RedCV: Basic Thresholding Operations

Note: The explanation below can be found in the book Learning OpenCV 
by Bradski and Kaehler.

Thresholding is a basic technique for separating out regions of an image corresponding to objects which we want to analyze. This separation is based on the variation of intensity between the object pixels and the background pixels. To differentiate the pixels we are interested, we perform a comparison of each pixel intensity value with respect to a threshold. Once we have separated the important pixels, we can set them with a determined value to identify them.

Types of thresholding

RedCV offers the function rcvThreshold to perform thresholding operations. This function allows 5 refinements that effectuate different types of thresholding operations. This function requires grayscale image and thus applies an automatic conversion to colored image.

rcvThreshold/binary: if the intensity of the pixel is higher than thresh, then the new pixel intensity is set to maxValue (i.e., 255)

rcvThreshold/binaryInv: If the intensity of the pixel is higher than thresh, then the new pixel intensity is set to 0. Otherwise, it is set to maxValue.

rcvThreshold/trunc: The maximum intensity value for the pixels is thresh. if pixel is greater, then its value is truncated.

rcvThreshold/toZero: If the pixel is lower than thresh, the new pixel value will be set to 0.

rcvThreshold/toZeroInv: If the pixel is greater than thresh, the new pixel value will be set to 0.

Code sample

As can be seen, the code is easy to understand and illustrates the elegance of Red language. Thresholding operations are defined into the ops block as an ordered list. Selected operation op is called by using Do instruction. Since maxVal is only required by /binary and /binaryInv refinements, the slider controlling maxValue is hidden in other cases.

    Title:   "Thresholding Operations"
    Author:  "Francois Jouen"
    File:    %threshold2.red
    Needs:   'View
]

#include %../../libs/redcv.red ; for redCV functions
margins: 10x10
defSize: 512x512
img1: rcvCreateImage defSize
dst:  rcvCreateImage defSize
isFile: false
thresh: 128
maxValue: 255

ops: [
    [rcvThreshold/binary img1 dst thresh maxValue]
    [rcvThreshold/binaryInv img1 dst thresh maxValue]
    [rcvThreshold/trunc img1 dst thresh maxValue]
    [rcvThreshold/toZero img1 dst thresh maxValue]
    [rcvThreshold/toZeroInv img1 dst thresh maxValue]
]


loadImage: does [
    isFile: false
    canvas/image/rgb: black
    canvas/size: 0x0
    tmp: request-file
    if not none? tmp [
        fileName: to string! to-local-file tmp
        win/text: fileName
        img1: rcvLoadImage tmp
        dst:  rcvCloneImage img1
        ; update faces
        if img1/size/x >= defSize/x [
            win/size/x: img1/size/x + 20
            win/size/y: img1/size/y + 260
        ] 
        either (img1/size/x = img1/size/y) [bb/size: 60x60] [bb/size: 80x60]
        canvas/size: img1/size
        canvas/image/size: canvas/size  
        canvas/offset/x: (win/size/x - img1/size/x) / 2
        bb/image: img1
        canvas/image: dst
        isFile: true
        do op: ops/1
        r1/data: true
        r2/data: false
        r3/data: false
        r4/data: false
        r5/data: false
    ]
]

; ***************** Test Program ****************************
view win: layout [
        title "Basic Thresholding Operations"
        origin margins space margins
        button 60 "Load"        [loadImage] 
                    
        button 60 "Quit"        [rcvReleaseImage img1 
                                rcvReleaseImage dst Quit]
        return
        bb: base 80x60 img1
        return
        r1: radio "Binary"          [sl2/visible?: true  do op: ops/1]
        r2: radio "Binary Inverted" [sl2/visible?: true  do op: ops/2]    
        r3: radio "Truncate"        [sl2/visible?: false do op: ops/3]   
        r4: radio "To 0"            [sl2/visible?: false do op: ops/4]
        r5: radio "To 0 Inverted"   [sl2/visible?: false do op: ops/5]       
        return
        text 100 "Threshold value"  
        sl1: slider 340 [thresh: to integer! face/data * 255 f1/data: form thresh do op]
        f1: field 50 "128" 
        return
        text 100 "Maximal value" 
        sl2: slider 340 [maxValue: to integer! face/data * 255 f2/data: form maxValue do op]
        f2: field 50 "255"
        return
        canvas: base 512x512 dst    
        do [r1/data: true sl1/data: 0.5 sl2/data: 1.0]
]

And the result