OpenCV - cvPutText is adding noise to my images

Question

I am completely new with OpenCV. I started reading Learing OpenCV a few days ago and now I would like to do some tests.

I wanted to program an ASCII art conversor. I have done the image iteration and grouped pixels by squares to calculate each region brightness to associate it to a respective character. So now I need to know how bright is a character in order to link each image region to a character. My idea was to make another program that gives me the brightness of some characters. The approach is as follows:

• Create an image of size 100*100 (for example) - black background and 1 channel
• Draw the character somewhere in it in white color (cvPutText)
• Iterate the image and count the number of white pixels
• Normalize the result

I think that my program is ok, but I'm getting strange results... Just to check it, I am displaying the images with each character and something strange is going on. The image shows what I am talking about.

This is the code I am using:

... 
char c = 'a'; //or whatever
IplImage *img = cvCreateImage(cvSize(150, 150), IPL_DEPTH_8U, 1);

char buffer[7];
sprintf(buffer, "%c", c);

CvFont font;
cvInitFont(&font, CV_FONT_HERSHEY_SIMPLEX, 0.5, 0.5);
cvPutText(img, buffer, cvPoint(2, 15), &font, cvScalar(255));

//iteration to calculate brightness will go here

cvNamedWindow(buffer);
cvShowImage(buffer, img);
cvMoveWindow(buffer, 50*(c-first), 20);
...

I am developing on MacOS Lion (if it helps...). Thank you for reading (and helping!).

Answer 1

You did not initialize the image content: you've just allocate the memory and you see some kind of memory garbage.

Just add cvSet(img, cvScalarAll(0)) to set your image to black.