Extract RGB from JPEG image, using libjpeg C++ Library - c++

Question

I am using the libjpeg library to read and copy a jpeg into an editing program that I am writing in C++
I have a display buffer which is a vector of a datatype called ColorData
All ColorData consists of is 3 floats (RGB)
Here is my code that opens the jpeg files
PixelBuffer * IOManager::load_jpg_to_pixel_buffer(const char *file_name){
struct jpeg_decompress_struct cinfo;
FILE * infile;
JSAMPARRAY buffer;
if ((infile = fopen(file_name, "rb")) == NULL) {
std::cout << "Could not open the jpg file: " << file_name << std::endl;
return nullptr;
}
struct jpeg_error_mgr jerr;
cinfo.err = jpeg_std_error(&jerr);
jpeg_create_decompress(&cinfo);
jpeg_stdio_src(&cinfo, infile);
jpeg_read_header(&cinfo, TRUE);
jpeg_start_decompress(&cinfo);
int width = static_cast<int>(cinfo.output_width);
int height = static_cast<int>(cinfo.output_height);
std::cout << typeid(cinfo.colormap).name() << std::endl;
std::cout << "Width: " << width << "Height: " << height << std::endl;
PixelBuffer * image_buffer = new PixelBuffer(width, height, ColorData());
std::cout << cinfo.output_components << std::endl;
buffer = (*cinfo.mem->alloc_sarray)
((j_common_ptr) &cinfo, JPOOL_IMAGE, cinfo.output_width * cinfo.output_components, 1);
/* Step 6: while (scan lines remain to be read) */
/* jpeg_read_scanlines(...); */
/* Here we use the library's state variable cinfo.output_scanline as the
* loop counter, so that we don't have to keep track ourselves.
*/
while (cinfo.output_scanline < cinfo.output_height) {
/* jpeg_read_scanlines expects an array of pointers to scanlines.
* Here the array is only one element long, but you could ask for
* more than one scanline at a time if that's more convenient.
*/
(void) jpeg_read_scanlines(&cinfo, buffer, 1);
/* Assume put_scanline_someplace wants a pointer and sample count. */
}
return nullptr;
}
How can I get the RGB value from the jpeg using the libjpeg?

Answer 1

The RGB values are in buffer. It's actually an array of arrays, so you have to index buffer[0].
Something like this:
while (cinfo.output_scanline < cinfo.output_height)
{
(void) jpeg_read_scanlines(&cinfo, buffer, 1);
// get the pointer to the row:
unsigned char* pixel_row = (unsigned char*)(buffer[0]);
// iterate over the pixels:
for(int i = 0; i < cinfo.output_width; i++)
{
// convert the RGB values to a float in the range 0 - 1
float red = (float)(*pixel_row++) / 255.0f;
float green = (float)(*pixel_row++) / 255.0f;
float blue = (float)(*pixel_row++) / 255.0f;
}
}
This is assuming cinfo.output_components is 3.