OSDev.org

The Place to Start for Operating System Developers
It is currently Sun Dec 04, 2022 9:20 am

All times are UTC - 6 hours




Post new topic Reply to topic  [ 4 posts ] 
Author Message
 Post subject: How exactly do I generate the bitmask for font rendering?
PostPosted: Tue May 24, 2022 10:30 pm 
Offline

Joined: Sat Apr 30, 2022 5:57 am
Posts: 17
Post is to do with this page in the wiki: https://wiki.osdev.org/VGA_Fonts#Displaying_a_character

Specifically, the optimized version of the character displaying routine. There is no explanation of how exactly this bitmap is generated. (Or hardcoded)

I don't know if I am an idiot and have missed some computer graphics term for bitmask that implies some structure, or if the page is just unclear on how to do things.


Top
 Profile  
 
 Post subject: Re: How exactly do I generate the bitmask for font rendering
PostPosted: Wed May 25, 2022 2:40 am 
Offline
Member
Member
User avatar

Joined: Sat Mar 31, 2012 3:07 am
Posts: 4487
Location: Chichester, UK
I'm not quite sure what your problem is. The section "Decoding of bitmap fonts" shows you the structure of the character bitmap.

Edit: Ah - I think you're referring to the mask_table array. Sorry, I've no idea where that came from.


Top
 Profile  
 
 Post subject: Re: How exactly do I generate the bitmask for font rendering
PostPosted: Wed May 25, 2022 3:01 am 
Offline
Member
Member

Joined: Wed Mar 30, 2011 12:31 am
Posts: 624
While the math seems to have been left as an exercise for the reader, the mask table is basically the same data as the bitmap - just with whole bytes filled in instead of individual bits. Or rather, with whole pixels filled in - whatever that pixel size may be.

A brief explanation of the idea is that you have your bitmap representation of a glyph, made up of a byte per row, with bits representing columns, and you take this bitmap and expand it out so that each bit is now 8 bits. Note that for an 8-bit bitmap row, this means a mask row is 8 bytes - the example code seems to assume 32-bit values, so you need to two of them, but I'm lazy and will use 64-bit values. So a row of 01101100b becomes 0x00FFFF00_FFFF0000. Except... we're probably on a little-endian machine, so this is actually backwards. It should be byte reversed to 0x0000FFFF_00FFFF00. We do this for every row in our bitmap and then the masking can be used to write multiple pixels at a time, which is probably faster than individual calls to a set-pixel function.

_________________
toaruos on github | toaruos.org | gitlab | twitter | bim - a text editor


Top
 Profile  
 
 Post subject: Re: How exactly do I generate the bitmask for font rendering
PostPosted: Wed May 25, 2022 3:56 am 
Offline

Joined: Sat Apr 30, 2022 5:57 am
Posts: 17
klange wrote:
While the math seems to have been left as an exercise for the reader, the mask table is basically the same data as the bitmap - just with whole bytes filled in instead of individual bits. Or rather, with whole pixels filled in - whatever that pixel size may be.

A brief explanation of the idea is that you have your bitmap representation of a glyph, made up of a byte per row, with bits representing columns, and you take this bitmap and expand it out so that each bit is now 8 bits. Note that for an 8-bit bitmap row, this means a mask row is 8 bytes - the example code seems to assume 32-bit values, so you need to two of them, but I'm lazy and will use 64-bit values. So a row of 01101100b becomes 0x00FFFF00_FFFF0000. Except... we're probably on a little-endian machine, so this is actually backwards. It should be byte reversed to 0x0000FFFF_00FFFF00. We do this for every row in our bitmap and then the masking can be used to write multiple pixels at a time, which is probably faster than individual calls to a set-pixel function.


Ah, so 0b00100110 becomes 0x00FFFF00, 0x00FF0000. Just checking if I understood your explanation correctly.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC - 6 hours


Who is online

Users browsing this forum: DotBot [Bot], PetalBot [Bot] and 6 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group