Blend Modes Draft
This is a draft as I need to confirm everything behaves on hardware or if there are any quirks.
Layer Blending
Layer Blending on the ZX Spectrum Next allows the visual mixing of the Sprite, Layer 2, and ULA (or Tilemap) layers to produce lighting, shading, or other visual effects.
This is controlled primarily by the layer priority setting Sprite and Layers System Register ($15) and blending source selection ULA Control Register ($68).
Overview
Blending can be enabled using special layer modes referred to as SLU 6 and SLU 7. In these modes, colour values from a "blending" layer (ULA or Tilemap) are added to Layer 2 pixel values and clamped to the 3-bit colour range [0–7].
Blending Modes (SLU 6 & 7)
Set by bits 4–2 of Sprite and Layers System Register ($15):
| SLU Value | Layer Order / Blending Description | Notes |
|---|---|---|
| 000 | S L U | Default mode: Sprites over Layer 2 over ULA |
| 001 | L S U | Layer 2 over Sprites over ULA |
| 010 | S U L | Sprites over ULA over Layer 2 |
| 011 | L U S | Layer 2 over ULA over Sprites |
| 100 | U S L | ULA over Sprites over Layer 2 |
| 101 | U L S | ULA over Layer 2 over Sprites |
| 110 | (U/T) S (T/U) ( B + L) | Blend layer + Layer 2 colours, clamped to [0–7] |
| 111 | (U/T) S (T/U) (B + L - 5) | Blend layer + Layer 2 - 5, colours clamped to [0–7] |
In both SLU 6 and 7:
- The blending source is selected by bits 6:5 of ULA Control Register ($68)
- The blended output replaces the separate display of ULA or Tilemap (whichever is selected)
Blend Source Selection ULA Control Register ($68)
ULA Control Register ($68) (104) — ULA Control
(R/W)
| Bit | Name | Description | Default |
|---|---|---|---|
| 7 | Disable ULA output | Turns off ULA layer entirely | 0 |
| 6–5 | Blending control for SLU 6 & 7 | Selects blend input source | 00 |
| 00 | Use ULA as blend layer | ||
| 01 | No blending (disable blend) | ||
| 10 | Use result of ULA + Tilemap | ||
| 11 | Use Tilemap as blend layer | ||
| 4 | Disable extended key matrix | Cancels entries in the 8x5 key matrix | 0 |
| 3 | ULA+ enable | Enables enhanced ULA+ mode | 0 |
| 2 | Half-pixel scroll | May enable smoother ULA scrolling | 0 |
| 1 | Reserved | Must be 0 | - |
| 0 | Stencil mode | Logical AND of ULA + Tilemap unless either is transparent | 0 |
Example Usage
To enable blending of the ULA layer with Layer 2 and show sprites on top:
- Set NextReg 0x15 bits 4:2 to 110 (SLU 6)
- Set NextReg 0x68 bits 6:5 to 00 (ULA as blend input)
For darker shading effects using SLU 7:
- Set NextReg 0x15 bits 4:2 to 111 (SLU 7)
- Set NextReg 0x68 bits 6:5 accordingly
Notes
- Colours are always clamped to 3-bit RGB values [0–7] per channel.
- SLU 6 blends normally: Blend + Layer2
- SLU 7 blends with subtraction: Blend + Layer2 - 5 (for dimming)
- Blending applies only when the chosen blend source is active and visible.
- Sprites are unaffected by the blending math and remain on top (as per SLU order).