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fix: Use hash-based block dithering for BW-boundary letterbox fills

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Pixel-level Bayer dithering in the 171-254 gray range creates a
high-frequency checkerboard in the BW pass that causes e-ink display
crosstalk during HALF_REFRESH, washing out cover images. Replace with
2x2 hash-based block dithering for this specific gray range — each
block gets a uniform level (2 or 3) via a spatial hash, avoiding
single-pixel alternation while approximating the target gray. Standard
Bayer dithering remains for all other gray ranges.

Also removes all debug instrumentation from the investigation.

Co-authored-by: Cursor <cursoragent@cursor.com>
This commit is contained in:
cottongin
2026-02-13 14:49:42 -05:00
parent ea11d2f7d3
commit 0c71e0b13f
1 changed files with 77 additions and 20 deletions
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97
src/activities/boot_sleep/SleepActivity.cpp
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@@ -70,6 +70,39 @@ uint8_t quantizeBayerDither(int gray, int x, int y) {
}
}
// Check whether a gray value would produce a dithered mix that crosses the
// level-2 / level-3 boundary. This is the ONLY boundary where some dithered
// pixels are BLACK (level ≤ 2) and others are WHITE (level 3) in the BW pass,
// creating a high-frequency checkerboard that causes e-ink display crosstalk
// and washes out adjacent content during HALF_REFRESH.
// Gray values 171-254 produce a level-2/level-3 mix via Bayer dithering.
bool bayerCrossesBwBoundary(uint8_t gray) {
return gray > 170 && gray < 255;
}
// Hash-based block dithering for BW-boundary gray values (171-254).
// Each blockSize×blockSize pixel block gets a single uniform level (2 or 3),
// determined by a deterministic spatial hash. The proportion of level-3 blocks
// approximates the target gray. Unlike Bayer, the pattern is irregular
// (noise-like), making it much less visually obvious at the same block size.
// The hash is purely spatial (depends only on x, y, blockSize) so it produces
// identical levels across BW, LSB, and MSB render passes.
static constexpr int BW_DITHER_BLOCK = 2;
uint8_t hashBlockDither(uint8_t avg, int x, int y) {
const int bx = x / BW_DITHER_BLOCK;
const int by = y / BW_DITHER_BLOCK;
// Fast mixing hash (splitmix32-inspired)
uint32_t h = (uint32_t)bx * 2654435761u ^ (uint32_t)by * 2246822519u;
h ^= h >> 16;
h *= 0x45d9f3bu;
h ^= h >> 16;
// Proportion of level-3 blocks needed to approximate the target gray
const float ratio = (avg - 170.0f) / 85.0f;
const uint32_t threshold = (uint32_t)(ratio * 4294967295.0f);
return (h < threshold) ? 3 : 2;
}
// --- Edge average cache ---
// Caches the computed edge averages alongside the cover BMP so we don't rescan on every sleep.
constexpr uint8_t EDGE_CACHE_VERSION = 2;
@@ -195,7 +228,6 @@ LetterboxFillData computeEdgeAverages(const Bitmap& bitmap, int imgX, int imgY,
data.avgA = countTop > 0 ? static_cast<uint8_t>(sumTop / countTop) : 128;
data.avgB = countBot > 0 ? static_cast<uint8_t>(sumBot / countBot) : 128;
data.valid = true;
} else if (imgX > 0) {
// Left/right letterboxing -- compute overall average of first/last EDGE_SAMPLE_DEPTH columns
data.horizontal = false;
@@ -230,9 +262,9 @@ LetterboxFillData computeEdgeAverages(const Bitmap& bitmap, int imgX, int imgY,
data.valid = true;
}
bitmap.rewindToData();
free(outputRow);
free(rowBytes);
bitmap.rewindToData();
return data;
}
