问题
How can I read the color value of 24bit BMP images at all the pixel [h*w] in C or C++ on Windows [better without any 3rd party library]. I got Dev-C++
A working code will be really appreciated as I\'ve never worked on Image reading & have come to SO after Googling [if you can google better than me, plz provide a link].
回答1:
You can try this one:
unsigned char* readBMP(char* filename)
{
int i;
FILE* f = fopen(filename, "rb");
unsigned char info[54];
fread(info, sizeof(unsigned char), 54, f); // read the 54-byte header
// extract image height and width from header
int width = *(int*)&info[18];
int height = *(int*)&info[22];
int size = 3 * width * height;
unsigned char* data = new unsigned char[size]; // allocate 3 bytes per pixel
fread(data, sizeof(unsigned char), size, f); // read the rest of the data at once
fclose(f);
for(i = 0; i < size; i += 3)
{
unsigned char tmp = data[i];
data[i] = data[i+2];
data[i+2] = tmp;
}
return data;
}
Now data should contain the (R, G, B) values of the pixels. The color of pixel (i, j) is stored at data[3 * (i * width + j)], data[3 * (i * width + j) + 1] and data[3 * (i * width + j) + 2].
In the last part, the swap between every first and third pixel is done because windows stores the color values as (B, G, R) triples, not (R, G, B).
回答2:
Code of readBMP function after padding fix:
unsigned char* ReadBMP(char* filename)
{
int i;
FILE* f = fopen(filename, "rb");
if(f == NULL)
throw "Argument Exception";
unsigned char info[54];
fread(info, sizeof(unsigned char), 54, f); // read the 54-byte header
// extract image height and width from header
int width = *(int*)&info[18];
int height = *(int*)&info[22];
cout << endl;
cout << " Name: " << filename << endl;
cout << " Width: " << width << endl;
cout << "Height: " << height << endl;
int row_padded = (width*3 + 3) & (~3);
unsigned char* data = new unsigned char[row_padded];
unsigned char tmp;
for(int i = 0; i < height; i++)
{
fread(data, sizeof(unsigned char), row_padded, f);
for(int j = 0; j < width*3; j += 3)
{
// Convert (B, G, R) to (R, G, B)
tmp = data[j];
data[j] = data[j+2];
data[j+2] = tmp;
cout << "R: "<< (int)data[j] << " G: " << (int)data[j+1]<< " B: " << (int)data[j+2]<< endl;
}
}
fclose(f);
return data;
}
回答3:
Here is a working C++ version of the answer:
#include <fstream>
#include <iostream>
#include <string>
#include <array>
#include <vector>
#include <iterator>
std::vector<char> readBMP(const std::string &file)
{
static constexpr size_t HEADER_SIZE = 54;
std::ifstream bmp(file, std::ios::binary);
std::array<char, HEADER_SIZE> header;
bmp.read(header.data(), header.size());
auto fileSize = *reinterpret_cast<uint32_t *>(&header[2]);
auto dataOffset = *reinterpret_cast<uint32_t *>(&header[10]);
auto width = *reinterpret_cast<uint32_t *>(&header[18]);
auto height = *reinterpret_cast<uint32_t *>(&header[22]);
auto depth = *reinterpret_cast<uint16_t *>(&header[28]);
std::cout << "fileSize: " << fileSize << std::endl;
std::cout << "dataOffset: " << dataOffset << std::endl;
std::cout << "width: " << width << std::endl;
std::cout << "height: " << height << std::endl;
std::cout << "depth: " << depth << "-bit" << std::endl;
std::vector<char> img(dataOffset - HEADER_SIZE);
bmp.read(img.data(), img.size());
auto dataSize = ((width * 3 + 3) & (~3)) * height;
img.resize(dataSize);
bmp.read(img.data(), img.size());
char temp = 0;
for (auto i = dataSize - 4; i >= 0; i -= 3)
{
temp = img[i];
img[i] = img[i+2];
img[i+2] = temp;
std::cout << "R: " << int(img[i] & 0xff) << " G: " << int(img[i+1] & 0xff) << " B: " << int(img[i+2] & 0xff) << std::endl;
}
return img;
}
回答4:
I've created a BitMap class that works for bmp files that have 24 bits per pixel. If the bmp isn't compatible, you should get a relevant error.
It follows along almost exactly with the Wikipedia article. (The one problem is that it doesn't work with files that have a pixel array offset that is greater than 255. This is noted in the code and should be easily fixable.)
I've been using this with bmp files created by mspaint.
Here is an example usage
example.cpp
#include "bmp.h"
int main() {
// load the file. The constructor now does most of the work
BitMap example_bmp("examplefile.bmp");
// get the vector <R,G,B> for the pixel at (1,1)
std::vector<unsigned int> example_vector = example_bmp.getPixel(1,1);
}
example_vector now contains the rgb (in that order) values of the pixel at coordinate (1,1) indexed from the top of the image, going down. Indices start at 0. See the Wikipedia examples.
