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Fix BMP rendering gamma/brightness (#302)

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1. Refactor Bitmap.cpp/h to expose the options for FloydSteinberg and
brightness/gamma correction at runtime
2. Fine-tune the thresholds for Floyd Steiberg and simple quantization
to better match the display's colors

Turns out that 2 is enough to make the images render properly, so the
brightness boost and gamma adjustment doesn't seem necessary currently
(at least for my test image).
This commit is contained in:
Jonas Diemer 2026-01-12 12:36:19 +01:00 committed by GitHub
parent 66b100c6ca
commit 0165fab581
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GPG Key ID: B5690EEEBB952194
6 changed files with 371 additions and 414 deletions
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166
lib/GfxRenderer/Bitmap.cpp
View File

@ -8,119 +8,15 @@
// ============================================================================ // ============================================================================
// Note: For cover images, dithering is done in JpegToBmpConverter.cpp // Note: For cover images, dithering is done in JpegToBmpConverter.cpp
// This file handles BMP reading - use simple quantization to avoid double-dithering // This file handles BMP reading - use simple quantization to avoid double-dithering
constexpr bool USE_FLOYD_STEINBERG = false; // Disabled - dithering done at JPEG conversion constexpr bool USE_ATKINSON = true; // Use Atkinson dithering instead of Floyd-Steinberg
constexpr bool USE_NOISE_DITHERING = false; // Hash-based noise dithering
// Brightness adjustments:
constexpr bool USE_BRIGHTNESS = false; // true: apply brightness/gamma adjustments
constexpr int BRIGHTNESS_BOOST = 20; // Brightness offset (0-50), only if USE_BRIGHTNESS=true
constexpr bool GAMMA_CORRECTION = false; // Gamma curve, only if USE_BRIGHTNESS=true
// ============================================================================ // ============================================================================
// Integer approximation of gamma correction (brightens midtones)
static inline int applyGamma(int gray) {
if (!GAMMA_CORRECTION) return gray;
const int product = gray * 255;
int x = gray;
if (x > 0) {
x = (x + product / x) >> 1;
x = (x + product / x) >> 1;
}
return x > 255 ? 255 : x;
}
// Simple quantization without dithering - just divide into 4 levels
static inline uint8_t quantizeSimple(int gray) {
if (USE_BRIGHTNESS) {
gray += BRIGHTNESS_BOOST;
if (gray > 255) gray = 255;
gray = applyGamma(gray);
}
return static_cast<uint8_t>(gray >> 6);
}
// Hash-based noise dithering - survives downsampling without moiré artifacts
static inline uint8_t quantizeNoise(int gray, int x, int y) {
if (USE_BRIGHTNESS) {
gray += BRIGHTNESS_BOOST;
if (gray > 255) gray = 255;
gray = applyGamma(gray);
}
uint32_t hash = static_cast<uint32_t>(x) * 374761393u + static_cast<uint32_t>(y) * 668265263u;
hash = (hash ^ (hash >> 13)) * 1274126177u;
const int threshold = static_cast<int>(hash >> 24);
const int scaled = gray * 3;
if (scaled < 255) {
return (scaled + threshold >= 255) ? 1 : 0;
} else if (scaled < 510) {
return ((scaled - 255) + threshold >= 255) ? 2 : 1;
} else {
return ((scaled - 510) + threshold >= 255) ? 3 : 2;
}
}
// Main quantization function
static inline uint8_t quantize(int gray, int x, int y) {
if (USE_NOISE_DITHERING) {
return quantizeNoise(gray, x, y);
} else {
return quantizeSimple(gray);
}
}
// Floyd-Steinberg quantization with error diffusion and serpentine scanning
// Returns 2-bit value (0-3) and updates error buffers
static inline uint8_t quantizeFloydSteinberg(int gray, int x, int width, int16_t* errorCurRow, int16_t* errorNextRow,
bool reverseDir) {
// Add accumulated error to this pixel
int adjusted = gray + errorCurRow[x + 1];
// Clamp to valid range
if (adjusted < 0) adjusted = 0;
if (adjusted > 255) adjusted = 255;
// Quantize to 4 levels (0, 85, 170, 255)
