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VERA rendering pipeline is not properly documented #286
Description
I would like some documentation regarding how the VERA pipeline works.
Based on what I found regarding sprites in the VERA programming guide, I THINK the order is:
- Background
- Layer0
- Layer1
Sprites can be rendered at these locations:
- Disabled
- Between background and layer0
- Between layer0 and layer1
- In front of layer1
For sprites within the same location, sprites with its properties at a lower address in ram is rendered on top of sprites with its properties higher in ram.
The background itself, is defined by the "Border Color" (DC_BORDER) register.
Once this is confirmed (or corrected), I think this info belongs somewhere inside the VERA Programmer's reference.
In addition to this, I suggest to document how the different screen modes uses the layers, since users may want to start with a screen mode, and do VERA config on top of it. Not sure where this implementation detail should be documented though. I think this info may be in the same category as the memory map describing HOW the x16 stores data in video ram, and thus, I think these details should be stored the same place.
Activity
stople commentedon Feb 18, 2026
My first explanation seem to be inaccurate. Trying again, with what I found: (TLDR)
Sprites are first rendered to a line buffer, and there are a limited number of sprites that can be rendered pr line:
The line budget is 801 pr line.
The line can have up to 640 pixels. There are 128 entries in the sprite table.
While drawing the screen, sprites are written to a line buffer.
This line buffer stores color, Z and collision info for each pixel.
The sprite with lowest address is evaluated first.
If the sprite is on the line, the pixels are evaluated:
The composer evaluates the Z of each sprite pixel of the sprite line buffer, and draws the color at the relevant place.
Screen mode $80 uses layer1 for 16 color 1bpp text with transparent background, on top of pixel data which uses layer0.
Screen mode 0 uses layer1 for 16 color 1bpp text with white text on blue background, hiding layer 0 which is disabled.