (transparency)
IplImage *im1_a = cvCreateImage(cvGetSize(im1_bgr), 8, 1);
// Set the alpha value
cvSet(im1_a, cvScalar(128), NULL);

// Merge the 4 channel to an BGRA image
IplImage *im1_bgra = cvCreateImage(cvGetSize(im1_bgr), 8, 4);
cvMerge(im1_b, im1_g, im1_r, im1_a, im1_bgra);