How to convert a jpg to yuv in C? - c

Question

Even though a question of this nature sounds very similar, I am having problems in converting a jpg image to yuv in C (without using opencv).
This is what I have understood as of now, how to solve this problem :
Identify the structure of file formats for jpg and yuv. i.e what each byte in the file actually contains. This is what I think jpg format looks like.
With the above structure I tried to read a jpg file and tried to decipher its 18th and 19th bytes. I did type cast them to both char and int but I don`t get any meaningful values for width and height of the image.
Once I have read these values, I should be able to convert them from jpg to yuv. I was looking at this resource.
Appropriately, construct yuv image and write it to a (.yuv) file.
Kindly help me by pointing me to appropriate resources. I will keep updating my progress on this post. Thanks in advance.

Answer 1

Usually the image is already stored in YUV (or, to be more precise: YCbCr).
When reading the file, the jpeg reader usually converts YUV to RGB. Converting back will reduce quality somewhat.
In libTurboJpeg (http://libjpeg-turbo.virtualgl.org/) you can read the jpeg without color conversion. Check https://github.com/libjpeg-turbo/libjpeg-turbo/blob/master/turbojpeg.h -
it has the tjDecompressToYUV function which gives you the 3 colorspaces on 3 different output buffers.

Answer 2

Not sure what you have against opencv, maybe ImageMagick is acceptable to you? It is installed on most Linux distors and is available for OSX, and Windows. It has C bindings, and also a command-line version that I am showing here. So you can create an image like this:
# Create test image
convert -size 100x100 \
\( xc:red xc:lime xc:blue +append \) \
\( xc:cyan xc:magenta xc:yellow +append \) \
-append image.jpg
Now convert to YUV and write to 3 separate files:
convert image.jpg -colorspace yuv -separate bands.jpg
bands-0.jpg (Y)
bands-1.jpg (U)
bands-2.jpg(V)
Or, closer to what you ask, write all three bands YUV into a binary file:
convert image.jpg -colorspace yuv rgb:yuv.bin

Answer 3

Based on https://en.wikipedia.org/wiki/YUV#Y.27UV444_to_RGB888_conversion
Decoding a JPEG, well in pure C without libraries ... the following code is somewhat straightforward ...
https://bitbucket.org/Halicery/firerainbow-progressive-jpeg-decoder/src
Assuming you have the jpeg decoded to rgb using the above or a library (using a library is likely easier).
int width = (width of the image);
int height = (height of the image);
byte *mydata = (pointer to rgb pixels);
byte *cursor;
size_t byte_count = (length of the pixels .... i.e. width x height x 3);
int n;
for (cursor = mydata, n = 0; n < byte_count; cursor += 3, n += 3)
{
int red = cursor[0], green = cursor[1], blue = cursor[2];
int y = 0.299 * red + 0.587 * green + 0.114 * blue;
int u = -0.147 * red + -0.289 * green + 0.436 * blue;
int v = 0.615 * red + -0.515 * green + -0.100 * blue;
cursor[0] = y, cursor[1] = u, cursor[2] = v;
}
// At this point, the entire image has been converted to yuv ...
And write that to file ...
FILE* fout = fopen ("myfile.yuv, "wb");
if (fout) {
fwrite (mydata, 1, byte_count, fout);
fclose (fout);
}