cv::Scalar not displaying expected color

Question

On an image frame, I use

void ellipse(Mat& img, Point center, Size axes, double angle, double startAngle, double endAngle, const Scalar& color, int thickness=1, int lineType=8, int shift=0)

to draw an ellipse and I want to set the ellipse color to green [ RGB value : (165, 206, 94) ]. So I set the parameter const Scalar& color to

cv::Scalar(94.0, 206.0, 165.0, 0.0); // as BGR order, suppose the value is 0.0 - 255.0
cv::Scalar(94.0/255.0, 206.0/255.0, 165.0/255.0, 0.0); // suppose the value is 0.0 - 1.0

I also tried RGB alternative.

CV_RGB(165.0, 206.0, 94.0); // as RGB order, suppose the value is 0.0 - 255.0
CV_RGB(165.0/255.0, 206.0/255.0, 94.0/255.0); // suppose the value is 0.0 - 1.0

But the color being displayed is white [ RGB value (255, 255, 255) ] , not the desired green one.

What I missed at this point? Any suggestion please. Thank you.

EDIT:

Let me put whole related code here. According to OpenCV iOS - Video Processing, this is the CvVideoCamera config in - (void)viewDidLoad;:

self.videoCamera = [[CvVideoCamera alloc] initWithParentView:imgView];
[self.videoCamera setDelegate:self];
self.videoCamera.defaultAVCaptureDevicePosition = AVCaptureDevicePositionFront;
self.videoCamera.defaultAVCaptureSessionPreset = AVCaptureSessionPreset352x288;
self.videoCamera.defaultAVCaptureVideoOrientation = AVCaptureVideoOrientationPortrait;
self.videoCamera.defaultFPS = 30;
self.videoCamera.grayscaleMode = NO;
[self.videoCamera adjustLayoutToInterfaceOrientation:UIInterfaceOrientationPortrait];

Then after [self.videoCamera start]; called, the (Mat&)image would be captured and can be processed in the CvVideoCameraDelegate method - (void)processImage:(Mat&)image; and here are the code to draw an ellipse:

- (void)processImage:(Mat&)image {

NSLog(@"image.type(): %d", image.type()); // got 24

// image.convertTo(image, CV_8UC3); // try to convert image type, but with or without this line result the same

NSLog(@"image.type(): %d", image.type()); // also 24

cv::Scalar colorScalar = cv::Scalar( 94, 206, 165 );
cv::Point center( image.size().width*0.5, image.size().height*0.5 );
cv::Size size( 100, 100 );
cv::ellipse( image, center, size, 0, 0, 360, colorScalar, 4, 8, 0 );

}

Eventually, the ellipse is still in white, not the desired green one.

Durston

Check the rest part of your code and make sure you are not changing the color value in somewhere else. The portion you have provided is perfectly showing the expected green color in my machine. – Cryo

maybe you have float type of image? which have [0 1] values range? – Selfsacrifice

@Barshan Das I've checked and commented the rest of code but the color still the same. Which one is work for you? – Durston

@Selfsacrifice What do you mean by "image"? Is it the image as the parameter in the method ellipse()? If so, it's Mat& image, how can I know its type is float or not? – Durston

you can use mat.type() – Selfsacrifice

Answer 1

Set alpha to 255 can fix this problem. Scalar(94,206,165,255)

Heraldic

Answer 2

As mrgloom points correctly in the comment, it might be because of type of your image [ the Mat object where you want to draw, i.e Mat &img in ellipse() function]. cv::Scalar(94, 206, 165) is the desired green color for 8UC3 type images. Setting these values in 32FC3 image will result in white color.

Cryo

Sorry please clarify me. I've got image.type() = 24. Does this result cv::Scalar(94, 206, 165) in white color? If so, what Mat image type should I convert to for getting the correct green color? Can I just use src.convertTo(dst, <suitable_image_type>);? Thank you. – Durston

yes you can convert the src by the convertTo function. Show the code of how you are building the src matrix. – Cryo

I've tried to convert image type with no luck. Please see the related code in the edition. Thanks again. – Durston

Answer 3

you can use

src.convertTo(src, CV_8UC3);

Where CV_8UC3 means that you use 8 bits unsigned char and 3 color image representation. More information you can find here OpenCV docs after that your ellipse should be green, if it doesn't help post the whole code.

Descendant

Tried your answer without success. Please see the whole related code in the edition. Thank you. – Durston

Answer 4

I was having similar problem and I have managed to fix it by first converting image to BGR. So in your case processImage function would look like as:

-(void)processImage:(Mat&)image
{
    cvtColor(image, image, CV_RGBA2BGR);
    cv::Scalar colorScalar = cv::Scalar( 94, 206, 165 );
    cv::Point center( image.size().width*0.5, image.size().height*0.5 );
    cv::Size size( 100, 100 );
    cv::ellipse( image, center, size, 0, 0, 360, colorScalar, 4, 8, 0 );
}

The only line which I have included in your code is:

cvtColor(image, image, CV_RGBA2BGR);

If you also log channel, depth and type information in the above function as follows:

NSLog(@"Before conversion");
NSLog(@"channels %d", image.channels());
NSLog(@"depth %d", image.depth());
NSLog(@"type %d", image.type());
NSLog(@"element size %lu", image.elemSize());
cvtColor(image, image, CV_RGBA2BGR);
NSLog(@"After conversion");
NSLog(@"channels %d", image.channels());
NSLog(@"depth %d", image.depth());
NSLog(@"type %d", image.type());
NSLog(@"element size %lu", image.elemSize());

you will see before conversion:

channels 4
depth 0
type 24
element size 4

which I think is CV_8UC4 and after conversion it becomes:

channels 3
depth 0
type 16
element size 3

which is CV_8UC3.

I guess one of the reason why it does not work without cvtColor is that the opencv drawing functions don't support alpha transparency when the target image is 4-channel as mentioned in opencv documentation. So by converting CV_RGBA2BGR we take out alpha channel. However having said that I do not managed to get it work if I do:

cvtColor(image, image, CV_RGBA2RGB);

In this Red and Blue colors are inverted in the image. So although it seems to work but I am not sure if it is the actual reason.

Yahiya