打开 pgm 图像 16 bpp

Question

我需要将每像素 16 位的 pgm 图像转换为每像素 8 位的 pgm 图像，但我无法读取 16 bpp 的 pgm 图像，我不明白我做错了什么。这里的代码：

#include "pgm.h"

#include <iterator>
#include <algorithm>
#include <fstream>
#include <sstream>

using namespace std;

bool convert16to8bit(const std::string& inFilename, mat<uint8_t>& img, const std::string& outFilename){
    mat<uint16_t> imgTemp;

    ifstream is(inFilename, ios::binary);
    if (!is)
        return false;

    string magic;
    is >> magic;
    if (magic != "P2" && magic != "P5")
        return false;

    size_t rows, cols, nlevels;
    is >> cols >> rows >> nlevels;

    if (nlevels < 255)
        return false;

    is.get();

    img = mat<uint8_t>(rows, cols);
    if (nlevels == 255){
        if (magic == "P5"){
            is.unsetf(ios::skipws);
            img.data_.assign(istream_iterator<uint8_t>(is), istream_iterator<uint8_t>());
        }else
            img.data_.assign(istream_iterator<int>(is), istream_iterator<int>());
    }
    else{
        imgTemp = mat<uint16_t>(rows, cols);
        if (magic == "P5"){
            is.unsetf(ios::skipws);
            imgTemp.data_.assign(istream_iterator<uint16_t>(is), istream_iterator<uint16_t>());
        }
        else
            imgTemp.data_.assign(istream_iterator<int>(is), istream_iterator<int>());

        for (size_t r = 0; r < rows; r++)
            for (size_t c = 0; c < cols; c++)
                img(r, c) = imgTemp(r, c);
    }

    stringstream ss; 
    ss << outFilename << ".pgm";
    ofstream os(ss.str(), ios::binary);
    if (!os)
        return false;
    os << magic <<"\n" << cols << " " << rows << "\n255\n";
    if (magic == "P2") 
        copy(begin(img), end(img), ostream_iterator<int>(os, " ")); 
    else 
        copy(begin(img), end(img), ostream_iterator<uint8_t>(os));

    return true;
}


//mat.h

#if !defined MAT_H 
#define MAT_H

#include <vector>

template <typename T> 
struct mat {
    size_t rows_, cols_; 
    std::vector<T> data_;

    mat(size_t rows = 0, size_t cols = 0) : rows_(rows), cols_(cols), data_(rows*cols) {}

    const size_t rows() const { return rows_; } 
    const size_t cols() const { return cols_; }

    const T& operator()(size_t r, size_t c) const { return data_[r*cols_ + c]; } 
    T& operator()(size_t r, size_t c) { return data_[r*cols_ + c]; }

    auto begin() -> decltype(data_.begin()) { return data_.begin(); } 
    auto end() -> decltype(data_.end()) { return data_.end(); } 
    auto begin() const -> decltype(data_.begin()) { return data_.begin(); } 
    auto end() const -> decltype(data_.end()) { return data_.end(); }
};
#endif // MAT_H

此代码在尝试执行指令 img(r, c) = imgTemp ( r, c)（错误：向量下标超出范围）时产生错误，使用 pgm 16bpp 但使用 pgm 8bpp 我可以毫无问题地打开它，并毫无错误地将其重新创建为原始文件。我认为问题出在 istream_iterator < uint16_t > 因为 istream_iterator < uint8_t > 有效（使用 pgm 8bit）。有什么解决办法吗？

感谢您的帮助

Answer 1

我找到了解决问题的方法。这是正确的代码，如果它对某人有用的话：

#include "pgm.h"

#include <iterator>
#include <algorithm>
#include <fstream>
#include <sstream>

using namespace std;

bool convert16to8bit(const std::string& inFilename, mat<uint8_t>& img, const std::string& outFilename){
    mat<uint16_t> imgTemp;

    ifstream is(inFilename, ios::binary);
    if (!is)
        return false;

    string magic;
    is >> magic;
    if (magic != "P2" && magic != "P5")
        return false;

    size_t rows, cols, nlevels;
    is >> cols >> rows >> nlevels;

    if (nlevels < 255)
        return false;

    is.get();

    img = mat<uint8_t>(rows, cols);
    if (nlevels == 255){
        if (magic == "P5"){
            is.unsetf(ios::skipws);
            img.data_.assign(istream_iterator<uint8_t>(is), istream_iterator<uint8_t>());
        }else
            img.data_.assign(istream_iterator<int>(is), istream_iterator<int>());
    }
    else{
        imgTemp = mat<uint16_t>(rows, cols);
        if (magic == "P5"){

            //here is the line changed
            is.read(reinterpret_cast<char *>(&imgTemp.data_[0]), imgTemp.rows()*imgTemp.cols() * 2);          
        }
        else
            imgTemp.data_.assign(istream_iterator<int>(is), istream_iterator<int>());

        for (size_t r = 0; r < rows; r++)
            for (size_t c = 0; c < cols; c++)
                img(r, c) = imgTemp(r, c);
    }

    stringstream ss; 
    ss << outFilename << ".pgm";
    ofstream os(ss.str(), ios::binary);
    if (!os)
        return false;
    os << magic <<"\n" << cols << " " << rows << "\n255\n";
    if (magic == "P2") 
        copy(begin(img), end(img), ostream_iterator<int>(os, " ")); 
    else 
        copy(begin(img), end(img), ostream_iterator<uint8_t>(os));

    return true;
}

打开 pgm 图像 16 bpp

Open a pgm image 16 bpp

c++

pgm