Uri TauberandRyan Mercado
a2f2eea79e
## Summary
* **What is the goal of this PR?**
Replace the pixel-by-pixel `fillRectDither` implementation with a new
byte-aligned `fillRectImpl` that eliminates per-pixel
`rotateCoordinates` calls and Read-Modify-Write bitwise loops, yielding
a significant rendering speedup on ESP32 E-ink framebuffers.
Ported from @rhythmerc's crosspoint-reader fork commit 27ea625.
* **What changes are included?**
- **`GfxRenderer.cpp` — `fillRectDither` refactor:** The existing
`if/else if` chain is replaced with a `switch` statement that delegates
each `Color` case to the new `fillRectImpl<Color>()` template,
eliminating runtime branching.
- **`GfxRenderer.cpp` — new `fillRectImpl<C>()` template:** Core of the
optimization. Key behaviors:
- Clips the rectangle in logical space upfront.
- Rotates only **2 opposing corner points** (top-left and bottom-right)
into physical framebuffer space instead of rotating every pixel
individually.
- Derives physical-space `byteStart`/`byteEnd` and precomputes
`headMask` / `tailMask` for MSB-first partial-byte boundaries,
performing RMW only on the edge bytes.
- **Solid fills (`Black` / `White`):** Uses `memset` for all interior
full-byte runs per row — no per-pixel writes.
- **Dithered fills (`LightGray` / `DarkGray`):** Precomputes both parity
variants of `blackMask` (even/odd `py`) **outside** the row loop,
eliminating the previously re-evaluated 8-bit construction loop on every
physical row. Interior full bytes are then written with a single
`memset(whiteMask)`.
- Uses `if constexpr` throughout to dispatch on `Color` at compile time,
generating zero runtime branches per template instantiation.
- **`GfxRenderer.h`:** Declares the new private `fillRectImpl<Color>()`
template method with an explanatory doc-comment.
- **Explicit template instantiations** added for all four active `Color`
variants (`Black`, `White`, `LightGray`, `DarkGray`).
## Additional Context
* **Performance:** The primary motivation is ESP32 E-ink framebuffer
performance. The old path called `rotateCoordinates` and did a full RMW
for every single pixel in the rectangle. The new path calls
`rotateCoordinates` exactly **twice** per fill regardless of rectangle
size, then operates at byte granularity — a complexity reduction from
O(W×H) coordinate transforms to O(1).
* **Dither correctness:** The `blackMask` precomputation relies on the
dither pattern having period 2 in both logical X and Y, which makes the
per-row byte pattern repeat with period 2 in `py`. Reviewers should
verify the `lxBase`/`lyBase` derivations for all four orientations
(`Portrait`, `PortraitInverted`, `LandscapeClockwise`,
`LandscapeCounterClockwise`) match the inverse of `rotateCoordinates`.
* **Edge case — single-byte rows:** When `byteStart == byteEnd`, the
head and tail masks are ANDed together into a single `rectMask` to avoid
double-masking the same byte. This path should be tested with narrow
rectangles (width < 8px).
* **No behavioral change for `Color::Clear`:** The `Clear` case exits
early via `if constexpr` and is a no-op, matching the original behavior.
---
### AI Usage
While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.
Did you use AI tools to help write this code? _**< YES >**_
---------
Co-authored-by: Ryan Mercado <rmercado@firstdollar.com>
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