Confused about libjpeg: jpeg_read_scanlines - c++

Question

I'm confused about the way libjpeg jpeg_read_scanlines works. It's my understanding that it decompresses a JPEG, row by row, and creates a decompressed pixel buffer.
Typical usage is something like:
jpeg_decompress_struct cinfo;
...
unsigned char* image = new unsigned char[cinfo.image_width * cinfo.image_height];
unsigned char* ptr = image;
int row_stride = cinfo.image_width;
while (cinfo.output_scanline < cinfo.image_height)
{
jpeg_read_scanlines(&cinfo, &ptr, 1);
ptr += row_stride;
}
Question: I'm confused about the output buffer size. In all example code I see which uses jpeg_read_scanlines, the size of the output buffer is width X height, where width and height refer to the dimensions of the JPEG file. So for a 10x10 JPEG file we'd have a 100 byte output buffer.
But... isn't the size of each RGB pixel 3 bytes (24-bit)? So shouldn't the uncompressed data actually be width X height X 3 bytes?
Why isn't it?
I notice that with code which uses jpeg_write_scanlines, the buffer to be compressed IS width X height X 3. So why is the buffer used with jpeg_read_scanlines only width X height?

Answer 1

You are only reading 1 line at a time with the line
jpeg_read_scanlines(&cinfo, &ptr, 1);
so you only needed the line
unsigned char* image = new unsigned char[cinfo.image_width * cinfo.image_height];
to be
unsigned char* image = new unsigned char[cinfo.image_width * cinfo.image_components];
The start of the buffer is being re-used for every scanline. Most of your current buffer is actually unused.

Answer 2

For RGB data, output_components will be 3 (R,G,B).
Here's some related documentation from libjpeg.txt:
output_width image width and height, as scaled
output_height
out_color_components # of color components in out_color_space
output_components # of color components returned per pixel
colormap the selected colormap, if any
actual_number_of_colors number of entries in colormap
output_components is 1 (a colormap index) when quantizing colors; otherwise it
equals out_color_components. It is the number of JSAMPLE values that will be
emitted per pixel in the output arrays.
Typically you will need to allocate data buffers to hold the incoming image.
You will need output_width * output_components JSAMPLEs per scanline in your
output buffer, and a total of output_height scanlines will be returned.