Rotating images/graphics

[the ink monitor]

I read somewhere awhile back that when you rotate a graphic in PSP and say you want to rotate it some more that it was best to undo the first rotate and start again because the PC(?) had to recalculate the position and in doing so a little bit of the data/definition was lost, so if you kept on rotating this loss would be made worse as it would be multiplied up.

Now, I also seem to remember that this 'Golden Rule' of undoing rotates applies only to small amounts up to 90degrees and that whole jumps of 90, 180 and 270 degrees were quite OK because they are straightforward mirrors, flips or right angle turns.

Hope I've explained that clearly!

[allicorn]

That's about right.

Because pixels are square, if you rotate a picture in way that means each individual pixel can just move to a new place, then that rotation doesn't reduce the quality of the image if you keep doing it over and over.

Multiples of 90 degrees. Basically, if you imagine that the picture was just a solid colored square - any rotation you make where you wouldn't actually be able to tell whether it had rotated or not by looking at - that's safe, and you can do it as many times as you like.

All other angles (whether they be small or large) that aren't an exact multiple of 90' result in pixels changing not just moving. If you keep rotating, those changes add up and you soon realise your image is all a bit more blurry than it started out. The trick - as you point out - is to undo the last rotate before doing the next one.

You can see this in effect very easily if you make a really small new image, say 10x10, fill it white, zoom right in as far as you can go and then draw a single black line - just one pixel wide - down the center.

Now rotate the image 180'. The pixel line still looks absolutely perfect.

Now rotate it 10' (or some other value) and you'll see how PSP cleverly changes and interpolates the colors of individual pixels to preserve the impression of a smooth-edged line despite the fact that pixels are always just little square tiles.

Try rotating a few more times and you'll the line get softer and softer but an undo and then a second rotate will not cause the same reduction in quality.

Alli

[the ink monitor]

Ahhhhh! As I thought, and thank you allicorn - just one more thing (just to show my ignorance!) - so it's the software that is making the adjustment and not, as I thought, the computer?

I thought I had read that it was the computer that couldn't er, compute the change.

[allicorn]

Interesting question

Kinda both I guess. The computer crunches the numbers but it's the software that decides exactly which sums to do to work out what color the pixels need to be after the rotation.

The limitation, and the ultimate reason why this all has to work the way it does, is that the computer - all computers - build up the picture on the screen out of pixels that are basically just square tiles. They might be so tiny you can barely see a single one - but they're all square.

So - any line or edge that can run along in line with the edges of the square pixels - a straight vertical or horizontal line - is always going to be the sharpest, most accurate line you can draw on the computer screen.

While any line with a slope to it, is to some degree made built up like a side-on view of a flight of stairs because of the square tiles it's made of.

A word you might have come across that kinda refers to being able to make out those "steps" in the sloping line is "jaggies". Each step is a "jaggie", because the square pixel tiles make the whole sloping line a bit jagged edged.

Alli

[the ink monitor]

Yes, I understand the jaggies, and thanks for the experiment which I have tried - and it certainly distorts!
Once you have rotated it 90 degrees in total, in 10 degree amounts, the line is barely distinguishable.

I tried it again with a Vector line and although it breaks up, it eventually snaps back together again after a quarter (9 x 10 degrees in total) turn.

Verrrrrrrrry interesting