hub75_framebuffer/

lib.rs

//! Framebuffer implementation for HUB75 LED matrix displays.
//!
//! ## How HUB75 LED Displays Work
//!
//! HUB75 RGB LED matrix panels are scanned, time-multiplexed displays that behave like a long
//! daisy-chained shift register rather than a random-access framebuffer.
//!
//! ### Signal names
//! - **R1 G1 B1 / R2 G2 B2** – Serial colour data for the upper and lower halves of the active scan line
//! - **CLK** – Shift-register clock; every rising edge pushes the six colour bits one pixel to the right
//! - **LAT / STB** – Latch; copies the shift-register contents to the LED drivers for the row currently selected by the address lines
//! - **OE** – Output-Enable (active LOW): LEDs are lit while OE is LOW and blanked when it is HIGH
//! - **A B C D (E)** – Row-address select lines (choose which pair of rows is lit)
//! - **VCC & GND** – 5 V power for panel logic and LED drivers
//!
//! ### Row-pair scanning workflow (e.g., 1/16-scan panel)
//! 1. While the panel is still displaying row pair N − 1, the controller shifts the six-bit colour data for row pair N into the chain (OE remains LOW so row N − 1 stays visible).
//! 2. After the last pixel is clocked in, the controller raises OE HIGH to blank the LEDs.
//! 3. With the panel blanked, it first changes the address lines to select row pair N, lets them settle for a few nanoseconds, and **then** pulses LAT to latch the freshly shifted data into the output drivers for that newly selected row.
//! 4. OE is immediately driven LOW again, lighting row pair N.
//! 5. Steps 1–4 repeat for every row pair fast enough (hundreds of Hz) that the human eye sees a steady image.
//!    - If the first row pair is being shifted, the panel continues showing the last row pair of the previous frame until the first blank-address-latch sequence occurs.
//!
//! ### Brightness and colour depth (Binary Code Modulation)
//! - Full colour is typically achieved using **Binary Code Modulation (BCM)**, also known as *Bit-Angle Modulation (BAM)*. Each bit-plane is displayed for a period proportional to its binary weight (1, 2, 4, 8 …), yielding 2ⁿ intensity levels per channel. See [Batsocks – LED dimming using Binary Code Modulation](https://www.batsocks.co.uk/readme/art_bcm_1.htm) for a deeper explanation.
//! - Because each LED is on for only a fraction of the total frame time, the driver can use relatively high peak currents without overheating while average brightness is preserved.
//!
//! ### Implications for software / hardware drivers
//! - You don't simply "write a pixel" once; you must continuously stream the complete refresh data at MHz-range clock rates.
//! - Precise timing of CLK, OE, address lines, and LAT is critical—especially the order: blank (OE HIGH) → set address → latch → un-blank (OE LOW).
//! - Microcontrollers typically employ DMA, PIO, or parallel GPIO tricks, and FPGAs use dedicated logic, to sustain the data throughput while leaving processing resources free.
//!
//! In short: a HUB75 panel is a high-speed shift-register chain that relies on rapid row-pair scanning and **Binary Code Modulation (BCM)** to create a bright, full-colour image. Keeping OE LOW almost all the time—blanking only long enough to change the address and pulse LAT—maximises brightness without visible artefacts.
//!
//! ## Framebuffer Implementations
//!
//! Four framebuffer layouts are provided, covering two hardware variants and
//! two BCM strategies:
//!
//! | Module | Word size | External latch? | BCM strategy |