@@ -245,40 +277,65 @@ void drawLetterboxFill(GfxRenderer& renderer, const LetterboxFillData& data, uin
const bool isSolid = (fillMode == CrossPointSettings::SLEEP_SCREEN_LETTERBOX_FILL::LETTERBOX_SOLID);
// For SOLID: snap to nearest e-ink level then convert to 2-bit
// For DITHERED: use the raw average, Bayer dithering produces the 2-bit level per pixel
const uint8_t colorA = isSolid ? snapToEinkLevel(data.avgA) : data.avgA;
const uint8_t colorB = isSolid ? snapToEinkLevel(data.avgB) : data.avgB;
const uint8_t solidA = colorA / 85; // only used for SOLID
const uint8_t solidB = colorB / 85;
// For DITHERED mode with gray values in 171-254 (the level-2/level-3 BW boundary):
// Pixel-level Bayer dithering creates a regular high-frequency checkerboard in
// the BW pass that causes e-ink display crosstalk during HALF_REFRESH.
//
// Solution: HASH-BASED BLOCK DITHERING. Each 2x2 pixel block gets a single
// level (2 or 3) determined by a spatial hash, with the proportion of level-3
// blocks tuned to approximate the target gray. The 2px minimum run avoids BW
// crosstalk, and the irregular hash pattern is much less visible than a regular
// Bayer grid at the same block size.
const bool hashA = !isSolid && bayerCrossesBwBoundary(data.avgA);
const bool hashB = !isSolid && bayerCrossesBwBoundary(data.avgB);
// For solid mode: snap to nearest e-ink level
const uint8_t levelA = isSolid ? snapToEinkLevel(data.avgA) / 85 : 0;
const uint8_t levelB = isSolid ? snapToEinkLevel(data.avgB) / 85 : 0;
if (data.horizontal) {
// Top letterbox
if (data.letterboxA > 0) {
for (int y = 0; y < data.letterboxA; y++)
for (int x = 0; x < renderer.getScreenWidth(); x++)
renderer.drawPixelGray(x, y, isSolid ? solidA : quantizeBayerDither(colorA, x, y));
for (int x = 0; x < renderer.getScreenWidth(); x++) {
uint8_t lv;
if (isSolid) lv = levelA;
else if (hashA) lv = hashBlockDither(data.avgA, x, y);
else lv = quantizeBayerDither(data.avgA, x, y);
renderer.drawPixelGray(x, y, lv);
}
}
// Bottom letterbox
if (data.letterboxB > 0) {
const int start = renderer.getScreenHeight() - data.letterboxB;
for (int y = start; y < renderer.getScreenHeight(); y++)
for (int x = 0; x < renderer.getScreenWidth(); x++)
renderer.drawPixelGray(x, y, isSolid ? solidB : quantizeBayerDither(colorB, x, y));
for (int x = 0; x < renderer.getScreenWidth(); x++) {
uint8_t lv;
if (isSolid) lv = levelB;
else if (hashB) lv = hashBlockDither(data.avgB, x, y);
else lv = quantizeBayerDither(data.avgB, x, y);
renderer.drawPixelGray(x, y, lv);
}
}
} else {
// Left letterbox
if (data.letterboxA > 0) {
for (int x = 0; x < data.letterboxA; x++)
for (int y = 0; y < renderer.getScreenHeight(); y++)
renderer.drawPixelGray(x, y, isSolid ? solidA : quantizeBayerDither(colorA, x, y));
for (int y = 0; y < renderer.getScreenHeight(); y++) {
uint8_t lv;
if (isSolid) lv = levelA;
else if (hashA) lv = hashBlockDither(data.avgA, x, y);
else lv = quantizeBayerDither(data.avgA, x, y);
renderer.drawPixelGray(x, y, lv);
}
}
// Right letterbox
if (data.letterboxB > 0) {
const int start = renderer.getScreenWidth() - data.letterboxB;
for (int x = start; x < renderer.getScreenWidth(); x++)
for (int y = 0; y < renderer.getScreenHeight(); y++)
renderer.drawPixelGray(x, y, isSolid ? solidB : quantizeBayerDither(colorB, x, y));
for (int y = 0; y < renderer.getScreenHeight(); y++) {
uint8_t lv;
if (isSolid) lv = levelB;
else if (hashB) lv = hashBlockDither(data.avgB, x, y);
else lv = quantizeBayerDither(data.avgB, x, y);
renderer.drawPixelGray(x, y, lv);
}
}
}
}