How can I read BMP pixel values into an array?

Question

I'm writing code in C++ (on Windows) and I'm trying to extract the pixel values of a grayscale bmp. I don't care about keeping any of the metadata, and just want to store the pixel values in a char array. I haven't been able to find a standard or "typical" way of doing this manually, so I'm wondering if there's perhaps a simple library that people use to load bitmaps into memory.

Thanks in advance!

Answer 1

Expanding on what Yola wrote, this should be able to read in and output a file. It is not well tested, but seems to work. It uses format of the file it reads in when it outputs.

#include <iostream>
#include <unistd.h>
#include <fstream>

using std::cout;
using std::endl;
using std::ofstream;
using std::ifstream;

#pragma pack(1)
#pragma once

typedef int LONG;
typedef unsigned short WORD;
typedef unsigned int DWORD;

typedef struct tagBITMAPFILEHEADER {
    WORD bfType;
    DWORD bfSize;
    WORD bfReserved1;
    WORD bfReserved2;
    DWORD bfOffBits;
} BITMAPFILEHEADER, *PBITMAPFILEHEADER;

typedef struct tagBITMAPINFOHEADER {
    DWORD biSize;
    LONG biWidth;
    LONG biHeight;
    WORD biPlanes;
    WORD biBitCount;
    DWORD biCompression;
    DWORD biSizeImage;
    LONG biXPelsPerMeter;
    LONG biYPelsPerMeter;
    DWORD biClrUsed;
    DWORD biClrImportant;
} BITMAPINFOHEADER, *PBITMAPINFOHEADER;

unsigned char** reds;
unsigned char** greens;
unsigned char** blues;
int rows;
int cols;

void ColorTest() {
    // Makes Red Rectangle in top left corner. Rectangle stretches to right alot
    for (int i = rows / 10; i < 3 * rows / 10; i++)
        for (int j = cols / 10; j < 7 * cols / 10; j++)
            reds[i][j] = 0xff;

// Makes small green box in bottom right
    for (int i = 8 * rows / 10; i < rows; i++)
        for (int j = 8 * cols / 10; j < cols; j++)
            greens[i][j] = 0xff;

// Makes White box in the middle of the screeene    
    for (int i = rows * 4 / 10; i < rows * 6 / 10; i++)
        for (int j = cols * 4 / 10; j < cols * 6 / 10; j++) {
            greens[i][j] = 0xff;
            reds[i][j] = 0xff;
            blues[i][j] = 0xff;
        }

// Blue verticle rectangle bottom left
    for (int i = rows * 6 / 10; i < rows; i++)
        for (int j = cols * 0; j < cols * 1 / 10; j++)
            blues[i][j] = 0xff;
}

void RGB_Allocate(unsigned char**& dude) {
    dude = new unsigned char*[rows];
    for (int i = 0; i < rows; i++)
        dude[i] = new unsigned char[cols];
}

bool FillAndAllocate(char*& buffer, const char* Picture, int& rows, int& cols, int& BufferSize) { //Returns 1 if executed sucessfully, 0 if not sucessfull
    std::ifstream file(Picture);

    if (file) {
        file.seekg(0, std::ios::end);
        std::streampos length = file.tellg();
        file.seekg(0, std::ios::beg);

        buffer = new char[length];
        file.read(&buffer[0], length);

        PBITMAPFILEHEADER file_header;
        PBITMAPINFOHEADER info_header;

        file_header = (PBITMAPFILEHEADER) (&buffer[0]);
        info_header = (PBITMAPINFOHEADER) (&buffer[0] + sizeof(BITMAPFILEHEADER));
        rows = info_header->biHeight;
        cols = info_header->biWidth;
        BufferSize = file_header->bfSize;
        return 1;
    }
    else {
        cout << "File" << Picture << " don't Exist!" << endl;
        return 0;
    }
}

void GetPixlesFromBMP24(unsigned char** reds, unsigned char** greens, unsigned char** blues, int end, int rows, int cols, char* FileReadBuffer) { // end is BufferSize (total size of file)
    int count = 1;
int extra = cols % 4; // The nubmer of bytes in a row (cols) will be a multiple of 4.
    for (int i = 0; i < rows; i++){
count += extra;
    for (int j = cols - 1; j >= 0; j--)
        for (int k = 0; k < 3; k++) {
                switch (k) {
                case 0:
                    reds[i][j] = FileReadBuffer[end - count++];
                    break;
                case 1:
                    greens[i][j] = FileReadBuffer[end - count++];
                    break;
                case 2:
                    blues[i][j] = FileReadBuffer[end - count++];
                    break;
                }
            }
            }
}

void WriteOutBmp24(char* FileBuffer, const char* NameOfFileToCreate, int BufferSize) {
    std::ofstream write(NameOfFileToCreate);
    if (!write) {
        cout << "Failed to write " << NameOfFileToCreate << endl;
        return;
    }
    int count = 1;
    int extra = cols % 4; // The nubmer of bytes in a row (cols) will be a multiple of 4.
    for (int i = 0; i < rows; i++){
        count += extra;
        for (int j = cols - 1; j >= 0; j--)
            for (int k = 0; k < 3; k++) {
                switch (k) {
                case 0: //reds
                    FileBuffer[BufferSize - count] = reds[i][j];
                    break;
                case 1: //green
                    FileBuffer[BufferSize - count] = greens[i][j];
                    break;
                case 2: //blue
                    FileBuffer[BufferSize - count] = blues[i][j];
                    break;
                }
                count++;
            }
            }
    write.write(FileBuffer, BufferSize);
}


int main(int args, char** cat) {
char* FileBuffer; int BufferSize;

#define Picture "ReadInPicture.bmp"
if (!FillAndAllocate(FileBuffer, Picture, rows, cols, BufferSize)){cout << "File read error" << endl; return 0;}
cout << "Rows: " << rows << " Cols: " << cols << endl;

RGB_Allocate(reds);
RGB_Allocate(greens);
RGB_Allocate(blues);
GetPixlesFromBMP24( reds,  greens, blues,BufferSize, rows, cols, FileBuffer);
ColorTest();
#define WriteOutFile "OutputPicture.bmp"
WriteOutBmp24(FileBuffer,  WriteOutFile,BufferSize);
    return 1;
}