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100 lines
4.5 KiB
Markdown
100 lines
4.5 KiB
Markdown
# Hires to Double-Hires Conversion
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## Background
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You can ignore color and just think about mapping to the 560x192 black and white
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display, since at that level HR and DHR are identical. NTSC magic translates the
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560x192 pattern into color, so if you get B&W correct you'll get color correct.
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(Mostly...)
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Hires uses the lower 7 bits per byte directly (bit 0 on the left of the screen).
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7 pixels/byte * 40 bytes per row gives 280 pixels per row. If the 8th bit is set
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that set of 7 pixels is shifted by half a pixel. This gives a resolution of 560
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pixels per row, although most patterns cannot be set.
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Double Hires also uses the lower 7 bits. 7 pixels/byte * 80 bytes per row gives
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560 pixels per row. The 8th bit in each byte is complete ignored. For added
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fun, bytes alternate between aux mem and main memory. But we want to reason
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about byte pairs anyway, since we're mapping one hires byte to 2 dhr bytes.
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## Conversion
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### Easy Case
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Hires bytes with the high bit set turn out to be the easy case. The source
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7 bits are basically just doubled up and then split with 7 going into each
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destination byte. We'll visualize that with the byte patterns for blue/orange:
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```
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source byte: 0xD5 | 0xAA
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source bits: 0b11010101 | 0b10101010
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hr pixels: 1 0 1 0: 1 0 1 | 0 1 0 1: 0 1 0
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dhr pixels: 11 00 11 0:0 11 00 11 | 00 11 00 1:1 00 11 00
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dest bits: 0b_1100110 0b_1100110 | 0b_1001100 0b_0011001
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dest bytes: 0x66 0x66 | 0x4C 0x19
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```
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To read that table, remember that bits/pixels are in reverse order.
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Also, the high destination bit doesn't matter; this wil prove useful later.
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### Hard Case
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When the high bit is _not_ set, the source pixels are shifted a
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half-pixel *right*, here shown by shifting the dhr pixels *left*.
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We'll visualize this using the byte patterns for violet/green:
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```
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source byte: 0x55 | 0x2A
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source bits: 0b01010101 | 0b00101010
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hr pixels: 1 0 1 0: 1 0 1 | 0 1 0 1: 0 1 0
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dhr pixels: (1)1 00 11 00: 11 00 11(?) | (0)0 11 00 11: 00 11 00(?)
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dest bits: 0b_0011001 0b_?110011 | 0b_1100110 0b_?001100
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dest bytes: 0x19 0x33 | ?<<7 | 0x66 0x06 | ?<<7
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```
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This means we're dropping a bit on the left - shown in parentheses -
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and have an unknown bit on the right - shown as ?. Visualizing it this
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way solves the mystery: when the high bit is clear in the hires bytes,
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a bit "spills" from each dhr pixel pair into the pixel pair to the left.
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Doing this will preserve the original 560 B&W pixel pattern.
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### Conversion Algorithm
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We pre-compute a table using this logic for all 256 source bytes to
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two destination bytes.
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This "spilling" means we cannot simply convert all 8192 bytes in order;
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we need to process each row 40 bytes at a time. (Bonus - we preserve
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screen holes!) Instead, we start at the right edge of the screen, get
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the source byte, and look up the two destination bytes - one table for
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the left (aux) byte, one for the right (main) byte.
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If the high bit of the source byte is _set_ we're in the easy case and
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so we're done - we have the two destination bytes.
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If the high bit of the source byte is _clear_ we're in the harder case.
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We need to spill a bit _in_ and a bit _out_. We saved the bit we're spilling
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_in_ from the previous iteration, so we `ORA` that into place. (Aside: that
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means we reset the spill bit both at the start of each row _and_ if the
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previous source byte had the high bit set.)
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To know what bit to spill _out_ we sneakily look at the otherwise unused
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high bit of the _destination byte_ from the table. The code which constructs
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the mapping table slips it into place there. (I said it would be useful.)
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## Edge Cases
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Astute readers will note that if high bits are clear in the source (i.e. the
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green/violet palette is used) we have some apparent problems.
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* For the right-most pixel we don't have anything to spill in; it will be left as 0.
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* The left-most pixel spills out and is ignored.
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So far as I can tell, this doesn't cause an actual problem in most cases. This
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effect is below the effective resolution of the hires display, should at most
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alter the NTSC color fringes that occur on the screen edges. Samples demonstrating
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actual problems are welcome.
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I also haven't experimented in detail, e.g. with things like the infamous
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"orange squeeze out" where vertical orange lines on byte boundaries disappear.
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In theory the same effect should be present here since we started from
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first-principles but practice may differ.
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