Difference between revisions of "Sprite and Layers System Register"

Revision as of 09:42, 28 December 2018

Number	$15
Readable	Yes
Writable	Yes
Short Description	Enables/disables Sprites and Lores Layer, and chooses priority of sprites and Layer 2.

Bitmapped as follows:

Bit	Function
7	Enable Lores Layer
6	If 1, the sprite rendering order is reversed, i.e. sprite 0 is on top (0 after reset)
5	If 1, the clipping works even in "over border" mode (doubling X-axis coordinates of clip window) (0 after reset)
4-2	Layers priority and mixing
1	Enable sprites over border (0 after reset)
0	Enable sprite visibility (0 after reset)

Bits 4-2	Priority and mixing of layers
%000	S L U (Sprites are at top, Layer 2 under, Enhanced ULA at bottom)
%001	L S U
%010	S U L
%011	L U S
%100	U S L
%101	U L S
%110	S (U+L) Colours from ULA and Layer 2 are added (per R/G/B channel) and clamped to 7.
%111	S (U+L-5) Similar as previous, but per R/G/B channel (U+L-5) is calculated and result is clamped by 0..7 range.

In layer mixing modes (%110 and %111) the "priority bit 7" of Layer 2 colour is propagated through the calculation, i.e. such pixel will raise above sprites even in mixing modes. If either U or L layer contains transparent colour pixel, the other layer pixel is drawn unchanged (skipping mixing calculation).

When both "over border" (bit 1) and "clipping over border" (bit 5) is enabled, the Sprite clipping window Clip Window Sprites Register ($19) X-axis coordinates are "doubled", and coordinates origin is moved from pixel [0,0] position to sprite [0,0] position ([-32,-32] pixel position). For example: setting clip coordinates as {x1:5, x2:155, y1:7, y2:248} will make sprites visible in area [10,7]..[311,248] (inclusively) in sprite coordinates, i.e. the first visible pixel is at X1*2 coordinate, and last visible pixel is at X2*2+1 coordinate.

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 In layer mixing modes (%110 and %111) the "priority bit 7" of Layer 2 colour is propagated through the calculation, i.e. such pixel will raise above sprites even in mixing modes. If either U or L layer contains ''transparent'' colour pixel, the other layer pixel is drawn unchanged (skipping mixing calculation).
-When both "over border" (bit 1) and "clipping over border" (bit 5) is enabled, the Sprite clipping window {{NextRegNo|$19}} '''X-axis''' coordinates are "doubled", and coordinates origin is moved from pixel [0,0] position to sprite [0,0] position ([-32,-32] pixel position). For example: setting clip coordinates as {x1:5, x2:155, y1:7, y2:248} will make sprites visible in area [10,7]..[309,248] (inclusively) in sprite coordinates.
+When both "over border" (bit 1) and "clipping over border" (bit 5) is enabled, the Sprite clipping window {{NextRegNo|$19}} '''X-axis''' coordinates are "doubled", and coordinates origin is moved from pixel [0,0] position to sprite [0,0] position ([-32,-32] pixel position). For example: setting clip coordinates as {x1:5, x2:155, y1:7, y2:248} will make sprites visible in area [10,7]..[311,248] (inclusively) in sprite coordinates, i.e. the first visible pixel is at X1*2 coordinate, and last visible pixel is at X2*2+1 coordinate.
-[[User:Ped7g|Ped7g]] ([[User talk:Ped7g|talk]]) 07:41, December 24, 2018 (UTC) note the doubled coordinates behave (core version 2.00.22 is described above) differently to normal coordinates, for example in X1=X2 case: normal clip window will display one pixel, but doubled coordinates will test X2*2-1 < X1*2, so no pixel should be displayed. This inconsistency has been raised in forum, and may change eventually (I would personally expect X2*2+1 boundary applied, so X1=X2 would produce 2-pixel-wide visible area, or even X2*2 producing odd-sized areas (up till full 320 sprite coordinate range) sounds reasonable), as new core was not yet released. So the details are "to be confirmed" by final version in published 2.00.22+ core.