how do i take a grayscale screenshot with c++?

Question

I have this piece of code here that takes a screenshot and as bitmaps and saves it as a BMP file. it works just fine but I want to be able to modify the bits and make it grayscale instead of an RGB map.

I found a way to do this but I had to save the BMPINFOHEADER and RGBQUAD array, then read it with the grayscale function it kind of worked, but the size stays the same.

I am very noob at this but I think the grayscale image should be a lot smaller than an RGB one.

is there a way of removing color from the bitmap right away without having to re-read it and modify it?

int CaptureBMP(LPCTSTR szFile)
{
    // Source[1]
    HDC hdcScr, hdcMem;
    HBITMAP hbmScr;
    BITMAP bmp;
    int iXRes, iYRes;

    // Create a normal DC and a memory DC for the entire screen. The
    // normal DC provides a "snapshot" of the screen contents. The
    // memory DC keeps a copy of this "snapshot" in the associated
    // bitmap.
    hdcScr = CreateDC("DISPLAY", NULL, NULL, NULL);


    hdcMem = CreateCompatibleDC(hdcScr);

    iXRes = GetDeviceCaps(hdcScr, HORZRES);
    iYRes = GetDeviceCaps(hdcScr, VERTRES);

    // Create a compatible bitmap for hdcScreen.
    hbmScr = CreateCompatibleBitmap(hdcScr, iXRes, iYRes);
    
    if (hbmScr == 0) return 0;

    // Select the bitmaps into the compatible DC.
    if (!SelectObject(hdcMem, hbmScr)) return 0;

    // Copy color data for the entire display into a
    // bitmap that is selected into a compatible DC.
    if (!StretchBlt(hdcMem,0, 0, iXRes, iYRes,hdcScr,0, 0, iXRes, iYRes,SRCCOPY)) return 0;

    // Source[2]
    PBITMAPINFO pbmi;
    WORD cClrBits;

    // Retrieve the bitmap's color format, width, and height.
    if (!GetObject(hbmScr, sizeof(BITMAP), (LPSTR)&bmp)) return 0;

    // Convert the color format to a count of bits.
    cClrBits = (WORD)(bmp.bmPlanes * bmp.bmBitsPixel);
    if (cClrBits == 1)
        cClrBits = 1;
    else if (cClrBits <= 4)
        cClrBits = 4;
    else if (cClrBits <= 8)
        cClrBits = 8;
    else if (cClrBits <= 16)
        cClrBits = 16;
    else if (cClrBits <= 24)
        cClrBits = 24;
    else cClrBits = 32;


    // Allocate memory for the BITMAPINFO structure. (This structure
    // contains a BITMAPINFOHEADER structure and an array of RGBQUAD
    // data structures.)
    if (cClrBits != 24)
        pbmi = (PBITMAPINFO)LocalAlloc(LPTR, sizeof(BITMAPINFOHEADER) + sizeof(RGBQUAD) * (1 << cClrBits));

    // There is no RGBQUAD array for the 24-bit-per-pixel format.
    else
        pbmi = (PBITMAPINFO)LocalAlloc(LPTR,sizeof(BITMAPINFOHEADER));

    // Initialize the fields in the BITMAPINFO structure.
    pbmi->bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
    pbmi->bmiHeader.biWidth = bmp.bmWidth;
    pbmi->bmiHeader.biHeight = bmp.bmHeight;
    pbmi->bmiHeader.biPlanes = bmp.bmPlanes;
    pbmi->bmiHeader.biBitCount = bmp.bmBitsPixel;
    if (cClrBits < 24)
        pbmi->bmiHeader.biClrUsed = (1 << cClrBits);

    // If the bitmap is not compressed, set the BI_RGB flag.
    pbmi->bmiHeader.biCompression = BI_RGB;

    // Compute the number of bytes in the array of color
    // indices and store the result in biSizeImage.
    pbmi->bmiHeader.biSizeImage = (pbmi->bmiHeader.biWidth + 7) / 8 * pbmi->bmiHeader.biHeight * cClrBits;

    // Set biClrImportant to 0, indicating that all of the
    // device colors are important.
    pbmi->bmiHeader.biClrImportant = 0;