uint8_t quantized;
int quantizedValue;
if (adjusted < 43) {
quantized = 0;
quantizedValue = 0;
} else if (adjusted < 128) {
quantized = 1;
quantizedValue = 85;
} else if (adjusted < 213) {
quantized = 2;
quantizedValue = 170;
} else {
quantized = 3;
quantizedValue = 255;
}
// Calculate error
int error = adjusted - quantizedValue;
// Distribute error to neighbors (serpentine: direction-aware)
if (!reverseDir) {
// Left to right
errorCurRow[x + 2] += (error * 7) >> 4; // Right: 7/16
errorNextRow[x] += (error * 3) >> 4; // Bottom-left: 3/16
errorNextRow[x + 1] += (error * 5) >> 4; // Bottom: 5/16
errorNextRow[x + 2] += (error) >> 4; // Bottom-right: 1/16
} else {
// Right to left (mirrored)
errorCurRow[x] += (error * 7) >> 4; // Left: 7/16
errorNextRow[x + 2] += (error * 3) >> 4; // Bottom-right: 3/16
errorNextRow[x + 1] += (error * 5) >> 4; // Bottom: 5/16
errorNextRow[x] += (error) >> 4; // Bottom-left: 1/16
}
return quantized;
}
Bitmap::~Bitmap() { Bitmap::~Bitmap() {
delete[] errorCurRow; delete[] errorCurRow;
delete[] errorNextRow; delete[] errorNextRow;
delete atkinsonDitherer;
delete fsDitherer;
} }
uint16_t Bitmap::readLE16(FsFile& f) { uint16_t Bitmap::readLE16(FsFile& f) {
@ -244,13 +140,14 @@ BmpReaderError Bitmap::parseHeaders() {
return BmpReaderError::SeekPixelDataFailed; return BmpReaderError::SeekPixelDataFailed;
} }
// Allocate Floyd-Steinberg error buffers if enabled // Create ditherer if enabled (only for 2-bit output)
if (USE_FLOYD_STEINBERG) { // Use OUTPUT dimensions for dithering (after prescaling)
delete[] errorCurRow; if (bpp > 2 && dithering) {
delete[] errorNextRow; if (USE_ATKINSON) {
errorCurRow = new int16_t[width + 2](); // +2 for boundary handling atkinsonDitherer = new AtkinsonDitherer(width);
errorNextRow = new int16_t[width + 2](); } else {
prevRowY = -1; fsDitherer = new FloydSteinbergDitherer(width);
}
} }
return BmpReaderError::Ok; return BmpReaderError::Ok;
@ -261,17 +158,6 @@ BmpReaderError Bitmap::readNextRow(uint8_t* data, uint8_t* rowBuffer) const {
// Note: rowBuffer should be pre-allocated by the caller to size 'rowBytes' // Note: rowBuffer should be pre-allocated by the caller to size 'rowBytes'
if (file.read(rowBuffer, rowBytes) != rowBytes) return BmpReaderError::ShortReadRow; if (file.read(rowBuffer, rowBytes) != rowBytes) return BmpReaderError::ShortReadRow;
// Handle Floyd-Steinberg error buffer progression
const bool useFS = USE_FLOYD_STEINBERG && errorCurRow && errorNextRow;
if (useFS) {
if (prevRowY != -1) {
// Sequential access - swap buffers
int16_t* temp = errorCurRow;
errorCurRow = errorNextRow;
errorNextRow = temp;
memset(errorNextRow, 0, (width + 2) * sizeof(int16_t));
}
}
prevRowY += 1; prevRowY += 1;
uint8_t* outPtr = data; uint8_t* outPtr = data;
@ -282,12 +168,18 @@ BmpReaderError Bitmap::readNextRow(uint8_t* data, uint8_t* rowBuffer) const {
// Helper lambda to pack 2bpp color into the output stream // Helper lambda to pack 2bpp color into the output stream
auto packPixel = [&](const uint8_t lum) { auto packPixel = [&](const uint8_t lum) {
uint8_t color; uint8_t color;
if (useFS) { if (atkinsonDitherer) {
// Floyd-Steinberg error diffusion color = atkinsonDitherer->processPixel(adjustPixel(lum), currentX);
color = quantizeFloydSteinberg(lum, currentX, width, errorCurRow, errorNextRow, false); } else if (fsDitherer) {
color = fsDitherer->processPixel(adjustPixel(lum), currentX);
} else { } else {
if (bpp > 2) {
// Simple quantization or noise dithering // Simple quantization or noise dithering
color = quantize(lum, currentX, prevRowY); color = quantize(adjustPixel(lum), currentX, prevRowY);
} else {
// do not quantize 2bpp image
color = static_cast<uint8_t>(lum >> 6);
}
} }
currentOutByte |= (color << bitShift); currentOutByte |= (color << bitShift);
if (bitShift == 0) { if (bitShift == 0) {
@ -345,6 +237,11 @@ BmpReaderError Bitmap::readNextRow(uint8_t* data, uint8_t* rowBuffer) const {
return BmpReaderError::UnsupportedBpp; return BmpReaderError::UnsupportedBpp;
