问题
Trying to read and write to/from a PPM Image file (.ppm) in the only way I know how:
std::istream& operator >>(std::istream &inputStream, PPMObject &other)
{
inputStream.seekg(0, ios::end);
int size = inputStream.tellg();
inputStream.seekg(0, ios::beg);
other.m_Ptr = new char[size];
while (inputStream >> other.m_Ptr >> other.width >> other.height >> other.maxColVal)
{
other.magicNum = (string) other.m_Ptr;
}
return inputStream;
}
My values correspond to the actual file. So I cheerfully attempt to write the data:
std::ostream& operator <<(std::ostream &outputStream, const PPMObject &other)
{
outputStream << "P6" << " "
<< other.width << " "
<< other.height << " "
<< other.maxColVal << " "
;
outputStream << other.m_Ptr;
return outputStream;
}
I am making sure to open the file using std::ios::binary for both reading and writing:
int main ()
{
PPMObject ppmObject = PPMObject();
std::ifstream image;
std::ofstream outFile;
image.open("C:\\Desktop\\PPMImage.ppm", std::ios::binary);
image >> ppmObject;
image.clear();
image.close();
outFile.open("C:\\Desktop\\NewImage.ppm", std::ios::binary);
outFile << ppmObject;
outFile.clear();
outFile.close();
return 0;
}
Logic Error:
I am only writing a portion of the image. There is no problem with the header or opening the file manually.
Class public member variables:
The m_Ptr member variable is a char * and height, width maxColrVal are all integers.
Attempted Solution:
Using inputStream.read and outputStream.write to read and write data but I don't know how and what I have tried doesn't work.
Since my char * m_Ptr contains all of the pixel data. I can iterate through it:
for (int I = 0; I < other.width * other.height; I++) outputStream << other.m_Ptr[I];
But this causes a run-time error for some reason..
回答1:
Based on http://fr.wikipedia.org/wiki/Portable_pixmap, P6 is a binary image. This reads a single image. Note that no checking is performed. This needs to be added.
std::istream& operator >>(std::istream &inputStream, PPMObject &other)
{
inputStream >> other.magicNum;
inputStream >> other.width >> other.height >> other.maxColVal;
inputStream.get(); // skip the trailing white space
size_t size = other.width * other.height * 3;
other.m_Ptr = new char[size];
inputStream.read(other.m_Ptr, size);
return inputStream;
}
This code writes a single image.
std::ostream& operator <<(std::ostream &outputStream, const PPMObject &other)
{
outputStream << "P6" << "\n"
<< other.width << " "
<< other.height << "\n"
<< other.maxColVal << "\n"
;
size_t size = other.width * other.height * 3;
outputStream.write(other.m_Ptr, size);
return outputStream;
}
m_Ptr contains only the RGB pixel values.
I tested the code on an image I downloaded from the web (http://igm.univ-mlv.fr/~incerti/IMAGES/COLOR/Aerial.512.ppm) and using the following structure PPMObject it worked.
struct PPMObject
{
std::string magicNum;
int width, height, maxColVal;
char * m_Ptr;
};
来源:https://stackoverflow.com/questions/28896001/read-write-to-ppm-image-file-c