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⌂ ∞ ▽ Elite on the 6502

Drawing the screen: fadeColours

[NES version, Bank 3]

       Name: fadeColours                                             [Show more]
       Type: Variable
   Category: Drawing the screen
    Summary: Lookup table that converts a NES colour to the same colour but
             with a smaller brightness value
  Deep dive: Views and view types in NES Elite

    Context: See this variable in context in the source code
 References: This variable is used as follows:
             * FadeColours uses fadeColours




 Colours on the NES are stored as hue and value, using an HSV model but without
 the saturation. Specifically the hue (i.e. blue, red etc.) is stored in the
 low nibble, while the value (i.e. the brightness) is stored in bits 4 and 5
 of the high nibble. Bits 6 and 7 are unused and are always zero.

 This means that given a colour value in hexadecimal, it is in the form &vh
 where v is the value (brightness) and h is the hue. We can therefore alter the
 brightness of a colour by increasing or decreasing the high nibble between
 0 and 3, with &0h being the darkest and &3h being the brightest.

 The NES only supports 54 of the 64 possible colours in this scheme, with
 colours &vE and &vF all being black, as well as $0D. The convention is to use
 $0F for all these variants of black.

 Given a colour &vh, the table entry at fadeColours + &vh contains the same
 colour but with a reduced brightness in &v. Specifically, it returns the
 colour with a brightness of &v - 1. We can therefore use this table to fade a
 colour to black, which will take up to four steps depending on the brightness
 of the starting colour. See the FadeColours routine for an example.



.fadeColours

 EQUB $0F, $0F, $0F, $0F    ; Fade colours with value &0v to black ($0F)
 EQUB $0F, $0F, $0F, $0F
 EQUB $0F, $0F, $0F, $0F
 EQUB $0F, $0F, $0F, $0F

 EQUB $00, $01, $02, $03    ; Fade colours with value &1v to &0v
 EQUB $04, $05, $06, $07
 EQUB $08, $09, $0A, $0B
 EQUB $0C, $0F, $0F, $0F

 EQUB $10, $11, $12, $13    ; Fade colours with value &2v to &1v
 EQUB $14, $15, $16, $17
 EQUB $18, $19, $1A, $1B
 EQUB $1C, $0F, $0F, $0F

 EQUB $20, $21, $22, $23    ; Fade colours with value &3v to &2v
 EQUB $24, $25, $26, $27
 EQUB $28, $29, $2A, $2B
 EQUB $2C, $0F, $0F, $0F