} }
if (atkinsonDitherer)
atkinsonDitherer->nextRow();
else if (fsDitherer)
fsDitherer->nextRow();
// Flush remaining bits if width is not a multiple of 4 // Flush remaining bits if width is not a multiple of 4
if (bitShift != 6) *outPtr = currentOutByte; if (bitShift != 6) *outPtr = currentOutByte;
@ -356,12 +253,9 @@ BmpReaderError Bitmap::rewindToData() const {
return BmpReaderError::SeekPixelDataFailed; return BmpReaderError::SeekPixelDataFailed;
} }
// Reset Floyd-Steinberg error buffers when rewinding // Reset dithering when rewinding
if (USE_FLOYD_STEINBERG && errorCurRow && errorNextRow) { if (fsDitherer) fsDitherer->reset();
memset(errorCurRow, 0, (width + 2) * sizeof(int16_t)); if (atkinsonDitherer) atkinsonDitherer->reset();
memset(errorNextRow, 0, (width + 2) * sizeof(int16_t));
prevRowY = -1;
}
return BmpReaderError::Ok; return BmpReaderError::Ok;
} }

10
lib/GfxRenderer/Bitmap.h
View File

@ -2,6 +2,10 @@
#include <SdFat.h> #include <SdFat.h>
#include <cstdint>
#include "BitmapHelpers.h"
enum class BmpReaderError : uint8_t { enum class BmpReaderError : uint8_t {
Ok = 0, Ok = 0,
FileInvalid, FileInvalid,
@ -28,7 +32,7 @@ class Bitmap {
public: public:
static const char* errorToString(BmpReaderError err); static const char* errorToString(BmpReaderError err);
explicit Bitmap(FsFile& file) : file(file) {} explicit Bitmap(FsFile& file, bool dithering = false) : file(file), dithering(dithering) {}
~Bitmap(); ~Bitmap();
BmpReaderError parseHeaders(); BmpReaderError parseHeaders();
BmpReaderError readNextRow(uint8_t* data, uint8_t* rowBuffer) const; BmpReaderError readNextRow(uint8_t* data, uint8_t* rowBuffer) const;
@ -44,6 +48,7 @@ class Bitmap {
static uint32_t readLE32(FsFile& f); static uint32_t readLE32(FsFile& f);
FsFile& file; FsFile& file;
bool dithering = false;
int width = 0; int width = 0;
int height = 0; int height = 0;
bool topDown = false; bool topDown = false;
@ -56,4 +61,7 @@ class Bitmap {
mutable int16_t* errorCurRow = nullptr; mutable int16_t* errorCurRow = nullptr;
mutable int16_t* errorNextRow = nullptr; mutable int16_t* errorNextRow = nullptr;
mutable int prevRowY = -1; // Track row progression for error propagation mutable int prevRowY = -1; // Track row progression for error propagation
mutable AtkinsonDitherer* atkinsonDitherer = nullptr;
mutable FloydSteinbergDitherer* fsDitherer = nullptr;
}; };

90
lib/GfxRenderer/BitmapHelpers.cpp Normal file
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@ -0,0 +1,90 @@
#include "BitmapHelpers.h"
#include <cstdint>
// Brightness/Contrast adjustments:
constexpr bool USE_BRIGHTNESS = false; // true: apply brightness/gamma adjustments
constexpr int BRIGHTNESS_BOOST = 10; // Brightness offset (0-50)
constexpr bool GAMMA_CORRECTION = false; // Gamma curve (brightens midtones)
constexpr float CONTRAST_FACTOR = 1.15f; // Contrast multiplier (1.0 = no change, >1 = more contrast)
constexpr bool USE_NOISE_DITHERING = false; // Hash-based noise dithering
// Integer approximation of gamma correction (brightens midtones)
// Uses a simple curve: out = 255 * sqrt(in/255) ≈ sqrt(in * 255)
static inline int applyGamma(int gray) {
if (!GAMMA_CORRECTION) return gray;
// Fast integer square root approximation for gamma ~0.5 (brightening)
// This brightens dark/mid tones while preserving highlights
const int product = gray * 255;
// Newton-Raphson integer sqrt (2 iterations for good accuracy)
int x = gray;
if (x > 0) {
x = (x + product / x) >> 1;
x = (x + product / x) >> 1;
}
return x > 255 ? 255 : x;
}
// Apply contrast adjustment around midpoint (128)
// factor > 1.0 increases contrast, < 1.0 decreases
static inline int applyContrast(int gray) {
// Integer-based contrast: (gray - 128) * factor + 128
// Using fixed-point: factor 1.15 ≈ 115/100
constexpr int factorNum = static_cast<int>(CONTRAST_FACTOR * 100);
int adjusted = ((gray - 128) * factorNum) / 100 + 128;
if (adjusted < 0) adjusted = 0;
if (adjusted > 255) adjusted = 255;
return adjusted;
