How to rotate yuv420 data?

Question

I need to know how to rotate an image, which is in yuv420p format by 90 degrees. The option of converting this to rgb, rotating and again reconverting to yuv is not feasible. Even an algorithm would help.

Regards, Anirudh.

Answer 1

I suppose it is not planar YUV, if it is it already it's quite easy (skip first and last steps). You meant to have YUV 4:2:0 planar, but then I do not understand why you have difficulties.

1. convert it to a planar first: allocate space for planes and put bytes at right places according to the packed YUV format you have.
2. rotate the Y, U, V planes separately. The "color" (U, V) information for each block then shall be kept the same.
3. recombine the planes to reobtain the right packed YUV you had at the beginning

This always works fine if your image dimensions are multiple of 4. If not, then take care...

Answer 2

In case the image is yuv420 planar, this is how the image data is encoded. Planar meaning the y section is first, followed by U section and then with V section.

Considering the width of the image w, and height of the image h.

1. The total size of the image is w*h*3/2

2. The Y section also called luminescence occupies w*h.

3. there is a U pixel and V pixel for every 2x2 block in Y section.

4. the U section comes next, occupies (w/2)*(h/2) and is laid at an offset w*h from beginning of the image.

5. the V section follows, occupies (w/2)*(h/2) and is laid at an offset of (w*h)+((w*h)/4).

In order to rotate the image by 90 degrees, you essentially copy this w*h array to an array of h*w

As mentioned in above post, you simply need to copy each of the 3 above Y, U, V blocks separately.

1. Start with the Y section. The 1st pixel to be copied is at (h-1)*w in Source Array, copy this to (0,0) of destination array. The 2nd pixel is at (h-2)*w and so on...

2. Remember that the U and V sections are only (w/2)*(h/2)

3. Next copy the U section. The first pixel to be copied is at (w*h)+(((h/2)-1)*(w/2)) in Source Array, copy this to (h*w)+(0,0) in the Destination Array. The 2nd pixel is at (w*h)+(((h/2)-2)*(w/2)) and so on...

4. Finally copy the V section. The first pixel to be copied is at ((w*h)+(w*h/4))+(((h/2)-1)*(w/2)) in Source Array, copy this to (h*w)+(w*h/4)+(0,0) in the Destination Array. The 2nd pixel is at ((w*h)+(w*h/4))+(((h/2)-2)*(w/2)) and so on...

The Destination Array obtained in this way contains the 90 degree rotated image.

Answer 3

I think YUV420p is indeed planar.

Try and take a look at AviSynth's source code. The turn (rotate) functions are in turn.cpp and turnfunc.cpp

http://www.avisynth.org/