how do i take a grayscale screenshot with c++? - 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;
}

Answer 1

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!\n";
}