}
// Combined brightness/contrast/gamma adjustment
int adjustPixel(int gray) {
if (!USE_BRIGHTNESS) return gray;
// Order: contrast first, then brightness, then gamma
gray = applyContrast(gray);
gray += BRIGHTNESS_BOOST;
if (gray > 255) gray = 255;
if (gray < 0) gray = 0;
gray = applyGamma(gray);
return gray;
}
// Simple quantization without dithering - divide into 4 levels
// The thresholds are fine-tuned to the X4 display
uint8_t quantizeSimple(int gray) {
if (gray < 45) {
return 0;
} else if (gray < 70) {
return 1;
} else if (gray < 140) {
return 2;
} else {
return 3;
}
}
// Hash-based noise dithering - survives downsampling without moiré artifacts
// Uses integer hash to generate pseudo-random threshold per pixel
static inline uint8_t quantizeNoise(int gray, int x, int y) {
uint32_t hash = static_cast<uint32_t>(x) * 374761393u + static_cast<uint32_t>(y) * 668265263u;
hash = (hash ^ (hash >> 13)) * 1274126177u;
const int threshold = static_cast<int>(hash >> 24);
const int scaled = gray * 3;
if (scaled < 255) {
return (scaled + threshold >= 255) ? 1 : 0;
} else if (scaled < 510) {
return ((scaled - 255) + threshold >= 255) ? 2 : 1;
} else {
return ((scaled - 510) + threshold >= 255) ? 3 : 2;
}
}
// Main quantization function - selects between methods based on config
uint8_t quantize(int gray, int x, int y) {
if (USE_NOISE_DITHERING) {
return quantizeNoise(gray, x, y);
} else {
return quantizeSimple(gray);
}
}

233
lib/GfxRenderer/BitmapHelpers.h Normal file
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@ -0,0 +1,233 @@
#pragma once
#include <cstring>
// Helper functions
uint8_t quantize(int gray, int x, int y);
uint8_t quantizeSimple(int gray);
int adjustPixel(int gray);
// Atkinson dithering - distributes only 6/8 (75%) of error for cleaner results
// Error distribution pattern:
// X 1/8 1/8
// 1/8 1/8 1/8
// 1/8
// Less error buildup = fewer artifacts than Floyd-Steinberg
class AtkinsonDitherer {
public:
explicit AtkinsonDitherer(int width) : width(width) {
errorRow0 = new int16_t[width + 4](); // Current row
errorRow1 = new int16_t[width + 4](); // Next row
errorRow2 = new int16_t[width + 4](); // Row after next
}
~AtkinsonDitherer() {
delete[] errorRow0;
delete[] errorRow1;
delete[] errorRow2;
}
// **1. EXPLICITLY DELETE THE COPY CONSTRUCTOR**
AtkinsonDitherer(const AtkinsonDitherer& other) = delete;
// **2. EXPLICITLY DELETE THE COPY ASSIGNMENT OPERATOR**
AtkinsonDitherer& operator=(const AtkinsonDitherer& other) = delete;
uint8_t processPixel(int gray, int x) {
// Add accumulated error
int adjusted = gray + errorRow0[x + 2];
if (adjusted < 0) adjusted = 0;
if (adjusted > 255) adjusted = 255;
// Quantize to 4 levels
uint8_t quantized;
int quantizedValue;
if (false) { // original thresholds
if (adjusted < 43) {
quantized = 0;
quantizedValue = 0;
} else if (adjusted < 128) {
quantized = 1;
quantizedValue = 85;
} else if (adjusted < 213) {
quantized = 2;
quantizedValue = 170;
} else {
quantized = 3;
quantizedValue = 255;
}
} else { // fine-tuned to X4 eink display
if (adjusted < 30) {
quantized = 0;
quantizedValue = 15;
} else if (adjusted < 50) {
quantized = 1;
quantizedValue = 30;
} else if (adjusted < 140) {
quantized = 2;
quantizedValue = 80;
} else {
quantized = 3;
quantizedValue = 210;
}
}
// Calculate error (only distribute 6/8 = 75%)
int error = (adjusted - quantizedValue) >> 3; // error/8
// Distribute 1/8 to each of 6 neighbors
errorRow0[x + 3] += error; // Right
errorRow0[x + 4] += error; // Right+1
errorRow1[x + 1] += error; // Bottom-left
errorRow1[x + 2] += error; // Bottom
errorRow1[x + 3] += error; // Bottom-right
errorRow2[x + 2] += error; // Two rows down
return quantized;
}
void nextRow() {
int16_t* temp = errorRow0;
errorRow0 = errorRow1;
errorRow1 = errorRow2;
errorRow2 = temp;
memset(errorRow2, 0, (width + 4) * sizeof(int16_t));
}
void reset() {
memset(errorRow0, 0, (width + 4) * sizeof(int16_t));
memset(errorRow1, 0, (width + 4) * sizeof(int16_t));
memset(errorRow2, 0, (width + 4) * sizeof(int16_t));
