Re: [Algorithms] OT: How to read a bitmap of arbitrary size?

[Klaus H. <k_h...@os...>]

Well, this is where it gets very much off-topic :) So I'll try to answer
your question with a single mail. If you still don't get it, then please
mail me off-line.

First off, let's define:
bpr = bytes_per_pixel * pixels_per_row

Now assume that you want to round bpr *down* to the nearest 4, if and only
if bpr is not a multiple of 4. This can be done, by setting the two
least-significant bits of bpr to 0. Assuming that you use a postive 32-bit
integer, you can achieve this as follows:

bpr_rounded_down = bpr & (~3);
<=>
bpr_rounded_down = bpr & 0xFFFFFFFC;

However, in order to compute the pitch, you need to round *up* to the
nearest 4, if and only if bpr is not a multiple of 4. In order to do this,
we first add 3 to bpr, and call the result bpr'. Examples:

bpr = 16 => bpr' = bpr + 3 = 19
bpr = 17 => bpr' = bpr + 3 = 20
bpr = 18 => bpr' = bpr + 3 = 21
bpr = 19 => bpr' = bpr + 3 = 22
bpr = 20 => bpr' = bpr + 3 = 23

Now you use the above approach, and round bpr' *down* to the nearest 4
(which results in the pitch you are looking for):

bpr = 16 => pitch = bpr' & (~3) = 16
bpr = 17 => pitch = bpr' & (~3) = 20
bpr = 18 => pitch = bpr' & (~3) = 20
bpr = 19 => pitch = bpr' & (~3) = 20
bpr = 20 => pitch = bpr' & (~3) = 20

So the formula for the pitch is:
pitch = (bpr + 3) & (~3);

Another way to implement this would be:
pitch = (bpr + 3) - ((bpr + 3) % 4);
<=>
pitch = (bpr + 3) - ((bpr + 3) & 3);

HTH,
Niki

PS: I'm not going to answer the question, why you need to add 3, instead of
4 :)
PPS: Binary operations, like AND, OR, XOR, NOT, et cetera, are very useful,
and it makes a lot of sense to learn more about them.

----- Original Message -----
From: Pai-Hung Chen <pa...@ac...>
To: <gda...@li...>
Sent: Sunday, August 13, 2000 8:31 PM
Subject: Re: [Algorithms] OT: How to read a bitmap of arbitrary size?


> Hi,
>
> That explains everything! :-)  Thanks a lot!  But could you explain what's
> the meaning of ((bytes_per_pixel * pixels_per_row) + 3) & (~3),
specifically
> the ...+ 3) & (~3) part?
>
> Thank you,
>
> Pai-Hung Chen