Here is the header file:
#ifndef BMP_H
#define BMP_H
#include <iostream>
#include <vector>
#include <fstream>
class BitMap {
private:
unsigned char m_bmpFileHeader[14];
unsigned int m_pixelArrayOffset;
unsigned char m_bmpInfoHeader[40];
int m_height;
int m_width;
int m_bitsPerPixel;
int m_rowSize;
int m_pixelArraySize;
unsigned char* m_pixelData;
char * m_copyname;
const char * m_filename;
public:
BitMap(const char * filename);
~BitMap();
std::vector<unsigned int> getPixel(int i,int j);
void makeCopy(char * filename);
void writePixel(int i,int j, int R, int G, int B);
void swapPixel(int i, int j, int i2, int j2);
void dispPixelData();
int width() {return m_width;}
int height() {return m_height;}
int vd(int i, int j);
int hd(int i, int j);
bool isSorted();
};
BitMap::BitMap( const char * filename) {
using namespace std;
m_filename = filename;
ifstream inf(filename);
if(!inf) {
cerr<<"Unable to open file: "<<filename<<"\n";
}
//unsigned char m_bmpFileHeader[14];
unsigned char a;
for(int i =0;i<14;i++) {
inf>>hex>>a;
m_bmpFileHeader[i] = a;
}
if(m_bmpFileHeader[0]!='B' || m_bmpFileHeader[1]!='M') {
cerr<<"Your info header might be different!\nIt should start with 'BM'.\n";
}
/*
THE FOLLOWING LINE ONLY WORKS IF THE OFFSET IS 1 BYTE!!!!! (it can be 4 bytes max)
That should be fixed now.
old line was
m_pixelArrayOffset = m_bmpFileHeader[10];
*/
unsigned int * array_offset_ptr = (unsigned int *)(m_bmpFileHeader + 10);
m_pixelArrayOffset = *array_offset_ptr;
if( m_bmpFileHeader[11] != 0 || m_bmpFileHeader[12] !=0 || m_bmpFileHeader[13] !=0 ) {
std::cerr<< "You probably need to fix something. bmp.h("<<__LINE__<<")\n";
}
//unsigned char m_bmpInfoHeader[40];
for(int i=0;i<40;i++) {
inf>>hex>>a;
m_bmpInfoHeader[i]=a;
}
int * width_ptr = (int*)(m_bmpInfoHeader+4);
int * height_ptr = (int*)(m_bmpInfoHeader+8);
m_width = *width_ptr;
m_height = *height_ptr;
printf("W: %i, H: %i", m_width, m_height);
m_bitsPerPixel = m_bmpInfoHeader[14];
if(m_bitsPerPixel!=24) {
cerr<<"This program is for 24bpp files. Your bmp is not that\n";
}
int compressionMethod = m_bmpInfoHeader[16];
if(compressionMethod!=0) {
cerr<<"There's some compression stuff going on that we might not be able to deal with.\n";
cerr<<"Comment out offending lines to continue anyways. bpm.h line: "<<__LINE__<<"\n";
}
m_rowSize = int( floor( (m_bitsPerPixel*m_width + 31.)/32 ) ) *4;
m_pixelArraySize = m_rowSize* abs(m_height);
m_pixelData = new unsigned char [m_pixelArraySize];
inf.seekg(m_pixelArrayOffset,ios::beg);
for(int i=0;i<m_pixelArraySize;i++) {
inf>>hex>>a;
m_pixelData[i]=a;
}
}
BitMap::~BitMap() {
delete[] m_pixelData;
}
void BitMap::dispPixelData() {
for(int i=0;i<m_pixelArraySize;i++) {
std::cout<<(unsigned int)m_pixelData[i]<<" ";
}
std::cout<<"\n";
}
// output is in rgb order.
std::vector<unsigned int> BitMap::getPixel(int x, int y) {
if(x<m_width && y<m_height) {
std::vector<unsigned int> v;
v.push_back(0);
v.push_back(0);
v.push_back(0);
y = m_height -1- y; //to flip things
//std::cout<<"y: "<<y<<" x: "<<x<<"\n";
v[0] = (unsigned int) ( m_pixelData[ m_rowSize*y+3*x+2 ] ); //red
v[1] = (unsigned int) ( m_pixelData[ m_rowSize*y+3*x+1 ] ); //greed
v[2] = (unsigned int) ( m_pixelData[ m_rowSize*y+3*x+0 ] ); //blue
return v;
}
else {std::cerr<<"BAD INDEX\n";std::cerr<<"X: "<<x<<" Y: "<<y<<"\n";}
}
void BitMap::makeCopy(char * filename) {
std::ofstream copyfile(filename);
std::ifstream infile(m_filename);
m_copyname = filename;
unsigned char c;
while(infile) {
infile>>c;
copyfile<<c;
}
}
// changes the file
void BitMap::writePixel(int x,int y, int R, int G, int B) {
std::fstream file(m_filename);
y = m_height -1- y; // to flip things.
