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Update apple2_hires.md

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tools/bitmap editor/apple2_hires.md
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@ -269,9 +269,9 @@ The results are very informative. Let's zoom in a bit.
![screenshot](img/apple2_hires_poke0-128detail.png)
The first line of pixels is black, corresponding to 0.
POKEing 1 in the next line produced a violet dot in x=0, while POKEing 2 resulted in a green dot in x=1 and finally, POKEing 3 created two white dots, one in position 0 and the other in position 1.
`POKE`ing 1 in the next line produced a violet dot in x=0, while `POKE`ing 2 resulted in a green dot in x=1 and finally, POKEing 3 created two white dots, one in position 0 and the other in position 1.
From these 4 POKE, we can already see that
From these 4 `POKE`, we can already see that
#### 1. Plotting is inverted compared to the order of the representation of a binary value.
| dec | binary | result |
@ -302,4 +302,39 @@ To understand what color a pixel is going to be rendered, one must consider the
This might be summarised as the following:
* if the pixel is off, it's rendered black except if both its neighbours are on, in which case it's rendered using the color of its neighbours' columns
* if the pixel is on, it's rendered white except if both its neighbours are off, in which cas it's rendered using the color of his own column
* if the pixel is on, it's rendered white except if both its neighbours are off, in which cas it's rendered using the color of his own column.
And we can continue our observations:
#### 6. It's impossible to plot more than one pair of consecutive colored pixels: colored pixels are always odd in number
#### 7. To plot only two consecutive colored pixels, they must be surrounded by two white pixels on one side and two black pixels on the other side
#### 8. Single dot (then colored) pixels must be surrounded by two pairs of black pixels but the minimum distance between two single dot pixels of the same color is 3 black pixels. The minimum distance between two single dot pixels of different colors is 2 pixels.
Now what about values above 128 ?
Let's edit line 20 of the previous program
20 FOR Y = 0 TO 160
![screenshot](img/apple2_hires_poke0-160detail.png)
Yes ! New colors !
So, the 7th bit switches to a different color palette. Pixels in this palette follow the same rules as the previous palette. But we can add more observations.
#### 9. A second palette is selected when the 7th bit (AKA the "hi-bit") is ON
#### 10. Blue is on even columns and orange/red is on odd columns ... HEY WAIT !! LOOK CLOSELY !
#### 11. Blue pixels are displayed in-between the columns of the violet/green pixels while red pixels are displayed in-between the columns of the green/violet pixels.
How weird is that ?
Let's try this:
10 HGR: N=128: YY=0
20 FOR Y = 0 TO 127
30 A = INT(Y/64): REM A-ZONE
40 B = INT( (Y - 64 * A) / 8): REM B-ZONE
50 C = INT(Y - 64 * A - 8 * B): REM C-ZONE
60 P = 8192 + A * 40 + B * 128 + C * 1024: REM STARTING ADDRESS IN RAM
70 POKE P,YY+N
80 N = 128-N: IF N=0 THEN YY=YY+1
90 NEXT
![screenshot](apple2_hires_poke_colorsdetail.png)
What this does is plotting increasing values but every other line we add 128 to see the equivalent of the second palette.