}
private:
int width;
int16_t* errorRow0;
int16_t* errorRow1;
int16_t* errorRow2;
};
// Floyd-Steinberg error diffusion dithering with serpentine scanning
// Serpentine scanning alternates direction each row to reduce "worm" artifacts
// Error distribution pattern (left-to-right):
// X 7/16
// 3/16 5/16 1/16
// Error distribution pattern (right-to-left, mirrored):
// 1/16 5/16 3/16
// 7/16 X
class FloydSteinbergDitherer {
public:
explicit FloydSteinbergDitherer(int width) : width(width), rowCount(0) {
errorCurRow = new int16_t[width + 2](); // +2 for boundary handling
errorNextRow = new int16_t[width + 2]();
}
~FloydSteinbergDitherer() {
delete[] errorCurRow;
delete[] errorNextRow;
}
// **1. EXPLICITLY DELETE THE COPY CONSTRUCTOR**
FloydSteinbergDitherer(const FloydSteinbergDitherer& other) = delete;
// **2. EXPLICITLY DELETE THE COPY ASSIGNMENT OPERATOR**
FloydSteinbergDitherer& operator=(const FloydSteinbergDitherer& other) = delete;
// Process a single pixel and return quantized 2-bit value
// x is the logical x position (0 to width-1), direction handled internally
uint8_t processPixel(int gray, int x) {
// Add accumulated error to this pixel
int adjusted = gray + errorCurRow[x + 1];
// Clamp to valid range
if (adjusted < 0) adjusted = 0;
if (adjusted > 255) adjusted = 255;
// Quantize to 4 levels (0, 85, 170, 255)
uint8_t quantized;
int quantizedValue;
if (false) { // original thresholds
if (adjusted < 43) {
quantized = 0;
quantizedValue = 0;
} else if (adjusted < 128) {
quantized = 1;
quantizedValue = 85;
} else if (adjusted < 213) {
quantized = 2;
quantizedValue = 170;
} else {
quantized = 3;
quantizedValue = 255;
}
} else { // fine-tuned to X4 eink display
if (adjusted < 30) {
quantized = 0;
quantizedValue = 15;
} else if (adjusted < 50) {
quantized = 1;
quantizedValue = 30;
} else if (adjusted < 140) {
quantized = 2;
quantizedValue = 80;
} else {
quantized = 3;
quantizedValue = 210;
}
}
// Calculate error
int error = adjusted - quantizedValue;
// Distribute error to neighbors (serpentine: direction-aware)
if (!isReverseRow()) {
// Left to right: standard distribution
// Right: 7/16
errorCurRow[x + 2] += (error * 7) >> 4;
// Bottom-left: 3/16
errorNextRow[x] += (error * 3) >> 4;
// Bottom: 5/16
errorNextRow[x + 1] += (error * 5) >> 4;
// Bottom-right: 1/16
errorNextRow[x + 2] += (error) >> 4;
} else {
// Right to left: mirrored distribution
// Left: 7/16
errorCurRow[x] += (error * 7) >> 4;
// Bottom-right: 3/16
errorNextRow[x + 2] += (error * 3) >> 4;
// Bottom: 5/16
errorNextRow[x + 1] += (error * 5) >> 4;
// Bottom-left: 1/16
errorNextRow[x] += (error) >> 4;
}
return quantized;
}
// Call at the end of each row to swap buffers
void nextRow() {
// Swap buffers
int16_t* temp = errorCurRow;
errorCurRow = errorNextRow;
errorNextRow = temp;
// Clear the next row buffer
memset(errorNextRow, 0, (width + 2) * sizeof(int16_t));
rowCount++;
}
// Check if current row should be processed in reverse
bool isReverseRow() const { return (rowCount & 1) != 0; }
// Reset for a new image or MCU block
void reset() {
memset(errorCurRow, 0, (width + 2) * sizeof(int16_t));
memset(errorNextRow, 0, (width + 2) * sizeof(int16_t));
rowCount = 0;
}
private:
int width;
int rowCount;
int16_t* errorCurRow;
int16_t* errorNextRow;
};

280
lib/JpegToBmpConverter/JpegToBmpConverter.cpp
View File

@ -7,6 +7,8 @@
#include <cstdio> #include <cstdio>
#include <cstring> #include <cstring>
#include "BitmapHelpers.h"
// Context structure for picojpeg callback // Context structure for picojpeg callback
struct JpegReadContext { struct JpegReadContext {
FsFile& file; FsFile& file;
@ -23,282 +25,12 @@ constexpr bool USE_8BIT_OUTPUT = false; // true: 8-bit grayscale (no quantizati
constexpr bool USE_ATKINSON = true; // Atkinson dithering (cleaner than F-S, less error diffusion) constexpr bool USE_ATKINSON = true; // Atkinson dithering (cleaner than F-S, less error diffusion)