int blueOffset = m_pixelArrayOffset+m_rowSize*y+3*x+0;
// writes to the file
file.seekg(blueOffset,std::ios::beg);
file<< (unsigned char)B;
file.seekg(blueOffset+1,std::ios::beg);
file<< (unsigned char)G;
file.seekg(blueOffset+2,std::ios::beg);
file<< (unsigned char)R;
// edits data in pixelData array
m_pixelData[m_rowSize*y+3*x+2] = (unsigned char)R;
m_pixelData[m_rowSize*y+3*x+1] = (unsigned char)G;
m_pixelData[m_rowSize*y+3*x+0] = (unsigned char)B;
}
// changes the file
void BitMap::swapPixel(int i, int j, int i2, int j2) {
std::vector<unsigned int> p1 = (*this).getPixel(i,j);
std::vector<unsigned int> p2 = (*this).getPixel(i2,j2);
(*this).writePixel(i,j,p2[0],p2[1],p2[2]);
(*this).writePixel(i2,j2,p1[0],p1[1],p1[2]);
}
#endif
回答5:
I can't comment on the top level answer because I don't have enough stackoverflow rep yet, but I just wanted to point out one very critical bug with that implementation.
Some bitmaps can be written with a negative height, so when you try to allocate your image data buffer, your code will crash with std::bad_alloc. Bitmaps with negative height means that the image data is stored top to bottom instead of the traditional bottom to top. Therefore, a slightly better version of the top level answer is (still not including portability for systems with different endianness and size of bytes):
unsigned char* readBMP(char* filename)
{
int i;
FILE* f = fopen(filename, "rb");
unsigned char info[54];
fread(info, sizeof(unsigned char), 54, f); // read the 54-byte header
// extract image height and width from header
int width, height;
memcpy(&width, info + 18, sizeof(int));
memcpy(&height, info + 22, sizeof(int));
int heightSign = 1;
if (height < 0){
heightSign = -1;
}
int size = 3 * width * abs(height);
unsigned char* data = new unsigned char[size]; // allocate 3 bytes per pixel
fread(data, sizeof(unsigned char), size, f); // read the rest of the data at once
fclose(f);
if(heightSign == 1){
for(i = 0; i < size; i += 3)
{
//code to flip the image data here....
}
}
return data;
}
回答6:
You have to read the bitmap header first. After got to the data offset which you will find in the bitmap headers and read the pixels line by line, make care about the padding in bmp file format.
take a look on msdn http://msdn.microsoft.com/en-us/library/aa452883.aspx
http://msdn.microsoft.com/en-us/library/windows/desktop/dd318229(v=vs.85).aspx
回答7:
A simple, OS-portable Python solution is provided in How can I read the RGB value of a given pixel in Python?. Adapts to multiple image formats, takes care of padding and so on.
回答8:
I tried the codes above by didil and I quoted below as a reference (sorry and I did not have enough reputation to make a comment).
The code compiles OK, but crashes during the for loop iteration. I think this has something to do with 'i' being a uint32_t, instead of int. When 'i' reaches zero, the for loop is still valid and 'i' gets decremented by 3 which becomes a negative value. Since 'i' is a uint32_t, its value becomes a positive value and is greater than 0. As such, the for loop never finishes and causes the execution to crash when 'i' points to beyond the boundary of img storage.
#include <fstream>
#include <iostream>
#include <string>
#include <array>
#include <vector>
#include <iterator>
std::vector<char> readBMP(const std::string &file)
{
static constexpr size_t HEADER_SIZE = 54;
std::ifstream bmp(file, std::ios::binary);
std::array<char, HEADER_SIZE> header;
bmp.read(header.data(), header.size());
auto fileSize = *reinterpret_cast<uint32_t *>(&header[2]);
auto dataOffset = *reinterpret_cast<uint32_t *>(&header[10]);
auto width = *reinterpret_cast<uint32_t *>(&header[18]);
auto height = *reinterpret_cast<uint32_t *>(&header[22]);
auto depth = *reinterpret_cast<uint16_t *>(&header[28]);
std::cout << "fileSize: " << fileSize << std::endl;
std::cout << "dataOffset: " << dataOffset << std::endl;
std::cout << "width: " << width << std::endl;
std::cout << "height: " << height << std::endl;
std::cout << "depth: " << depth << "-bit" << std::endl;
std::vector<char> img(dataOffset - HEADER_SIZE);
bmp.read(img.data(), img.size());
auto dataSize = ((width * 3 + 3) & (~3)) * height;
img.resize(dataSize);
bmp.read(img.data(), img.size());
char temp = 0;
for (auto i = dataSize - 4; i >= 0; i -= 3)
{
temp = img[i];
img[i] = img[i+2];
img[i+2] = temp;
std::cout << "R: " << int(img[i] & 0xff) << " G: " << int(img[i+1] & 0xff) << " B: " << int(img[i+2] & 0xff) << std::endl;
}
return img;
}
来源：https://stackoverflow.com/questions/9296059/read-pixel-value-in-bmp-file