//! |--------|-----------|-----------------|--------------|
//! | [`plain`] | 16-bit | No | Threshold frames |
//! | [`latched`] | 8-bit | Yes | Threshold frames |
//! | [`bitplane::plain`] | 16-bit | No | True bitplane |
//! | [`bitplane::latched`] | 8-bit | Yes | True bitplane |
//!
//! ### Plain vs. Latched
//! - **Plain** packs all HUB75 signals (address, latch, OE, colour) into each
//!   16-bit word. No extra hardware beyond a parallel output peripheral.
//! - **Latched** uses 8-bit words with a separate external latch circuit to
//!   hold the row address and gate the pixel clock, halving per-entry memory.
//!
//! ### Threshold Frames vs. True Bitplane
//! The two BCM strategies differ in how they store colour data and how the DMA
//! chain must be configured to render it.
//!
//! **Threshold frames** (`plain`, `latched`) -- the driver compares each
//! channel's 8-bit value against per-frame thresholds and stores the resulting
//! on/off bits. For a colour depth of `BITS`, this produces
//! `2^BITS - 1` frames. Frame *n* is displayed for a duration proportional to
//! `2^n`. Memory grows exponentially with colour depth.
//!
//! **True bitplane** (`bitplane::plain`, `bitplane::latched`) -- each of
//! `PLANES` planes (typically 8) stores one bit of every colour channel
//! directly. To render, configure the DMA descriptor chain so that each
//! plane's data is output `2^(7 - plane_index)` times (plane 0 = MSB is
//! scanned 128 times, plane 7 = LSB is scanned once). Memory scales linearly
//! with the number of planes.
//!
//! All four variants have configurable row and column dimensions, support
//! `embedded-graphics` via the `DrawTarget` trait, and expose per-plane
//! pointers for DMA setup through the [`FrameBuffer`] trait.
//!
//! ## Multiple Panels
//! Use [`tiling::TiledFrameBuffer`] to drive several HUB75 panels as one large
//! virtual display. Combine it with a pixel-remapping policy such as
//! [`tiling::ChainTopRightDown`] and any of the framebuffer flavours above.
//! The wrapper exposes a single `embedded-graphics` canvas, so for example a
//! 3 × 3 stack of 64 × 32 panels simply looks like a 192 × 96 screen while
//! all coordinate translation happens transparently.
//!
//! ## Available Feature Flags
//!
//! ### `skip-black-pixels` Feature (disabled by default)
//! When enabled, calls to `set_pixel()` with `Color::BLACK` return early without
//! writing to the framebuffer. This provides a significant performance boost for
//! UI applications that frequently draw black pixels (backgrounds, clearing, etc.)
//! by assuming the framebuffer was already cleared.
//!
//! **Important**: This optimization assumes that black pixels represent "no change"
//! rather than "explicitly set to black". By default, black pixels are written
//! normally to ensure correct overwrite behavior. To enable the optimization:
//!
//! ```toml
//! [dependencies]
//! hub75-framebuffer = { version = "0.10.0", features = ["skip-black-pixels"] }
//! ```
//!
//! ### `esp32-ordering` Feature (required for original ESP32 only)
//! **Required** when targeting the original ESP32 chip (not ESP32-S3 or other variants).
//! This feature adjusts byte ordering to accommodate the quirky requirements of the
//! ESP32's I²S peripheral in 8-bit and 16-bit modes. The original ESP32 has different
//! byte ordering requirements compared to other ESP32 variants (S2, S3, C3, etc.),
//! which do **not** need this feature.
//!
//! ```toml
//! [dependencies]
//! hub75-framebuffer = { version = "0.10.0", features = ["esp32-ordering"] }