constexpr bool USE_FLOYD_STEINBERG = false; // Floyd-Steinberg error diffusion (can cause "worm" artifacts) constexpr bool USE_FLOYD_STEINBERG = false; // Floyd-Steinberg error diffusion (can cause "worm" artifacts)
constexpr bool USE_NOISE_DITHERING = false; // Hash-based noise dithering (good for downsampling) constexpr bool USE_NOISE_DITHERING = false; // Hash-based noise dithering (good for downsampling)
// Brightness/Contrast adjustments:
constexpr bool USE_BRIGHTNESS = true; // true: apply brightness/gamma adjustments
constexpr int BRIGHTNESS_BOOST = 10; // Brightness offset (0-50)
constexpr bool GAMMA_CORRECTION = true; // Gamma curve (brightens midtones)
constexpr float CONTRAST_FACTOR = 1.15f; // Contrast multiplier (1.0 = no change, >1 = more contrast)
// Pre-resize to target display size (CRITICAL: avoids dithering artifacts from post-downsampling) // Pre-resize to target display size (CRITICAL: avoids dithering artifacts from post-downsampling)
constexpr bool USE_PRESCALE = true; // true: scale image to target size before dithering constexpr bool USE_PRESCALE = true; // true: scale image to target size before dithering
constexpr int TARGET_MAX_WIDTH = 480; // Max width for cover images (portrait display width) constexpr int TARGET_MAX_WIDTH = 480; // Max width for cover images (portrait display width)
constexpr int TARGET_MAX_HEIGHT = 800; // Max height for cover images (portrait display height) constexpr int TARGET_MAX_HEIGHT = 800; // Max height for cover images (portrait display height)
// ============================================================================ // ============================================================================
// Integer approximation of gamma correction (brightens midtones)
// Uses a simple curve: out = 255 * sqrt(in/255) ≈ sqrt(in * 255)
static inline int applyGamma(int gray) {
if (!GAMMA_CORRECTION) return gray;
// Fast integer square root approximation for gamma ~0.5 (brightening)
// This brightens dark/mid tones while preserving highlights
const int product = gray * 255;
// Newton-Raphson integer sqrt (2 iterations for good accuracy)
int x = gray;
if (x > 0) {
x = (x + product / x) >> 1;
x = (x + product / x) >> 1;
}
return x > 255 ? 255 : x;
}
// Apply contrast adjustment around midpoint (128)
// factor > 1.0 increases contrast, < 1.0 decreases
static inline int applyContrast(int gray) {
// Integer-based contrast: (gray - 128) * factor + 128
// Using fixed-point: factor 1.15 ≈ 115/100
constexpr int factorNum = static_cast<int>(CONTRAST_FACTOR * 100);
int adjusted = ((gray - 128) * factorNum) / 100 + 128;
if (adjusted < 0) adjusted = 0;
if (adjusted > 255) adjusted = 255;
return adjusted;
}
// Combined brightness/contrast/gamma adjustment
static inline int adjustPixel(int gray) {
if (!USE_BRIGHTNESS) return gray;
// Order: contrast first, then brightness, then gamma
gray = applyContrast(gray);
gray += BRIGHTNESS_BOOST;
if (gray > 255) gray = 255;
if (gray < 0) gray = 0;
gray = applyGamma(gray);
return gray;
}
// Simple quantization without dithering - just divide into 4 levels
static inline uint8_t quantizeSimple(int gray) {
gray = adjustPixel(gray);
// Simple 2-bit quantization: 0-63=0, 64-127=1, 128-191=2, 192-255=3
return static_cast<uint8_t>(gray >> 6);
}
// Hash-based noise dithering - survives downsampling without moiré artifacts
// Uses integer hash to generate pseudo-random threshold per pixel
static inline uint8_t quantizeNoise(int gray, int x, int y) {
gray = adjustPixel(gray);
// Generate noise threshold using integer hash (no regular pattern to alias)
uint32_t hash = static_cast<uint32_t>(x) * 374761393u + static_cast<uint32_t>(y) * 668265263u;
hash = (hash ^ (hash >> 13)) * 1274126177u;
const int threshold = static_cast<int>(hash >> 24); // 0-255
// Map gray (0-255) to 4 levels with dithering
const int scaled = gray * 3;
if (scaled < 255) {
return (scaled + threshold >= 255) ? 1 : 0;
} else if (scaled < 510) {
return ((scaled - 255) + threshold >= 255) ? 2 : 1;
} else {
return ((scaled - 510) + threshold >= 255) ? 3 : 2;
