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Padding calculation in I10 Sprite Drawing / Bitmap Loading
Posted: November 3rd, 2017, 5:50 pm
by HavocVulture
I just completed the homework for I10, but while working on it I was confused by the padding formula. In the video the calculation used is:
Code: Select all
const int padding = (4 - (width * 3) % 4) % 4;
Seemed a bit overwrought to me so I replaced it with:
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const int padding = (width * 3) % 4;
This worked with all the supplied 24-bit images. Is there something I'm missing? Is there a case where the simpler formula fails?
Re: Padding calculation in I10 Sprite Drawing / Bitmap Loadi
Posted: November 3rd, 2017, 7:22 pm
by albinopapa
Let's look at the possible values of width in regards to multiples of 4.
width = 12
(12 * 3) % 4 = 36 % 4 = 0 // Good
width = 13
(13 * 3) % 4 = 39 % 4 = 3 // Not good, we only need a padding of 1 here to make it 40
width = 14
(14 * 3) % 4 = 42 % 4 = 2 // Good
width = 15
(15 * 3) % 4 = 45 % 4 = 1 // Not good, we need a padding of 3 here to make it 48
Re: Padding calculation in I10 Sprite Drawing / Bitmap Loadi
Posted: November 3rd, 2017, 10:08 pm
by HavocVulture
Thanks for clearing that up!
Re: Padding calculation in I10 Sprite Drawing / Bitmap Loadi
Posted: November 4th, 2017, 3:42 am
by albinopapa
That only explains part of it, the 4- part, the reason for the last %4 is in cases where padding = 0, 4 - 0 = 4, but you don't need 4 padding bytes so %4 would make it 0.