//! ```
//!
//! ### `tail-closes-latch` Feature (plain framebuffers only)
//! Appends a single extra "tail" word at the end of the DMA buffer that drives the
//! LATCH signal LOW (de-asserted) on the final clock edge. Without this feature the
//! last word in each row asserts LATCH HIGH to latch shifted data into the LED
//! drivers, and the GPIO pins remain in that state after the DMA transfer completes.
//! Some hardware configurations (e.g. free-running DMA loops or peripherals that
//! continue clocking after the descriptor chain ends) can re-latch stale data or
//! glitch if LATCH is left asserted.
//!
//! Enabling `tail-closes-latch` adds one 16-bit `Entry` (for `plain`) or one entry
//! per bit-plane (for `bitplane::plain`) that parks the bus with LATCH=0 and
//! OE=BLANK, cleanly terminating the transfer. The cost is a single extra word per
//! DMA chunk, which is negligible compared to the frame data.
//!
//! ```toml
//! [dependencies]
//! hub75-framebuffer = { version = "0.10.0", features = ["tail-closes-latch"] }
//! ```
//!
//! ### Blanking delay features (`lead-blank-*` / `trail-blank-*`)
//!
//! Control the number of pixel-clock cycles of blanking (`OE` HIGH) inserted
//! around row-address changes. The lead blank controls how many cycles the
//! output is blanked *before* the row address is changed, and the trail blank
//! controls blanking *after* the row address is changed. Together they give
//! the address lines time to settle and prevent ghosting or "bleeding"
//! artifacts caused by the panel briefly displaying data on the wrong row
//! during the transition.
//!
//! | Feature          | Blanking cycles | Position              |
//! |------------------|-----------------|-----------------------|
//! | *(none)*         | 1 (default)     | before & after change |
//! | `lead-blank-1`   | 1               | before address change |
//! | `lead-blank-2`   | 2               | before address change |
//! | `lead-blank-4`   | 4               | before address change |
//! | `lead-blank-8`   | 8               | before address change |
//! | `lead-blank-16`  | 16              | before address change |
//! | `trail-blank-1`  | 1               | after address change  |
//! | `trail-blank-2`  | 2               | after address change  |
//! | `trail-blank-4`  | 4               | after address change  |
//! | `trail-blank-8`  | 8               | after address change  |
//! | `trail-blank-16` | 16              | after address change  |
//!
//! Higher values reduce ghosting at the cost of slightly less brightness (the
//! LEDs are on for less time per scan line). Start with the default and increase
//! only if you observe row-transition artifacts on your particular panel
//! hardware.
//!
//! ```toml
//! [dependencies]
//! hub75-framebuffer = { version = "0.10.0", features = ["lead-blank-4", "trail-blank-2"] }
//! ```
//!
//! **Note:** At most one `lead-blank-*` and one `trail-blank-*` feature may be
//! enabled at a time. If multiple are enabled for the same edge, compile-time cfg
//! conflicts will result.
//!
//! ### `defmt` Feature
//! Implements `defmt::Format` for framebuffer types so they can be emitted with
//! the `defmt` logging framework. No functional changes; purely adds a trait impl.
//!
//! ### `doc-images` Feature
//! Embeds documentation images when building docs on docs.rs. Not needed for
//! normal usage.
#![no_std]
#![warn(missing_docs)]
#![warn(clippy::all)]
#![warn(clippy::pedantic)]
#![allow(clippy::cast_possible_truncation)]
#![allow(clippy::cast_sign_loss)]

