[Newbies] Bitmap loading from array - representation

[Bert Freudenberg]

On Dec 1, 2007, at 20:34 , Pete wrote:

> Hi, I'm new to Smalltalk, coming from C++ and Java.
>
> I find Squeak very interesting, especially the
> features for rapid prototyping.
>
> I tried learning on the job, started with the
> Games-Chess class category, as I'm was formerly
> experienced in chess programming.
>
> My problem is that I dont understand how Bitmaps for
> Forms are defined:
>
> blackKingImage
> 	^((Color yxForm
> 	extent: 40 at 40
> 	depth: 2
> 	fromArray: #( 0 0 0 0 0 0 0 0 0 0 4194304 0 0
> 22020096 0 0 4194304 0 0 89391104 0 0 111411200 0
> 1398016 107216981 1426063360 22369600 107218261
> 1430257664 22456660 107222362 2772434944 89740885
> 111416741 1498415104 90527125 1162892885 1448083456
> 93672805 1095850325 1448083456 362108249 1431656790
> 2522087424 362190169 1435854230 2522087424 362190422
> 1452643686 2522087424 362112598 1431672169 1448345600
> 362112597 2505463146 2522087424 93760085 2505463145
> 1448083456 93678165 2526434665 1448083456 93673045
> 1704351141 1498415104 90527317 1700353429 1498415104
> 23418261 1700353429 1497366528 22631829 1499027029
> 1497366528 22631829 1503221333 1698693120 5657957
> 1503222101 1694498816 1463653 1499026773 2483027968
> 1414485 1499026774 1409286144 354986 2841291433
> 1342177280 87381 1431655765 1073741824 21845
> 1431655765 0 5802 2863311508 0 6485 1431655780 0 6485
> 1521046884 0 6485 1431655780 0 6826 2863311524 0 5461
> 1431655764 0 0 0 0 0 0 0)
> 	offset: 0 at 0)
> 	colorsFromArray: #(#( ) #(0.0 0.0 0.032) #(1.0 1.0
> 1.0) #( )  ))
>
> I searched the whole Web for an explanation, but found
> none.  It's also not clear to me why this array is
> size of 120, when we have an rectangular region of
> 40x40, and what is the difference of depth 1,2,4 ...


Depth is the number of bits per pixel. Depth 2 means 2 bits per  
pixel, hence 4 colors.

The array stores 32 bits per element. So at depth 2 you get 16 pixels  
per element. You need 3 elements per row for 40 pixels. 40 rows times  
3 words per row is 120. Mystery solved.

FYI, pixels are stored big-endian by default.

And I'm pretty sure Form even has a class comment, as should Bitmap.

- Bert -