用Bitblt快速读取像素颜色

Question

我正在做一个项目，在这个项目中，我必须用CreateCompatibleBitmap和Bitblt读取屏幕上像素的颜色。不幸的是，这个方法非常慢，我在一个循环中得到了大约60到100 ms的次数。我使用以下代码：

HDC hdc = GetDC(NULL), hdcMem = CreateCompatibleDC(hdc);
HBITMAP hBitmap = CreateCompatibleBitmap(hdc, ScreenX, ScreenY);
BITMAPINFOHEADER bmi = { 0 };
bmi.biSize = sizeof(BITMAPINFOHEADER);
bmi.biPlanes = 1;
bmi.biBitCount = 24;
bmi.biWidth = ScreenX;
bmi.biHeight = -ScreenY;
bmi.biCompression = BI_RGB;
SelectObject(hdcMem, hBitmap);
BitBlt(hdcMem, 0, 0, ScreenX, ScreenY, hdc, 0, 0, SRCCOPY);
GetDIBits(hdc, hBitmap, 0, ScreenY, ScreenData, (BITMAPINFO*)&bmi, DIB_RGB_COLORS);
DeleteObject(hBitmap);
DeleteDC(hdcMem);
ReleaseDC(NULL, hdc);

大部分时间(95%)用于Bitblt函数。我读到它花了很长时间，因为Bitblt必须转换颜色格式，但我不明白我如何才能避免这种情况。

我使用Windows 11，屏幕res是FHD1920x1080

有什么建议可以让我加速这个计划吗？

Answer 1

您可以使用CreateDIBSection创建一种位图，它允许您访问像素数据，而不是创建一个常规的“兼容”位图，然后使用GetDIBits将像素数据复制到缓冲区中，但是我运行了这样做的基准，这样可以节省时间，但并不是很大。我得到了大约10%的速度改善，这不是很多。

下面的代码，我写得这么快，所以可能犯了个错误。下面的变量dummy的要点是，由于没有输出，所以不会对所有内容进行优化。

#include <vector>
#include <iostream>
#include <Windows.h>
#include <chrono>

namespace c = std::chrono;

void get_screen_bytes1(void* ScreenData, int ScreenX, int ScreenY) {
    HDC hdc = GetDC(NULL), hdcMem = CreateCompatibleDC(hdc);
    HBITMAP hBitmap = CreateCompatibleBitmap(hdc, ScreenX, ScreenY);
    BITMAPINFOHEADER bmi = { 0 };
    bmi.biSize = sizeof(BITMAPINFOHEADER);
    bmi.biPlanes = 1;
    bmi.biBitCount = 24;
    bmi.biWidth = ScreenX;
    bmi.biHeight = -ScreenY;
    bmi.biCompression = BI_RGB;
    SelectObject(hdcMem, hBitmap);
    BitBlt(hdcMem, 0, 0, ScreenX, ScreenY, hdc, 0, 0, SRCCOPY);
    GetDIBits(hdc, hBitmap, 0, ScreenY, ScreenData, (BITMAPINFO*)&bmi, DIB_RGB_COLORS);
    DeleteObject(hBitmap);
    DeleteDC(hdcMem);
    ReleaseDC(NULL, hdc);
}

struct bitmap_info {
    HBITMAP handle;
    uint8_t* data;
};

bitmap_info get_screen_bytes2(int wd, int hgt) {
    HDC hdc_scr = GetDC(NULL);
    BITMAPINFO bmi;
    memset(&bmi, 0, sizeof(BITMAPINFO));
    bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    bmi.bmiHeader.biWidth = wd;
    bmi.bmiHeader.biHeight = -hgt; 
    bmi.bmiHeader.biPlanes = 1;
    bmi.bmiHeader.biBitCount = 24;
    bmi.bmiHeader.biCompression = BI_RGB;

    bitmap_info bi;
    bi.handle = CreateDIBSection(hdc_scr, &bmi, DIB_RGB_COLORS, (void**)bi.data, NULL, NULL);

    HDC hdc = CreateCompatibleDC(hdc_scr);
    auto hbm_old = SelectObject(hdc, bi.handle);
    BitBlt(hdc, 0, 0, wd, hgt, hdc_scr, 0, 0, SRCCOPY);
    SelectObject(hdc, hbm_old);
    DeleteDC(hdc);
    ReleaseDC(NULL, hdc_scr);

    return bi;
}

int main() {
    constexpr auto scr_wd = 2560;
    constexpr auto scr_hgt = 1440;

    std::vector<uint8_t> buffer(scr_wd * scr_hgt * 3);
    std::chrono::high_resolution_clock timer;

    int dummy;
    double sum = 0.0;
    int n = 100;
    for (int i = 0; i < n; ++i) {
        auto start = timer.now();
        get_screen_bytes1(buffer.data(), scr_wd, scr_hgt);
        sum += c::duration_cast<c::microseconds>(timer.now() - start).count();
        dummy += buffer[0];
    }
    double time1 = sum / n;
    std::cout << "get_screen_bytes1 => " << time1 << "\n";

    sum = 0.0;
    for (int i = 0; i < n; ++i) {
        auto start = timer.now();
        auto bmp_info = get_screen_bytes2( scr_wd, scr_hgt);
        sum += c::duration_cast<c::microseconds>(timer.now() - start).count();
        dummy += bmp_info.data[0];
        DeleteObject(bmp_info.handle);
    }
    double time2 = sum / n;
    std::cout << "get_screen_bytes2 => " << time2 << "\n";
    std::cout << dummy << "\n";
    std::cout << "pcnt speed improvement => " << time2 / time1 << "\n";

}