}
}
// Main quantization function - selects between methods based on config
static inline uint8_t quantize(int gray, int x, int y) {
if (USE_NOISE_DITHERING) {
return quantizeNoise(gray, x, y);
} else {
return quantizeSimple(gray);
}
}
// Atkinson dithering - distributes only 6/8 (75%) of error for cleaner results
// Error distribution pattern:
// X 1/8 1/8
// 1/8 1/8 1/8
// 1/8
// Less error buildup = fewer artifacts than Floyd-Steinberg
class AtkinsonDitherer {
public:
AtkinsonDitherer(int width) : width(width) {
errorRow0 = new int16_t[width + 4](); // Current row
errorRow1 = new int16_t[width + 4](); // Next row
errorRow2 = new int16_t[width + 4](); // Row after next
}
~AtkinsonDitherer() {
delete[] errorRow0;
delete[] errorRow1;
delete[] errorRow2;
}
uint8_t processPixel(int gray, int x) {
// Apply brightness/contrast/gamma adjustments
gray = adjustPixel(gray);
// Add accumulated error
int adjusted = gray + errorRow0[x + 2];
if (adjusted < 0) adjusted = 0;
if (adjusted > 255) adjusted = 255;
// Quantize to 4 levels
uint8_t quantized;
int quantizedValue;
if (adjusted < 43) {
quantized = 0;
quantizedValue = 0;
} else if (adjusted < 128) {
quantized = 1;
quantizedValue = 85;
} else if (adjusted < 213) {
quantized = 2;
quantizedValue = 170;
} else {
quantized = 3;
quantizedValue = 255;
}
// Calculate error (only distribute 6/8 = 75%)
int error = (adjusted - quantizedValue) >> 3; // error/8
// Distribute 1/8 to each of 6 neighbors
errorRow0[x + 3] += error; // Right
errorRow0[x + 4] += error; // Right+1
errorRow1[x + 1] += error; // Bottom-left
errorRow1[x + 2] += error; // Bottom
errorRow1[x + 3] += error; // Bottom-right
errorRow2[x + 2] += error; // Two rows down
return quantized;
}
void nextRow() {
int16_t* temp = errorRow0;
errorRow0 = errorRow1;
errorRow1 = errorRow2;
errorRow2 = temp;
memset(errorRow2, 0, (width + 4) * sizeof(int16_t));
}
void reset() {
memset(errorRow0, 0, (width + 4) * sizeof(int16_t));
memset(errorRow1, 0, (width + 4) * sizeof(int16_t));
memset(errorRow2, 0, (width + 4) * sizeof(int16_t));
}
private:
int width;
int16_t* errorRow0;
int16_t* errorRow1;
int16_t* errorRow2;
};
// Floyd-Steinberg error diffusion dithering with serpentine scanning
// Serpentine scanning alternates direction each row to reduce "worm" artifacts
// Error distribution pattern (left-to-right):
// X 7/16
// 3/16 5/16 1/16
// Error distribution pattern (right-to-left, mirrored):
// 1/16 5/16 3/16
// 7/16 X
class FloydSteinbergDitherer {
public:
FloydSteinbergDitherer(int width) : width(width), rowCount(0) {
errorCurRow = new int16_t[width + 2](); // +2 for boundary handling
errorNextRow = new int16_t[width + 2]();
}
~FloydSteinbergDitherer() {
delete[] errorCurRow;
delete[] errorNextRow;
}
// Process a single pixel and return quantized 2-bit value
// x is the logical x position (0 to width-1), direction handled internally
uint8_t processPixel(int gray, int x, bool reverseDirection) {
// Add accumulated error to this pixel
int adjusted = gray + errorCurRow[x + 1];
// Clamp to valid range
if (adjusted < 0) adjusted = 0;
if (adjusted > 255) adjusted = 255;
// Quantize to 4 levels (0, 85, 170, 255)
uint8_t quantized;
int quantizedValue;
if (adjusted < 43) {
quantized = 0;
quantizedValue = 0;
} else if (adjusted < 128) {
quantized = 1;
quantizedValue = 85;
} else if (adjusted < 213) {
quantized = 2;
quantizedValue = 170;
} else {
quantized = 3;
quantizedValue = 255;
}
// Calculate error
int error = adjusted - quantizedValue;
// Distribute error to neighbors (serpentine: direction-aware)
if (!reverseDirection) {
// Left to right: standard distribution
// Right: 7/16
errorCurRow[x + 2] += (error * 7) >> 4;
// Bottom-left: 3/16
errorNextRow[x] += (error * 3) >> 4;
// Bottom: 5/16
errorNextRow[x + 1] += (error * 5) >> 4;
// Bottom-right: 1/16
errorNextRow[x + 2] += (error) >> 4;
} else {
// Right to left: mirrored distribution
// Left: 7/16
errorCurRow[x] += (error * 7) >> 4;
// Bottom-right: 3/16