use embedded_graphics::draw_target::DrawTarget;
use embedded_graphics::pixelcolor::Rgb888;
use embedded_graphics::prelude::Point;

pub mod bitplane;
pub mod latched;
pub mod plain;
pub mod tiling;

/// Color type used in the framebuffer
pub type Color = Rgb888;

/// Word size configuration for the framebuffer
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum WordSize {
    /// 8-bit word size
    Eight,
    /// 16-bit word size
    Sixteen,
}

/// Computes the NROWS value from ROWS for `DmaFrameBuffer`
///
/// # Arguments
///
/// * `rows` - Total number of rows in the display
///
/// # Returns
///
/// Number of rows needed internally for `DmaFrameBuffer`
#[must_use]
pub const fn compute_rows(rows: usize) -> usize {
    rows / 2
}

/// Computes the number of frames needed for a given bit depth
///
/// This is used to determine how many frames are needed to achieve
/// the desired color depth through Binary Code Modulation (BCM).
///
/// # Arguments
///
/// * `bits` - Number of bits per color channel
///
/// # Returns
///
/// Number of frames required for the given bit depth
#[must_use]
pub const fn compute_frame_count(bits: u8) -> usize {
    (1usize << bits) - 1
}

/// Trait for read-only framebuffers.
pub trait FrameBuffer {
    /// The word type used by this framebuffer.
    type Word;

    /// Returns the word size configuration for this framebuffer
    fn get_word_size(&self) -> WordSize {
        match size_of::<Self::Word>() {
            1 => WordSize::Eight,
            2 => WordSize::Sixteen,
            _ => panic!("Unsupported word size"),
        }
    }

    /// Returns the number of BCM bit-planes in this framebuffer.
    ///
    /// Contiguous (threshold-based) framebuffers return `1` — the entire
    /// buffer is treated as a single plane.  True bit-plane framebuffers
    /// return the number of planes (typically equal to the colour depth in
    /// bits).
    fn plane_count(&self) -> usize;

    /// Returns a raw pointer and byte length for the given plane.
    ///
    /// For a single-plane framebuffer (`plane_count() == 1`), `plane_idx`
    /// must be `0` and the returned span covers the whole DMA-ready buffer.
    ///
    /// # Panics
    ///
    /// May panic if `plane_idx >= plane_count()`.
    fn plane_ptr_len(&self, plane_idx: usize) -> (*const u8, usize);
}

/// Trait for mutable framebuffers
///
/// This trait extends `FrameBuffer` with the ability to draw to the framebuffer
/// using the `embedded_graphics` drawing primitives.
pub trait MutableFrameBuffer:
    FrameBuffer + DrawTarget<Color = Color, Error = core::convert::Infallible>
{
}

/// Trait for all operations a user may want to call on a framebuffer.
pub trait FrameBufferOperations: FrameBuffer {
    /// Erase pixel colors while preserving control bits.
    /// This is much faster than `format()` and is the typical way to clear the display.
    fn erase(&mut self);

    /// Set a pixel in the framebuffer.
    fn set_pixel(&mut self, p: Point, color: Color);
}

#[cfg(test)]
mod tests {
    extern crate std;

    use std::format;

    use super::*;
    use embedded_graphics::pixelcolor::RgbColor;

    #[test]
    fn test_compute_rows() {
        // Test typical panel sizes
        assert_eq!(compute_rows(32), 16);
        assert_eq!(compute_rows(64), 32);
        assert_eq!(compute_rows(16), 8);
        assert_eq!(compute_rows(128), 64);

        // Test edge cases
        assert_eq!(compute_rows(2), 1);
        assert_eq!(compute_rows(0), 0);

        // Test that it always divides by 2
        for rows in [8, 16, 24, 32, 48, 64, 96, 128, 256] {
            assert_eq!(compute_rows(rows), rows / 2);
        }
    }

    #[test]
    fn test_compute_frame_count() {
        // Test common bit depths
        assert_eq!(compute_frame_count(1), 1); // 2^1 - 1 = 1
        assert_eq!(compute_frame_count(2), 3); // 2^2 - 1 = 3
        assert_eq!(compute_frame_count(3), 7); // 2^3 - 1 = 7
        assert_eq!(compute_frame_count(4), 15); // 2^4 - 1 = 15
        assert_eq!(compute_frame_count(5), 31); // 2^5 - 1 = 31
        assert_eq!(compute_frame_count(6), 63); // 2^6 - 1 = 63
        assert_eq!(compute_frame_count(7), 127); // 2^7 - 1 = 127
        assert_eq!(compute_frame_count(8), 255); // 2^8 - 1 = 255

        // Test the formula: (2^bits) - 1
        for bits in 1..=8 {
            let expected = (1usize << bits) - 1;
            assert_eq!(compute_frame_count(bits), expected);
        }
    }

    #[test]
    fn test_compute_frame_count_properties() {
        // Test that frame count grows exponentially
        assert!(compute_frame_count(2) > compute_frame_count(1));
        assert!(compute_frame_count(3) > compute_frame_count(2));
        assert!(compute_frame_count(4) > compute_frame_count(3));

