Save exr/pfm as little endian - c++

Question

I am load a bmp file into a CImg object and I save it into pfm file. Successful. And this .pfm file I am using it into another library, but this library doesn't accept big-endian, just little endian.
CImg<float> image;
image.load_bmp(_T("D:\\Temp\\memorial.bmp"));
image.normalize(0.0, 1.0);
image.save_pfm(_T("D:\\Temp\\memorial.pfm"));
So, how can I save bmp file to pfm file as little endian, not big endian .. it is possible ?
Later edit:
I have checked first 5 elements from .pfm header file. This is the result without invert_endianness:
CImg<float> image;
image.load_bmp(_T("D:\\Temp\\memorial.bmp"));
image.normalize(0.0, 1.0);
image.save_pfm(_T("D:\\Temp\\memorial.pfm"));
PF
512
768
1.0
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and this is the result with invert_endianness:
CImg<float> image;
image.load_bmp(_T("D:\\Temp\\memorial.bmp"));
image.invert_endianness();
image.normalize(0.0, 1.0);
image.save_pfm(_T("D:\\Temp\\memorial.pfm"));
PF
512
768
1.0
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Result is the same.

Answer 1

This is definitely not a proper answer but might work as a workaround for the time being.
I didn't find out how to properly invert the endianness using CImgs functions, so I modified the resulting file instead. It's a hack. The result opens fine in GIMP an looks very close to the original image, but I can't say if it works with the library you are using. It may be worth a try.
Comments in the code:
#include "CImg/CImg.h"
#include <algorithm>
#include <filesystem> // >= C++17 must be selected as Language Standard
#include <ios>
#include <iostream>
#include <iterator>
#include <fstream>
#include <string>
using namespace cimg_library;
namespace fs = std::filesystem;
// a class to remove temporary files
class remove_after_use {
public:
remove_after_use(const std::string& filename) : name(filename) {}
remove_after_use(const remove_after_use&) = delete;
remove_after_use& operator=(const remove_after_use&) = delete;
const char* c_str() const { return name.c_str(); }
operator std::string const& () const { return name; }
~remove_after_use() {
try {
fs::remove(name);
}
catch (const std::exception & ex) {
std::cerr << "remove_after_use: " << ex.what() << "\n";
}
}
private:
std::string name;
};
// The function to hack the file saved by CImg
template<typename T>
bool save_pfm_endianness_inverted(const T& img, const std::string& filename) {
remove_after_use tempfile("tmp.pfm");
// get CImg's endianness inverted image and save it to a temporary file
img.get_invert_endianness().save_pfm(tempfile.c_str());
// open the final file
std::ofstream os(filename, std::ios::binary);
// read "tmp.pfm" and modify
// The Scale Factor / Endianness line
if (std::ifstream is; os && (is = std::ifstream(tempfile, std::ios::binary))) {
std::string lines[3];
// Read the 3 PFM header lines as they happen to be formatted by
// CImg. Will maybe not work with another library.
size_t co = 0;
for (; co < std::size(lines) && std::getline(is, lines[co]); ++co);
if (co == std::size(lines)) { // success
// write the first two lines back unharmed:
os << lines[0] << '\n' << lines[1] << '\n';
if (lines[2].empty()) {
std::cerr << "something is wrong with the pfm header\n";
return false;
}
// add a '-' if it's missing, remove it if it's there:
if (lines[2][0] == '-') { // remove the - to invert
os << lines[2].substr(1);
}
else { // add a - to invert
os << '-' << lines[2] << '\n';
}
// copy all the rest as-is:
std::copy(std::istreambuf_iterator<char>(is),
std::istreambuf_iterator<char>{},
std::ostreambuf_iterator<char>(os));
}
else {
std::cerr << "failed reading pfm header\n";
return false;
}
}
else {
std::cerr << "opening files failed\n";
return false;
}
return true;
}
int main()
{
CImg<float> img("memorial.bmp");
img.normalize(0.f, 1.f);
std::cout << "saved ok: " << std::boolalpha
<< save_pfm_endianness_inverted(img, "memorial.pfm") << "\n";
}

Answer 2

Wanting to solve the same issue in classic C++ style (as language sake), I wrote:
BOOL CMyDoc::SavePfmEndiannessInverted(CImg<float>& img, const CString sFileName)
{
CString sDrive, sDir;
_splitpath(sFileName, sDrive.GetBuffer(), sDir.GetBuffer(), NULL, NULL);
CString sTemp;
sTemp.Format(_T("%s%sTemp.tmp"), sDrive, sDir);
sDrive.ReleaseBuffer();
sDir.ReleaseBuffer();
CRemoveAfterUse TempFile(sTemp);
img.get_invert_endianness().save_pfm(TempFile.c_str());
CFile fileTemp;
if (! fileTemp.Open(sTemp, CFile::typeBinary))
return FALSE;
char c;
UINT nRead = 0;
int nCount = 0;
ULONGLONG nPosition = 0;
CString sScale;
CByteArray arrHeader, arrData;
do
{
nRead = fileTemp.Read((char*)&c, sizeof(char));
switch (nCount)
{
case 0:
case 1:
arrHeader.Add(static_cast<BYTE>(c));
break;
case 2: // retrieve the '1.0' string
sScale += c;
break;
}
if ('\n' == c) // is new line
{
nCount++;
}
if (nCount >= 3) // read the header, so go out
{
nPosition = fileTemp.GetPosition();
break;
}
}while (nRead > 0);
if (nPosition > 1)
{
arrData.SetSize(fileTemp.GetLength() - nPosition);
fileTemp.Read(arrData.GetData(), (UINT)arrData.GetSize());
}
fileTemp.Close();
CFile file;
if (! file.Open(sFileName, CFile::typeBinary | CFile::modeCreate | CFile::modeReadWrite))
return FALSE;
CByteArray arrTemp;
ConvertCStringToByteArray(sScale, arrTemp);
arrHeader.Append(arrTemp);
arrHeader.Append(arrData);
file.Write(arrHeader.GetData(), (UINT)arrHeader.GetSize());
file.Close();
return TRUE;
}
But seem to not do the job, because the image result is darker
What I have done wrong here ? The code seem to me very clear, still, is not work as expected ...
Of course, this approach is more inefficient, I know, but as I said before, just for language sake.
I think it is nothing wrong with my code :)
Here is the trial:
CImg<float> image;
image.load_bmp(_T("D:\\Temp\\memorial.bmp"));
image.normalize(0.0f, 1.0f);
image.save_pfm(_T("D:\\Temp\\memorial.pfm"));
image.get_invert_endianness().save(_T("D:\\Temp\\memorial_inverted.pfm"));
and the memorial.pfm looks like this:
and memorial_inverted.pfm looks like this: