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Fixed a bug introduced by wrongly assuming that I could reverse some logic in part6

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This commit is contained in:
Adam Greenwood-Byrne 2020-07-15 22:09:40 +01:00
parent 6f8ea3ec5b
commit 735963ea89
2 changed files with 17 additions and 15 deletions
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28
part6-breakout/README.md
View file

@ -20,14 +20,14 @@ void drawChar(unsigned char ch, int x, int y, unsigned char attr, int zoom)
{
unsigned char *glyph = (unsigned char *)&font + (ch < FONT_NUMGLYPHS ? ch : 0) * FONT_BPG;
for (int i=0;i<(FONT_HEIGHT*zoom);i++) {
for (int i=1;i<=(FONT_HEIGHT*zoom);i++) {
for (int j=0;j<(FONT_WIDTH*zoom);j++) {
unsigned char mask = 1 << (j/zoom);
unsigned char col = (*glyph & mask) ? attr & 0x0f : (attr & 0xf0) >> 4;
drawPixel(x+j, y+i, col);
}
glyph += (i%zoom) ? FONT_BPL : 0;
glyph += (i%zoom) ? 0 : FONT_BPL;
}
}
```
@ -70,21 +70,23 @@ j=7 : (j / zoom) = 7 j=7 : (j / zoom) = 3
... ...
```
So, that sorts out our horizontal zoom. To achieve the vertical zoom, it's a similar solution. Instead of advancing our glyph pointer by the number of bytes per line on each iteration, we do it one in every `zoom` iterations by making use of the **modulo** operator (I think of this as "remainder after division"). If `i` divided by `zoom` has a non-zero remainder then we advance our glyph pointer, otherwise we leave it where it is!
So, that sorts out our horizontal zoom. To achieve the vertical zoom, it's a similar solution. Instead of advancing our glyph pointer by the number of bytes per line on each iteration, we do it one in every `zoom` iterations by making use of the **modulo** operator (I think of this as "remainder after division"). If `i` divided by `zoom` has no remainder then we advance our glyph pointer, otherwise we leave it where it is!
```c
zoom=2
i=0 : (i % zoom) = 0
i=1 : (i % zoom) = 1 -> advance the pointer
i=2 : (i % zoom) = 0
i=3 : (i % zoom) = 1 -> advance the pointer
i=4 : (i % zoom) = 0
i=5 : (i % zoom) = 1 -> advance the pointer
i=6 : (i % zoom) = 0
i=7 : (i % zoom) = 1 -> advance the pointer
i=1 : (i % zoom) = 1
i=2 : (i % zoom) = 0 -> advance the pointer
i=3 : (i % zoom) = 1
i=4 : (i % zoom) = 0 -> advance the pointer
i=5 : (i % zoom) = 1
i=6 : (i % zoom) = 0 -> advance the pointer
i=7 : (i % zoom) = 1
i=8 : (i % zoom) = 0 -> advance the pointer
...
```
By all means make these changes to _fb.c_ in your part5-framebuffer code and exercise them properly in _kernel.c_ to check that they work. Don't forget to update the function definition in _fb.h_ to include the `zoom` parameter too.
Perhaps you can see why we changed our loop to count from 1 rather than 0?
It is now trivial to modify `drawString` to take a `zoom` parameter and pass it through, so I won't document the changes here.
If you want, you can now make these changes to _fb.c_ in your part5-framebuffer code and exercise them properly in _kernel.c_ to check that they work. Don't forget to update the function definition in _fb.h_ to include the `zoom` parameter too.
It is also now trivial to modify `drawString` to take a `zoom` parameter and pass it through, so I won't document the changes here.

4
part6-breakout/fb.c
View file

@ -144,14 +144,14 @@ void drawChar(unsigned char ch, int x, int y, unsigned char attr, int zoom)
{
unsigned char *glyph = (unsigned char *)&font + (ch < FONT_NUMGLYPHS ? ch : 0) * FONT_BPG;
for (int i=0;i<(FONT_HEIGHT*zoom);i++) {
for (int i=1;i<=(FONT_HEIGHT*zoom);i++) {
for (int j=0;j<(FONT_WIDTH*zoom);j++) {
unsigned char mask = 1 << (j/zoom);
unsigned char col = (*glyph & mask) ? attr & 0x0f : (attr & 0xf0) >> 4;
drawPixel(x+j, y+i, col);
}
glyph += (i%zoom) ? FONT_BPL : 0;
glyph += (i%zoom) ? 0 : FONT_BPL;
}
}