    HANDLE hf;                  // file handle
    BITMAPFILEHEADER hdr;       // bitmap file-header
    PBITMAPINFOHEADER pbih;     // bitmap info-header
    LPBYTE lpBits;              // memory pointer
    DWORD dwTotal;              // total count of bytes
    DWORD cb;                   // incremental count of bytes
    BYTE* hp;                   // byte pointer
    DWORD dwTmp;

    pbih = (PBITMAPINFOHEADER)pbmi;
    lpBits = (LPBYTE)GlobalAlloc(GMEM_FIXED, pbih->biSizeImage);

    if (!lpBits) return 0;

    // Retrieve the color table (RGBQUAD array) and the bits
    // (array of palette indices) from the DIB.
    if (!GetDIBits(hdcMem, hbmScr, 0, (WORD)pbih->biHeight, lpBits, pbmi, DIB_RGB_COLORS)) return 0;

    // Create the .BMP file.
    hf = CreateFile(szFile, GENERIC_READ | GENERIC_WRITE, (DWORD)0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, (HANDLE)NULL);
    if (hf == INVALID_HANDLE_VALUE) return 0;

    hdr.bfType = 0x4d42;        // 0x42 = "B" 0x4d = "M"

    // Compute the size of the entire file.
    hdr.bfSize = (DWORD)(sizeof(BITMAPFILEHEADER) + pbih->biSize + pbih->biClrUsed * sizeof(RGBQUAD) + pbih->biSizeImage);
    hdr.bfReserved1 = 0;
    hdr.bfReserved2 = 0;

    // Compute the offset to the array of color indices.
    hdr.bfOffBits = (DWORD)sizeof(BITMAPFILEHEADER) +
        pbih->biSize + pbih->biClrUsed *
        sizeof(RGBQUAD);

    // Copy the BITMAPFILEHEADER into the .BMP file.
    if (!WriteFile(hf, (LPVOID)&hdr, sizeof(BITMAPFILEHEADER), (LPDWORD)&dwTmp, NULL)) return 0;

    // Copy the BITMAPINFOHEADER and RGBQUAD array into the file.
    if (!WriteFile(hf, (LPVOID)pbih, sizeof(BITMAPINFOHEADER)
        + pbih->biClrUsed * sizeof(RGBQUAD),
        (LPDWORD)&dwTmp, NULL))
        return 0;

    // Copy the array of color indices into the .BMP file.
    dwTotal = cb = pbih->biSizeImage;
    hp = lpBits;
    if (!WriteFile(hf, (LPSTR)hp, (int)cb, (LPDWORD)&dwTmp, NULL)) return 0;

    // Close the .BMP file.
    if (!CloseHandle(hf)) return 0;

    // Free memory.
    GlobalFree((HGLOBAL)lpBits);
    ReleaseDC(0, hdcScr);
    ReleaseDC(0, hdcMem);

    return 1;
}

user_17316773 · Answer

I used array conversion, which is mainly from the rgbRed, rgbGreen and rgbBlue components of the original true color map to the gray value Y of the gray image.

It can be obtained by using the following formula:

Y=0.299 * rgbRed+0.587 * rgbGreen+0.114 * rgbBlue

The processing code has been added to your code, you can refer to the following code, the output file size is 1/4 of the original.

#pragma pack(1)

typedef struct tag_color_32 {
   BYTE Red;
   BYTE Green;
   BYTE Blue;
   BYTE Alpha;
}color_32;

int CaptureBMP(LPCTSTR szFile)
{
// Source[1]
HDC hdcScr, hdcMem;
HBITMAP hbmScr;
BITMAP bmp;
.................
.................
// Copy the array of color indices into the .BMP file.
dwTotal = cb = pbih->biSizeImage;
hp = lpBits;
if (!WriteFile(hf, (LPSTR)hp, (int)cb, (LPDWORD)& dwTmp, NULL)) return 0;

// Close the .BMP file.
if (!CloseHandle(hf)) return 0;


/*********************edit 2022/1/7******************************/

FILE* originImg;
fopen_s(&originImg, "lena-32.bmp", "rb");
if (originImg == NULL) return 0;

int sizeFileHeader = sizeof(BITMAPFILEHEADER);
int sizeInfoHeader = sizeof(BITMAPINFOHEADER);

