VCL graphics code snippet to perform a threshold filter operation on a VCL TImage.

There are two types of ThresholdFilter operations: ThresholdFilterColor and ThresholdFilterNoColor. ThresholdFilterColor makes a pixel black if its color is below the threshold. ThresholdFilterNoColor additionally makes a pixel white if it is above or equal to the thresholdvalue. This example image show the difference between the two versions.

The tool ThresholdFilterer demonstrates the use of ThresholdFilter.

* View the code of 'ThresholdFilter' in plain text.

#include <cassert>

#include <vcl.h>

//If a pixel's grey value is below threshold, the pixel is made black

//Else it keeps it color

//From http://www.richelbilderbeek.nl/CppThresholdFilter.htm

void ThresholdFilterColor(

const TImage * const imageOriginal,

const int threshold,

TImage * const imageResult)

{

assert(imageOriginal!=0

&& "imageOriginal must not be NULL");

assert(imageOriginal->Picture !=0

&& "Picture of imageOriginal must not be NULL");

assert(imageOriginal->Picture->Bitmap !=0

&& "Bitmap of imageOriginal must not be NULL");

assert(imageOriginal->Picture->Bitmap->PixelFormat == pf24bit

&& "Bitmap of imageOriginal must be 24 bit");

assert(imageResult!=0

&& "imageResult must not be NULL");

assert(imageResult->Picture !=0

&& "Picture of imageResult must not be NULL");

assert(imageResult->Picture->Bitmap !=0

&& "Bitmap of imageResult must not be NULL");

assert(imageResult->Picture->Bitmap->PixelFormat == pf24bit

&& "Bitmap of imageResult must be 24 bit");

assert(threshold >= 0 && threshold < 256

&& "Threshold must be in range [0,255]");

const int width = imageOriginal->Picture->Bitmap->Width;

const int height = imageOriginal->Picture->Bitmap->Height;

imageResult->Picture->Bitmap->Width = width;

imageResult->Picture->Bitmap->Height = height;

for (int y=0; y!=height; ++y)

{

const unsigned char * const lineOriginal

= static_cast<unsigned char *>(imageOriginal->Picture->Bitmap->ScanLine[y]);

unsigned char * const lineResult

= static_cast<unsigned char *>(imageResult->Picture->Bitmap->ScanLine[y]);

for (int x=0; x!=width; ++x)

{

const int grey

= (lineOriginal[x*3+2] + lineOriginal[x*3+1] + lineOriginal[x*3+0]) / 3;

if (grey < threshold)

{

lineResult[x*3+2] = 0; //Red

lineResult[x*3+1] = 0; //Green

lineResult[x*3+0] = 0; //Blue

}

else

{

lineResult[x*3+2] = lineOriginal[x*3+2]; //Red

lineResult[x*3+1] = lineOriginal[x*3+1]; //Green

lineResult[x*3+0] = lineOriginal[x*3+0]; //Blue

}