Can't find error in pixel searching algorithm - c++

Question

Ok, I'm trying to make a program that finds the position of a colored pixel within the desktop. To do this I make a screenshot of the desktop then go through the pixels and search for the one that has the matching RGB as i need. The only problem is that my program returs strange coordonates X,Y for the found pixel...
#include <stdio.h>
#include <windows.h>
#include <atlimage.h>
#include <iostream>
using namespace std;
struct rgbcolor{
int red;
int green;
int blue;} myColor;
struct point{
int x;
int y;
};
point SearchPixel(int r,int g, int b){
CImage bitmapzor;
bitmapzor.Load(("C:\\1.bmp"));
COLORREF PixColor=0; //This is a color data
int R=0,G=0,B=0; //These are the channel values
BYTE* byteptr = (BYTE*)bitmapzor.GetBits();
int ok=0;
int pitch = bitmapzor.GetPitch(); //This is a pointer offset to get new line of the bitmap
//Go through every pixel and compare the RGB code
for (int i=0; i<bitmapzor.GetWidth();i++)
for (int j=0; j<bitmapzor.GetHeight();j++)
{
B= *(byteptr+pitch*j+3*i);
G= *(byteptr+pitch*j+3*i+1);
R= *(byteptr+pitch*j+3*i+2);
if(R==r&&G==g&&B==b)
{ point p;
p.x=i;
p.y=j;
cout<<"First pixel found at:\n X:"<<p.x<<"\n Y:"<<p.y<<"\n-----------------\n";
return p;
}
}
bitmapzor.Destroy(); //destroy the bitmap
point p;
p.x=-1;
p.y=-1;
cout<<"Pixel not found!\n";
return p;
}
bool ScreenCapture(int x, int y, int width, int height, char *filename){
// get a DC compat. w/ the screen
HDC hDc = CreateCompatibleDC(0);
// make a bmp in memory to store the capture in
HBITMAP hBmp = CreateCompatibleBitmap(GetDC(0), width, height);
// join em up
SelectObject(hDc, hBmp);
// copy from the screen to my bitmap
BitBlt(hDc, 0, 0, width, height, GetDC(0), x, y, SRCCOPY);
CImage image;
image.Attach(hBmp);
image.Save(("C:\\1.bmp"), Gdiplus::ImageFormatBMP);
SearchPixel(myColor.red,myColor.green,myColor.blue);
// free the bitmap memory
DeleteObject(hBmp);
return 1;
}
int main()
{ //RGB for the searched color
myColor.red=200;
myColor.green=191;
myColor.blue=231;
int count=0;
while(true){
ScreenCapture(0, 0, 1366, 768, "c:\\1.bmp");
count++;
cout<<"Number of searches:"<<count<<"\n\n";
Sleep(500);
}
system("pause");
return 0;
}

Answer 1

Well, this code could be simplified quite a lot, but first I'd suggest you try a much, much simpler case, like finding a known integer in a small array of integers. Once you have that working, move up to more complex cases.
EDIT:
You have enough knowledge to do this? Cristy, please don't take this the wrong way, but of all the programmers I've had to deal with, the worst have been the ones who thought they had nothing to learn. I didn't actually look for the bug in your code because your code is overcomplicated, and if you had gone from simple to complex when building it, you would have caught your error a lot sooner.

Answer 2

I suggest that you print out the RGB values of the first few pixel to make sure you're even grabbing the pixel data correctly. If your offsets are wrong, you'll never get this to work.

Answer 3

Ok, finally I found a mistake that I think is the cause of this problem.
The pointer arithmetics:
B= *(byteptr+pitch*j+3*i);
G= *(byteptr+pitch*j+3*i+1);
R= *(byteptr+pitch*j+3*i+2);
Are done for 24bits images, but the screenshot is a 32bits image that means I have to change the multiplier to 4 insteand of 3. That means:
B= *(byteptr+pitch*j+4*i);
G= *(byteptr+pitch*j+4*i+1);
R= *(byteptr+pitch*j+4*i+2);
I think... I'll be back with an edit if this works or not.
EDIT: Yes it works, it gives the corect pixel position. Thanks everyone for your help :) .