Thread: Rotate 8x8 bit image

Hi Guys
I’m looking to bitwise rotate an 8 byte array by 90 degrees clockwise, so this:

Code:

unsigned char shape[8] = {
0b00011000,
0b00111100,
0b01111110,
0b11111111,
0b00000000,
0b00000000,
0b00000000,
0b00000000;}

would become this...

Code:

0b00001000,
0b00001100,
0b00001110,
0b00001111,
0b00001111,
0b00001110,
0b00001100,
0b00001000;

It seems like it would be easy to rotate 180 degrees by simply reading from one array to another, indexing the bytes in reverse order,
but I can’t seem to wrap my head around doing it just 90 degrees.
Any ideas?
Cheers, Art.

It might be easier to see the pattern/algorithm for rotation if you use a different, array and (for now) don't think about the bit manipulation. Here's a 4 x 5:

Code:

a b c d e
f g h i j
k l m n o
p q r s t

Rotating 90 degrees clockwise gives

Code:

p k f a
q l g b
r m h c
s n i d
t o j e

So, you can see that for populating the new array, the top row (moving left to right) becomes the right column (moving top to bottom), the second row becomes the second-from-right column, etc. Assuming N rows and M columns in the original, you will then have M rows and N columns in the new array. Now, you need to find the pattern of mapping old coordinates to new. For example

Code:

a (0, 0) -> (0, 3)
b (0, 1) -> (1, 3)
c (0, 2) -> (2, 3)
d (0, 3) -> (3, 3)
e (0, 4) -> (4, 3)
...
l (2, 1) -> (1, 1)
m (2, 2) -> (2, 1)
n (2, 3) -> (3, 1)
...
q (3, 1) -> (1, 0)
r (3, 2) -> (2, 0)
s (3, 3) -> (3, 0)
t (3, 4) -> (4, 0)

Where coordinates are (old_row, old_col) -> (new_row, new_col)

If we say that the original matrix was N = 4 rows and M = 5 columns, the new matrix will be M rows (5) and N columns (4).

new_row is pretty easy, it just tracks the old_col. new_col is a bit trickier. It sort of tracks the old_row, but backwards. I'll let you figure that one out. You should express any formulae in terms of N and M instead of magic numbers like 4 or 5. You could apply similar logic for determining counter-clockwise rotations, or even 180 degree rotations.

The logic for doing this with bit manipulation is a bit trickier if you try to work it all into the loop, but you could create a function for storing a value in a particular bit location, the you just calculate new_row and new_col, and use your function to set the right bit:

Code:

value = get_bit(old_matrix, old_row, old_col);
new_row = old_col;
new_col = // you fill in this part
set_bit(new_matrix, new_row, new_col, value);

Hope that makes sense and helps you out.

Thanks
I had procedure worked out, but couldn’t get it rolled into a loop.
I was wasting time anding the bytes with different bit masks.

Simply look at a vertical row, say all bit 7 for example,
write that into the first byte, shift all of the source bits to the let & repeat till done