BITMAPFILEHEADER* bitmapFileHeader = new BITMAPFILEHEADER[sizeFileHeader + 1];
BITMAPINFOHEADER* bitmapInfoHeader = new BITMAPINFOHEADER[sizeInfoHeader + 1];

memset(bitmapFileHeader, 0, sizeFileHeader + 1);
memset(bitmapInfoHeader, 0, sizeInfoHeader + 1);
fread(bitmapFileHeader, sizeof(char), sizeFileHeader, originImg);
fseek(originImg, sizeFileHeader, 0);
fread(bitmapInfoHeader, sizeof(char), sizeInfoHeader, originImg);


int srcImageLineByteCount = (((bitmapInfoHeader->biWidth * 32) + 31) / 32) * 4; //Calculates the number of bytes of pixels per line of the original 32 bitmap
int grayImageLineByteCount = (((bitmapInfoHeader->biWidth) * 8 + 31) / 32) * 4; //Calculate the number of bytes of pixels in each row of 8-bit grayscale map

//************Bitmap header**********************
//Creates a two-dimensional array with high biHeight and width srcImageLineByteCount, and initializes the array
color_32* origImgData = new color_32[bitmapInfoHeader->biHeight * bitmapInfoHeader->biWidth];
for (int i = 0; i < bitmapInfoHeader->biHeight * bitmapInfoHeader->biWidth; i++)
{
    fread(origImgData + i, 4, 1, originImg);
}

fclose(originImg);
// palette
RGBQUAD* pRgbQuards = new RGBQUAD[256];
for (int i = 0; i < 256; i++)
{
    pRgbQuards[i].rgbBlue = i;
    pRgbQuards[i].rgbRed = i;
    pRgbQuards[i].rgbGreen = i;
    pRgbQuards[i].rgbReserved = 0;
}
//Modify headers
bitmapInfoHeader->biBitCount = 8;
bitmapInfoHeader->biClrUsed = 256;
bitmapInfoHeader->biSizeImage = (bitmapInfoHeader->biHeight) * grayImageLineByteCount;

//8 is a grayscale image with 256 RGBQUAD data structures. A color palette takes up 4 bytes of data, so the color palette length of 256 color images is 256*4 and 1024 bytes
bitmapFileHeader->bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER) + sizeof(RGBQUAD) * 256;  //The offset plus the size of the palette
bitmapFileHeader->bfSize = bitmapFileHeader->bfOffBits + bitmapInfoHeader->biSizeImage;
//Write the data
FILE* grayImg;
fopen_s(&grayImg, "lena-gray.bmp", "wb");
//Written to the file
fwrite(bitmapFileHeader, sizeof(char), sizeof(BITMAPFILEHEADER), grayImg);
fwrite(bitmapInfoHeader, sizeof(char), sizeof(BITMAPINFOHEADER), grayImg);
fwrite(pRgbQuards, sizeof(RGBQUAD), 256, grayImg);
//Grayscale map a two-dimensional array of bitmap data
for (int i = 0; i < bitmapInfoHeader->biHeight; i++)
{
    for (int j = 0; j < bitmapInfoHeader->biWidth; j++)    //The number of bytes of pixels per row of a grayscale image
    {
        float pr = origImgData[i * bitmapInfoHeader->biWidth + j].Red;
        float pg = origImgData[i * bitmapInfoHeader->biWidth + j].Green;
        float pb = origImgData[i * bitmapInfoHeader->biWidth + j].Blue;

        BYTE data = pr * 0.299 + pg * 0.587 + pb * 0.114;
        fwrite(&data, 1, 1, grayImg);

        for (int j = bitmapInfoHeader->biWidth; j < grayImageLineByteCount; j++) {
            BYTE data = 0;
            fwrite(&data, 1, 1, grayImg);
        }
    }
}

fclose(grayImg);

//Free memory.

delete[]origImgData;

/*******************edit 2022/1/7****************************/

// Free memory.
GlobalFree((HGLOBAL)lpBits);
ReleaseDC(0, hdcScr);
ReleaseDC(0, hdcMem);

return 1;
}
int main()
{
   CaptureBMP(L"lena-32");
   std::cout << "Hello World!
";
}

how do i take a grayscale screenshot with c++?

1 Answers