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Describe fixed-point types #26

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Aug 3, 2024
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Describe fixed-point types #26

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41 changes: 33 additions & 8 deletions include/twin_private.h
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Original file line number Diff line number Diff line change
Expand Up @@ -15,11 +15,40 @@
#define maybe_unused __attribute__((unused))

/*
* Post-transformed points are stored in 12.4 fixed point
* values
* Fixed-point type definitions
*
* "Qm.f" is a representation of a fixed-point type,
* where "m" bits are used for the integer part,
* "f" bits are used for the fractional part and 1 bit is used
* for the sign part. The total number of used bits is 1+m+f.
*
* twin_sfixed_t - A fixed-point type in the Q11.4 format.
*
* Hex Binary Decimal Actual
* Max 0x7fff 0111 1111 1111 1111 32767 2047.9375
* Min 0x8000 1000 0000 0000 0000 -32768 2048
*
* twin_dfixed_t - A fixed-point type in the Q23.8 format.
*
* Hex Binary
* Max 0x7fffffff 0111 1111 1111 1111 1111 1111 1111 1111
* Min 0x80000000 1000 0000 0000 0000 0000 0000 0000 0000
* Decimal Actual
* Max 2147483647 8388607.99609
* Min -2147483648 -8388608
*
* twin_gfixed_t - A fixed-point type in the Q1.6 format.
* And used in Glyph coordinates.
*
* Hex Binary Decimal Actual
* Max 0x7f 0111 1111 127 1.984375
* Min 0x80 1000 0000 -128 -2
*
* All of the above tables are based on the two's complement.
*/
typedef int16_t twin_sfixed_t; /* 12.4 format */
typedef int32_t twin_dfixed_t; /* 24.8 format (12.4 * 12.4) */
typedef int16_t twin_sfixed_t;
typedef int32_t twin_dfixed_t;
typedef int8_t twin_gfixed_t;

#define twin_sfixed_floor(f) ((f) & ~0xf)
#define twin_sfixed_trunc(f) ((f) >> 4)
Expand All @@ -46,10 +75,6 @@ typedef int32_t twin_dfixed_t; /* 24.8 format (12.4 * 12.4) */
#define TWIN_SFIXED_MIN (-0x7fff)
#define TWIN_SFIXED_MAX (0x7fff)

/*
* Glyph coordinates are stored in 2.6 fixed point
*/
typedef signed char twin_gfixed_t;

#define TWIN_GFIXED_ONE (0x40)

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