Confused about libjpeg: jpeg_read_scanlines

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?

You are reading the example incorrectly. Pay attention to the row_stride variable. — n. 1.8e9-where's-my-share m.
@n.m., in the example code row_stride is cinfo.output_width * cinfo.output_components;. And since cinfo.output_components is 1, row_stride is equivalent to cinfo.output_width — Channel72

Judge_Dredd Judge_Dredd · Accepted Answer · 2013-08-08T21:50:55

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.

Confused about libjpeg: jpeg_read_scanlines

2 Answers