GetPixel returns incorrect values

问题

This is my first post on this forum, please don't go hard on me if I didn't post it in the right place or if I did something wrong, I don't post in forums very much. So, I have this problem with GetPixel function. Basically, it should return the color decimal at x, y. The code I am about to post works perfectly on windows 7 32bit, but recently I bought a new laptop y50-70 with windows 8.1 64bit and the same code works completely different. I can't find any solution to the problem, can't even describe it. I think it could have something to do with desktop handle, HDC, GetDC(), GetPixel(), maybe even with my computer resolution, refresh rate or something like that... I have even recorded some videos which could help you understand the problem I am having because I can't even describe it correctly. It is like the real color is x = 219, y = 407 away from the place, where my mouse is pointing. The new laptop is 3 weeks old, I even tried to do system restore 1 time, but that didn't solve the problem.

Feel free to use this code, hope it will work fine for you:

#include <iostream>
#include <Windows.h>
using namespace std;

void Detect();

int main()
{
    Detect();

    return 0;
}

void Detect()
{
    POINT p;
    HDC hDC = GetDC(0);
    int x, y;

    while (!GetAsyncKeyState(VK_INSERT)) // Press insert to stop
    {
        GetCursorPos(&p);
        x = p.x;
        y = p.y;
        hDC = GetDC(0);
        cout << x << " " << y << " " << GetPixel(hDC, x, y) << endl;
        Sleep(50);
    }
    ReleaseDC(0, hDC);
}

Links to the problem below: https://youtu.be/q2H2M8WLHVI https://youtu.be/UcneHwXaGoM

If anyone could at least help somehow or tell what to do, where to go, I would very, very much appreaciate it. One of the main reasons why I started programming is because something like this, working with colours, conditions, etc... and now I can't advance further which is really sad. Hope to hear a reply. Thank you.

回答1:

This is probably an issue with DPI scaling.

If your new monitor has a higher-than-average number of dots per inch, then Windows, by default, will stretch the graphics. By default, Windows assumes that programs ignore the DPI. If Windows didn't stretch the graphics, then programs that didn't adjust for the DPI would have tiny text in tiny windows on high-density displays.

This is a bit of a hack. Some of the Windows APIs and messages that let you ask about the display and windows will translate the coordinates between an "average" 96 pixels-per-inch and whatever the actual DPI is for the monitor. Likewise, the APIs that let you size things do the opposite transformation. So this is all largely transparent to the program. But it's not perfect because not all the APIs can do the scaling in a consistent manner.

So my guess is that your laptop has a high-resolution display, GetPixel doesn't translate coordinates for DPI scaling, and the mouse position is transformed for DPI scaling. The result is that you're asking for a pixel that doesn't really line up with the mouse.

My suggested solution is the tell windows that your program is "DPI-aware". There are several ways to do this. The easiest in your case might be to call SetProcessDPIAware right at the beginning of your program. You could also do this be marking your program in the manifest. Depending on the compiler you're using, there might be a command line option to create the manifest you need automatically.

回答2:

Another alternative that has worked for me, is to change a setting in the Control Panel, to disable per-device DPI Scaling. Are you running multiple monitors?

Win 8.1 introduced a new setting in the Control Panel, which says "Let me choose one DPI scaling for all displays". By defualt this is turned off. Turning it on has worked for me.

See this video to find the setting: https://www.youtube.com/watch?v=sE3IUTPy1WA

I'm curious, so let me know if that works for you, when you don't use SetProccesDPIAware()

来源：https://stackoverflow.com/questions/29678763/getpixel-returns-incorrect-values

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getpixel