errorNextRow[x + 2] += (error * 3) >> 4;
// Bottom: 5/16
errorNextRow[x + 1] += (error * 5) >> 4;
// Bottom-left: 1/16
errorNextRow[x] += (error) >> 4;
}
return quantized;
}
// Call at the end of each row to swap buffers
void nextRow() {
// Swap buffers
int16_t* temp = errorCurRow;
errorCurRow = errorNextRow;
errorNextRow = temp;
// Clear the next row buffer
memset(errorNextRow, 0, (width + 2) * sizeof(int16_t));
rowCount++;
}
// Check if current row should be processed in reverse
bool isReverseRow() const { return (rowCount & 1) != 0; }
// Reset for a new image or MCU block
void reset() {
memset(errorCurRow, 0, (width + 2) * sizeof(int16_t));
memset(errorNextRow, 0, (width + 2) * sizeof(int16_t));
rowCount = 0;
}
private:
int width;
int rowCount;
int16_t* errorCurRow;
int16_t* errorNextRow;
};
inline void write16(Print& out, const uint16_t value) { inline void write16(Print& out, const uint16_t value) {
out.write(value & 0xFF); out.write(value & 0xFF);
out.write((value >> 8) & 0xFF); out.write((value >> 8) & 0xFF);
@ -623,12 +355,12 @@ bool JpegToBmpConverter::jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut) {
} }
} else { } else {
for (int x = 0; x < outWidth; x++) { for (int x = 0; x < outWidth; x++) {
const uint8_t gray = mcuRowBuffer[bufferY * imageInfo.m_width + x]; const uint8_t gray = adjustPixel(mcuRowBuffer[bufferY * imageInfo.m_width + x]);
uint8_t twoBit; uint8_t twoBit;
if (atkinsonDitherer) { if (atkinsonDitherer) {
twoBit = atkinsonDitherer->processPixel(gray, x); twoBit = atkinsonDitherer->processPixel(gray, x);
} else if (fsDitherer) { } else if (fsDitherer) {
twoBit = fsDitherer->processPixel(gray, x, fsDitherer->isReverseRow()); twoBit = fsDitherer->processPixel(gray, x);
} else { } else {
twoBit = quantize(gray, x, y); twoBit = quantize(gray, x, y);
} }
@ -686,12 +418,12 @@ bool JpegToBmpConverter::jpegFileToBmpStream(FsFile& jpegFile, Print& bmpOut) {
} }
} else { } else {
for (int x = 0; x < outWidth; x++) { for (int x = 0; x < outWidth; x++) {
const uint8_t gray = (rowCount[x] > 0) ? (rowAccum[x] / rowCount[x]) : 0; const uint8_t gray = adjustPixel((rowCount[x] > 0) ? (rowAccum[x] / rowCount[x]) : 0);
uint8_t twoBit; uint8_t twoBit;
if (atkinsonDitherer) { if (atkinsonDitherer) {
twoBit = atkinsonDitherer->processPixel(gray, x); twoBit = atkinsonDitherer->processPixel(gray, x);
} else if (fsDitherer) { } else if (fsDitherer) {
twoBit = fsDitherer->processPixel(gray, x, fsDitherer->isReverseRow()); twoBit = fsDitherer->processPixel(gray, x);
} else { } else {
twoBit = quantize(gray, x, currentOutY); twoBit = quantize(gray, x, currentOutY);
} }

4
src/activities/boot_sleep/SleepActivity.cpp
View File

@ -86,7 +86,7 @@ void SleepActivity::renderCustomSleepScreen() const {
if (SdMan.openFileForRead("SLP", filename, file)) { if (SdMan.openFileForRead("SLP", filename, file)) {
Serial.printf("[%lu] [SLP] Randomly loading: /sleep/%s\n", millis(), files[randomFileIndex].c_str()); Serial.printf("[%lu] [SLP] Randomly loading: /sleep/%s\n", millis(), files[randomFileIndex].c_str());
delay(100); delay(100);
Bitmap bitmap(file); Bitmap bitmap(file, true);
if (bitmap.parseHeaders() == BmpReaderError::Ok) { if (bitmap.parseHeaders() == BmpReaderError::Ok) {
renderBitmapSleepScreen(bitmap); renderBitmapSleepScreen(bitmap);
dir.close(); dir.close();
@ -101,7 +101,7 @@ void SleepActivity::renderCustomSleepScreen() const {
// render a custom sleep screen instead of the default. // render a custom sleep screen instead of the default.
FsFile file; FsFile file;
if (SdMan.openFileForRead("SLP", "/sleep.bmp", file)) { if (SdMan.openFileForRead("SLP", "/sleep.bmp", file)) {
Bitmap bitmap(file); Bitmap bitmap(file, true);
if (bitmap.parseHeaders() == BmpReaderError::Ok) { if (bitmap.parseHeaders() == BmpReaderError::Ok) {
Serial.printf("[%lu] [SLP] Loading: /sleep.bmp\n", millis()); Serial.printf("[%lu] [SLP] Loading: /sleep.bmp\n", millis());
renderBitmapSleepScreen(bitmap); renderBitmapSleepScreen(bitmap);