        // Test doubling property: each additional bit approximately doubles frame count
        for bits in 1..=7 {
            let current_frames = compute_frame_count(bits);
            let next_frames = compute_frame_count(bits + 1);
            // next_frames should be approximately 2 * current_frames + 1
            assert_eq!(next_frames, 2 * current_frames + 1);
        }
    }

    #[test]
    fn test_word_size_enum() {
        // Test enum values
        let eight = WordSize::Eight;
        let sixteen = WordSize::Sixteen;

        assert_ne!(eight, sixteen);
        assert_eq!(eight, WordSize::Eight);
        assert_eq!(sixteen, WordSize::Sixteen);
    }

    #[test]
    fn test_word_size_debug() {
        let eight = WordSize::Eight;
        let sixteen = WordSize::Sixteen;

        let eight_debug = format!("{:?}", eight);
        let sixteen_debug = format!("{:?}", sixteen);

        assert_eq!(eight_debug, "Eight");
        assert_eq!(sixteen_debug, "Sixteen");
    }

    #[test]
    fn test_word_size_clone_copy() {
        let original = WordSize::Eight;
        let cloned = original.clone();
        let copied = original;

        assert_eq!(original, cloned);
        assert_eq!(original, copied);
        assert_eq!(cloned, copied);
    }

    #[test]
    fn test_color_type_alias() {
        // Test that Color is an alias for Rgb888
        let red_color: Color = Color::RED;
        let red_rgb888: Rgb888 = Rgb888::RED;

        assert_eq!(red_color, red_rgb888);
        assert_eq!(red_color.r(), 255);
        assert_eq!(red_color.g(), 0);
        assert_eq!(red_color.b(), 0);

        // Test various colors
        let colors = [
            (Color::RED, (255, 0, 0)),
            (Color::GREEN, (0, 255, 0)),
            (Color::BLUE, (0, 0, 255)),
            (Color::WHITE, (255, 255, 255)),
            (Color::BLACK, (0, 0, 0)),
            (Color::CYAN, (0, 255, 255)),
            (Color::MAGENTA, (255, 0, 255)),
            (Color::YELLOW, (255, 255, 0)),
        ];

        for (color, (r, g, b)) in colors {
            assert_eq!(color.r(), r);
            assert_eq!(color.g(), g);
            assert_eq!(color.b(), b);
        }
    }

    #[test]
    fn test_color_construction() {
        // Test Color construction from RGB values
        let custom_color = Color::new(128, 64, 192);
        assert_eq!(custom_color.r(), 128);
        assert_eq!(custom_color.g(), 64);
        assert_eq!(custom_color.b(), 192);

        // Test that it behaves like Rgb888
        let rgb888_color = Rgb888::new(128, 64, 192);
        assert_eq!(custom_color, rgb888_color);
    }

    #[test]
    fn test_helper_functions_const() {
        // Test that helper functions can be used in const contexts
        const ROWS: usize = 32;
        const COMPUTED_NROWS: usize = compute_rows(ROWS);
        const BITS: u8 = 4;
        const COMPUTED_FRAME_COUNT: usize = compute_frame_count(BITS);

        assert_eq!(COMPUTED_NROWS, 16);
        assert_eq!(COMPUTED_FRAME_COUNT, 15);
    }

    #[test]
    fn test_realistic_panel_configurations() {
        // Test common HUB75 panel configurations
        struct PanelConfig {
            rows: usize,
            cols: usize,
            bits: u8,
        }

        let configs = [
            PanelConfig {
                rows: 32,
                cols: 64,
                bits: 3,
            }, // 32x64 panel, 3-bit color
            PanelConfig {
                rows: 64,
                cols: 64,
                bits: 4,
            }, // 64x64 panel, 4-bit color
            PanelConfig {
                rows: 32,
                cols: 32,
                bits: 5,
            }, // 32x32 panel, 5-bit color
            PanelConfig {
                rows: 16,
                cols: 32,
                bits: 6,
            }, // 16x32 panel, 6-bit color
        ];

        for config in configs {
            let nrows = compute_rows(config.rows);
            let frame_count = compute_frame_count(config.bits);

