Home > Article > Backend Development > How to Decode Compressed Images Using a Custom Input Stream in C ?
C stream input/output (IO) operations are commonly performed using the iostream library. While this library provides a solid foundation for most IO tasks, there are scenarios where you may need to implement custom input streams to handle data in non-standard formats.
Custom input streams are defined by extending the std::streambuf class and overriding its virtual methods, such as underflow() for reading and overflow() and sync() for writing. By overriding these functions, you can control the way the stream interacts with the underlying data source or destination.
Let's consider a specific example of implementing a custom input stream for decoding images that have been compressed using vertical XOR encoding.
<code class="cpp">class vxor_streambuf : public std::streambuf { public: // Constructor takes the original buffer and image width vxor_streambuf(std::streambuf *buffer, int width) : buffer(buffer), size(width / 2) { // Allocate memory for previous and current lines previous_line = new char[size]; memset(previous_line, 0, size); current_line = new char[size]; // Initialize streambuf member variables setg(0, 0, 0); setp(current_line, current_line + size); } // Destructor releases memory ~vxor_streambuf() { sync(); delete[] previous_line; delete[] current_line; } // Underflow() performs XOR decoding for reading std::streambuf::int_type underflow() { // Read line from original buffer streamsize read = buffer->sgetn(current_line, size); if (!read) return traits_type::eof(); // Perform vertical XOR decoding for (int i = 0; i < size; i += 1) { current_line[i] ^= previous_line[i]; previous_line[i] = current_line[i]; } // Update streambuf member variables setg(current_line, current_line, current_line + read); return traits_type::to_int_type(*gptr()); } // Overflow() performs XOR encoding for writing std::streambuf::int_type overflow(std::streambuf::int_type value) { int write = pptr() - pbase(); if (write) { // Perform vertical XOR encoding for (int i = 0; i < size; i += 1) { char tmp = current_line[i]; current_line[i] ^= previous_line[i]; previous_line[i] = tmp; } // Write line to original buffer streamsize written = buffer->sputn(current_line, write); if (written != write) return traits_type::eof(); } // Update streambuf member variables setp(current_line, current_line + size); if (!traits_type::eq_int_type(value, traits_type::eof())) sputc(value); return traits_type::not_eof(value); }; // Sync() flushes any pending data virtual int sync() { streambuf::int_type result = this->overflow(traits_type::eof()); buffer->pubsync(); return traits_type::eq_int_type(result, traits_type::eof()) ? -1 : 0; } private: streambuf *buffer; int size; char *previous_line; char *current_line; };</code>
<code class="cpp">ifstream infile("encoded_image.vxor"); vxor_istream in(infile, 288); // Create a new vxor_istream char data[144 * 128]; in.read(data, 144 * 128); // Read encoded data</code>
By following these guidelines and understanding the intricacies of streambuf manipulation, you can effectively implement custom input streams for various scenarios where standard IO operations fall short.
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