Rotating an image is a common task. This article reviews how to rotate a matrix using JavaCV.

These tutorials utilize GoatImage. The Image object used in the code examples comes from this library.

Related Articles:

• JavaCv Basics: IplImage
• JavaCv Basics: Contouring
• Cropping
• Splitting Objects
• JavaCV Basics: Basic Image Processing

Rotation Matrix

The rotation matrix  is used to map from one pixel position to another. The matrix, shown below, uses trigonometric functions.

Rotation is a linear transformation. A linear transformation uses a function to map from one matrix to another. In image processing, the matrix kernel is used to perform this mapping.

Rotation in JavaCV 3

Rotation in JavaCV 3 utilizes a generated rotation matrix and the warp affine function. The function getRotationMatrix2D generates a two dimensional matrix using a center Point2f, angle, and a scale.

/**
    * Rotate an image by a specified angle using an affine transformation.
    *
    * @param image      The image to rotate
    * @param angle      The angle to rotate by
    * @return           A rotated Image
    */
  def rotateImage(image : Image,angle : Double):Image={
    val srcMat = new Mat(image.image)
    val outMat = new Mat(srcMat.cols(),srcMat.rows(),srcMat.`type`())
    val cosv = Math.cos(angle)
    val sinv = Math.sin(angle)
    val width = image.image.width
    val height = image.image.height
    val cx = width/2
    val cy = height/2

    //(image.image.width*cosv + image.image.height*sinv, image.image.width*sinv + image.image.height*cosv);
    val rotMat : Mat = getRotationMatrix2D(new Point2f(cx.toInt,cy.toInt),angle,1)

    warpAffine(srcMat,outMat,rotMat,srcMat.size())
    new Image(new IplImage(outMat),image.name,image.itype)
  }

The Angle

The angle in OpenCV and thus JavaCV is centered at -45 degrees due to the use of vertical contouring. If an image is less than -45 degrees, adding 90 degrees will correct this offset.

val angle = if(minAreaRect.angle < -45.0) minAreaRect.angle + 90 else minAreaRect.angle

Conclusion

In this tutorial we reviewed the function in GoatImage for rotating images using OpenCv. The functions getRotationMatrix2D and warpAffine were introduced. Basic kernel processing was introduced as well.