            // Basic sanity checks for rows
            assert!(nrows > 0);
            assert!(nrows <= config.rows);
            assert_eq!(nrows * 2, config.rows);

            // Basic sanity checks for columns
            assert!(config.cols > 0);
            assert!(config.cols <= 256); // Reasonable upper limit for HUB75 panels

            // Frame count checks
            assert!(frame_count > 0);
            assert!(frame_count < 256); // Should be reasonable for typical bit depths

            // Frame count should grow with bit depth
            let prev_frame_count = compute_frame_count(config.bits - 1);
            assert!(frame_count > prev_frame_count);
        }
    }

    #[test]
    fn test_memory_calculations() {
        // Test that we can calculate memory requirements using helper functions
        const ROWS: usize = 64;
        const COLS: usize = 64;
        const BITS: u8 = 4;

        const NROWS: usize = compute_rows(ROWS);
        const FRAME_COUNT: usize = compute_frame_count(BITS);

        // These should be compile-time constants
        assert_eq!(NROWS, 32);
        assert_eq!(FRAME_COUNT, 15);

        // Verify the relationship between parameters
        assert_eq!(NROWS * 2, ROWS);
        assert_eq!(FRAME_COUNT, (1 << BITS) - 1);

        // Verify COLS is reasonable for memory calculations
        assert!(COLS > 0);
        assert!(COLS <= 256); // Reasonable limit for HUB75 panels
    }

    #[test]
    fn test_edge_cases() {
        // Test minimum values
        assert_eq!(compute_rows(2), 1);
        assert_eq!(compute_frame_count(1), 1);

        // Test maximum reasonable values
        assert_eq!(compute_rows(512), 256);
        assert_eq!(compute_frame_count(8), 255);

        // Test zero (though not practical)
        assert_eq!(compute_rows(0), 0);
    }

    // Note: We can't easily test the traits directly since they're abstract,
    // but they are thoroughly tested through their implementations in
    // the plain and latched modules.

    #[test]
    fn test_word_size_equality() {
        // Test all combinations of equality
        assert_eq!(WordSize::Eight, WordSize::Eight);
        assert_eq!(WordSize::Sixteen, WordSize::Sixteen);
        assert_ne!(WordSize::Eight, WordSize::Sixteen);
        assert_ne!(WordSize::Sixteen, WordSize::Eight);
    }

    #[test]
    fn test_bit_depth_limits() {
        // Test that our bit depth calculations work for the full range
        for bits in 1..=8 {
            let frame_count = compute_frame_count(bits);

            // Frame count should be positive
            assert!(frame_count > 0);

            // Frame count should be less than 2^bits
            assert!(frame_count < (1 << bits));

            // Frame count should be exactly (2^bits) - 1
            assert_eq!(frame_count, (1 << bits) - 1);
        }
    }

    #[test]
    fn test_documentation_examples() {
        // Test the example values from the documentation
        const ROWS: usize = 32;
        const COLS: usize = 64;
        const NROWS: usize = ROWS / 2;
        const BITS: u8 = 8;
        const FRAME_COUNT: usize = (1 << BITS) - 1;

        // Verify using our helper functions
        assert_eq!(compute_rows(ROWS), NROWS);
        assert_eq!(compute_frame_count(BITS), FRAME_COUNT);

        // Verify the values match documentation
        assert_eq!(ROWS, 32);
        assert_eq!(COLS, 64);
        assert_eq!(NROWS, 16);
        assert_eq!(FRAME_COUNT, 255);

        // Verify this matches typical panel dimensions
        assert!(COLS > 0);
        assert_eq!(NROWS * 2, ROWS);
    }
}