Golang : Changing a RGBA image number of channels with OpenCV

Problem:

Golang image packages deal with RGBA ( 4 channels ) and OpenCV deals with BGR ( 3 channels ).

You found out that the image frame returned by OpenCV's video capture from a camera only has 3 channels(BGR) - without the Alpha channel. You need to convert the image frame to BGRA ( 4 channels ).

Also, you would like to convert BGRA ( 4 channels ) to BGR (3 channels) as well. How to do that?

Solution:

First, create a new opencv.IplImage and with the number of channels that you want. If you want to convert the image to grayscale, set the number of channel to 1.

Next, use opencv.cvtColor() to convert the image.

Just a note, you don't have to stick to the limited constants found in Go-OpenCV package.

  const (
 CV_BGR2GRAY  = C.CV_BGR2GRAY
 CV_RGB2GRAY  = C.CV_RGB2GRAY
 CV_GRAY2BGR  = C.CV_GRAY2BGR
 CV_GRAY2RGB  = C.CV_GRAY2RGB
 CV_BGR2BGRA  = C.CV_BGR2BGRA
 CV_RGBA2BGRA = C.CV_RGBA2BGRA

 CV_BLUR_NO_SCALE = C.CV_BLUR_NO_SCALE
 CV_BLUR = C.CV_BLUR
 CV_GAUSSIAN = C.CV_GAUSSIAN
 CV_MEDIAN = C.CV_MEDIAN
 CV_BILATERAL = C.CV_BILATERAL

 CV_8U  = C.CV_8U
 CV_8S  = C.CV_8S
 CV_16U = C.CV_16U
 CV_16S = C.CV_16S
 CV_32S = C.CV_32S
 CV_32F = C.CV_32F
 CV_64F = C.CV_64F
  )

You can use the integer enumerations at [http://docs.opencv.org/3.1.0/df/d4e/group__imgproc__c.html][1] in opencv.cvtColor() function.

Here you go!

  func opencvImageBGRToBGRA(img *opencv.IplImage) opencv.IplImage {
  // The image frames from camera is in RGB (3 channels )
  // We need to convert the frames to RGBA (4 channels )
  // so that we can perform copy and paste the UTF8 strings
  // into the region of interest.
  // Using the ToImage() function will work, but will cause delay in refresh rate.
  // Use CvtColor() function for the best result

  w := img.Width()
  h := img.Height()

  // create an IplImage with 4 channels
  tmp := opencv.CreateImage(w, h, opencv.IPL_DEPTH_8U, 4)

  // upgrade BGR to BGRA ( 3 to 4 channels)
  opencv.CvtColor(img, tmp, opencv.CV_BGR2BGRA)
  return *tmp
 }

 func BGRAToBGR(img *opencv.IplImage) opencv.IplImage {
  w := img.Width()
  h := img.Height()

  // create a IplImage with 3 channels
  tmp := opencv.CreateImage(w, h, opencv.IPL_DEPTH_8U, 3)

  // downgrade BGRA to BGR ( 4 to 3 channels)
  opencv.CvtColor(img, tmp, 1)
  // why use integer value of 1?
  // see http://docs.opencv.org/3.1.0/df/d4e/group__imgproc__c.html
  return *tmp
 }

NOTES:

See request for RGB and RGB48 support in Golang discussion : https://groups.google.com/forum/#!topic/golang-nuts/z7W0nhBfCho