C语言GetPixel函数获得的值不正确？

Question

编写的图片识别程序，找到屏幕上的20个像素点然后和已保存的BMP文件作对比，每次GetPixel函数获得的值RGB均是255也就是白色，然后我把GetPixel函数单独拿出来对指定的窗口遍历又用SetPixel函数画了出来他的结果就是对的。
值正确的的代码如下：

#include <Windows.h>
#include <iostream>
#include <stdio.h>

using namespace std;

typedef struct
{
    BYTE b;
    BYTE g;
    BYTE r;
}RGB;

HWND hnd;
HDC hdc;
HWND hnd1;
HDC hdc1;
COLORREF image1;


RGB *cutimage=NULL;


int CutImage(int PointX,int PointY)
{

    cutimage=(RGB*)malloc(3);
    COLORREF image;
    image = GetPixel(hdc, PointX, PointY);
    cutimage->b = GetBValue(image);
    printf("b is:%d\t",cutimage->b);
    cutimage->g = GetGValue(image);
    printf("g is:%d\t",cutimage->g);
    cutimage->r = GetRValue(image);
    printf("r is:%d\t\n",cutimage->r);
    SetPixel(hdc1,PointX+10,PointY+200,image);
    return 1;
}

int main()
{
    hnd1=FindWindow(NULL,"无标题 - 画图");
    hdc = GetDC(hnd);
    hdc1=GetDC(hnd1);

    hnd=FindWindow(NULL,"阴阳师-网易游戏");
    image1 = GetPixel(hdc, 0, 0);
    SendMessage(hnd,MoveWindow(hnd,0,0,1000,570,true),0,0);
    int a = 0;
        system("pause");
    for (int i=0;i<20;i=i+1)
    {
        cout<<i<<endl;
        CutImage(405+i,297);
    }
    system("pause");
    return 1;
}

值错误的代码如下：

#include "ExternType.h"

    /*Test Start*/

RGB *BMP1 = (RGB*)malloc(60);
RGB *BMP2 = (RGB*)malloc(60);
RGB *BMP3 = (RGB*)malloc(60);
RGB *BMP4 = (RGB*)malloc(60);
RGB *cutimage = (RGB*)malloc(60);

RGB *F1 = BMP1;
RGB *F2 = BMP2;
RGB *F3 = BMP3;
RGB *F4 = BMP4;
RGB *F5 = cutimage;

HWND hnd;
HDC hdc;


    /*Test End*/

int main()
{
    system("pause");
    int i = 0;
    int PointX[4] = { 734,734,50,50 };
    int PointY[4] = { 465,465,59,59 };
    StartUp();
    do
    {
        cout << "----------------------------" << endl;
        CutImage(PointX[i], PointY[i]);
        i++;
        if (i == 4)
            i = 0;
        switch (Compares())
        {
        case 0: {cout << "没有匹配图片！" << endl; break; }
        case 1: {cout << "等待玩家！" << endl; break; }
        case 2: {cout << "点击开始！" << endl; break; }
        case 3: {cout << "战斗结束1" << endl; break; }
        case 4: {cout << "战斗结束2" << endl; break; }
        }
    } while (i != 5);




    /*Test Start*/


    /*Test End*/

    system("pause");
    free(F1);
    free(F2);
    free(F3);
    free(F4);
    free(F5);
    return 1;
}


#include "ExternType.h"

extern RGB *cutimage;
extern HWND hnd;
extern HDC hdc;

int CutImage(int PointX,int PointY)
{
    COLORREF image;
    for (int i = 0; i < 20; i++)
    {
        image = GetPixel(hnd, PointX+i, PointY);
        cutimage->b = GetBValue(image);
        cutimage->g = GetGValue(image);
        cutimage->r = GetRValue(image);
        *cutimage++;
    }
    for (int j = 0; j < 20; j++)
        *cutimage--;
    return 1;

}

这个问题想了很久了，网上说需要创建一个兼容的DC也试了一下还是不行。

Answer 1

DC的内容没有跟着屏幕更新，我把GetDC函数放到了main里之后一切就正常啦！不过还不太清楚原因是什么

int main()
{
    hnd1=FindWindow(NULL,"无标题 - 画图");
    hdc = GetDC(hnd);
    hdc1=GetDC(hnd1);

    hnd=FindWindow(NULL,"阴阳师-网易游戏");
    image1 = GetPixel(hdc, 0, 0);
    SendMessage(hnd,MoveWindow(hnd,0,0,1000,570,true),0,0);
    system("pause");
    int i = 0;
    int PointX[4] = { 734,734,50,50 };
    int PointY[4] = { 465,465,59,59 };
    StartUp();
    do
    {
        cout << "----------------------------" << endl;
        CutImage(PointX[i], PointY[i]);
        i++;
        if (i == 4)
            i = 0;
        switch (Compares())
        {
        case 0: {cout << "标志位"<<i<<"不匹配！" << endl; break; }
        case 1: {cout << "等待玩家！" << endl; break; }
        case 2: {cout << "点击开始！" << endl; break; }
        case 3: {cout << "战斗结束1" << endl; break; }
        case 4: {cout << "战斗结束2" << endl; break; }
        }
    } while (i != 5);




    /*Test Start*/


    /*Test End*/

    system("pause");
    free(F1);
    free(F2);
    free(F3);
    free(F4);
    free(F5);
    return 1;
}