Python/OpenCV: Converting images taken from capture

Question

Hello. I'm trying to convert images taken from a capture (webcam) and do some processing on them with OpenCV, but I'm having a difficult time..

When trying to convert the image to grayscale, the program crashes. (Python.exe has stopped working)

Here is the main snippet of my code:

newFrameImageGS = cv.CreateImage ((320, 240), cv.IPL_DEPTH_8U, 1)

for i in range(0,5):
    newFrameImage = cv.QueryFrame(ps3eye)
    cv.CvtColor(newFrameImage,newFrameImageGS,cv.CV_BGR2GRAY)
    golfSwing.append(newFrameImageGS)

When I try using cvConvertScale I get the assertion error:

src.size() == dst.size() && src.channels() == dst.channels()

which makes sense, but I'm pretty confused on how to go about converting the input images of my web cam into images that can be used by functions like cvUpdateMotionHistory() and cvCalcOpticalFlowLK()

Any ideas? Thanks.

UPDATE:

I converted the image to grayscale manually with this:

for row in range(0,newFrameImage.height):
            for col in range(0,newFrameImage.width):
                newFrameImageGS[row,col] = (newFrameImage8U[row,col][0] * 0.114 + # B
                                            newFrameImage8U[row,col][1] * 0.587 + # G
                                            newFrameImage8U[row,col][2] * 0.299)  # R

But this takes quite a while.. and i still can't figure out why cvCvtColor is causing the program to crash.

Answer 1

There is a common mistake here:

You're creating a single image in the newFrameImageGS variable before the loop, then overwrite its contents in the loop, which is then appended to a list. The result will not be what you would expect. The list will contain five references to the same image instance at the end, since only the object reference is appended to the list, no copy of the object made this way. This image will contain the very last frame, so you get five of that frame as a result, which is not what you want, I guess. Please review the Python tutorial if it is not clear for you. You can solve this by moving the first line of the above code into the body of the for loop.

Another possibilities if fixing the above would not help you:

The CvtColor function seems to be the correct one for conversion to grayscale, since it can convert to a different number of channels.

According to this manual the CvtColor function requires a destination image of the same data type as the source. Please double check that newFrameImage is a IPL_DEPTH_8U image.

Answer 2

For some reason, CvtColor caused the program to crash when the image depths where 8 bit. When I converted them to 32 bit, the program no longer crashed and everything seemed to work OK. I have no idea why this is, but at least it works now.

newFrameImage = cv.QueryFrame(ps3eye)

newFrameImage32F = cv.CreateImage((320, 240), cv.IPL_DEPTH_32F, 3)
cv.ConvertScale(newFrameImage,newFrameImage32F)

newFrameImageGS_32F = cv.CreateImage ((320,240), cv.IPL_DEPTH_32F, 1)
cv.CvtColor(newFrameImage32F,newFrameImageGS_32F,cv.CV_RGB2GRAY)

newFrameImageGS = cv.CreateImage ((320,240), cv.IPL_DEPTH_8U, 1)
cv.ConvertScale(newFrameImageGS_32F,newFrameImageGS)