c++ - How to compress to memory with libjpeg-turbo using jpeg_mem_dest -


i've tried following another answer can't seem right. have 8mib of rbgx bitmap convert jpeg in memory using libjpeg-turbo. if use jpeg_stdio_dest can write whole thing file, read file in, , it's right. however, trying use jpeg_mem_dest has been puzzle. have same set jpeg_stdio_dest, using mem seems 1 allocation of 4kib , never allocate more space.

i can't find docs further instruction on how use jpeg_mem_dest, , use direction.

void compress(std::vector<unsigned char>& input) {   jpeg_compress_struct cinfo{};   jpeg_error_mgr err{};    cinfo.err = jpeg_std_error(&err);   jpeg_create_compress(&cinfo); #if 0 // using open file* out works   jpeg_stdio_dest(&cinfo, out); #endif     cinfo.image_width = kwidth; // constants defined somewhere   cinfo.image_height = kheight;   cinfo.input_components = 4;   cinfo.in_color_space = jcs_ext_rgbx;    // what's wrong this?   unsigned char* buf{};   unsigned long buf_sz{};   jpeg_mem_dest(&cinfo, &buf, &buf_sz);    jpeg_set_defaults(&cinfo);   jpeg_set_quality(&cinfo, 70, true);    jpeg_start_compress(&cinfo, true);      while (cinfo.next_scanline < cinfo.image_height) {     auto row = static_cast<jsamprow>(&input[cinfo.next_scanline  * 4  * kwidth]);     jpeg_write_scanlines(&cinfo, &row, 1);     // prints 4096, , buf never changes      std::cout << "buf_sz: " << buf_sz                << " buf: " << static_cast<void*>(buf) << '\n';   }   jpeg_finish_compress(&cinfo);    // ...   // in reality, return compressed data }

yeah, not intuitive @ all. programmer proposed jpeg_mem_dest() tweak did not have choice, extending existing api not easy when wasn't designed support feature in first place. unobvious variables don't updated until after jpeg_finish_compress() call. relevant code in library is:

methoddef(void) term_mem_destination (j_compress_ptr cinfo) {   my_mem_dest_ptr dest = (my_mem_dest_ptr) cinfo->dest;    *dest->outbuffer = dest->buffer;   *dest->outsize = (unsigned long)(dest->bufsize - dest->pub.free_in_buffer); }

note word "term". function called indirectly through function pointer:

global(void) jpeg_finish_compress (j_compress_ptr cinfo) {   //...   /* write eoi, final cleanup */   (*cinfo->marker->write_file_trailer) (cinfo);   (*cinfo->dest->term_destination) (cinfo);   //... }

nothing can it. tweak std::cout code, move after loop accommodate way library works.

beware other gritty detail of function, not obvious. have free() buffer created. visible in provided cjpeg.c sample program, end of main().


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