feat: Add 4bit bmp support (#944)
## Summary * What is the goal of this PR? - Allow users to create custom sleep screen images with standard tools (ImageMagick, GIMP, etc.) that render cleanly on the e-ink display without dithering artifacts. Previously, avoiding dithering required non-standard 2-bit BMPs that no standard image editor can produce. ( see issue #931 ) * What changes are included? - Add 4-bit BMP format support to Bitmap.cpp (standard format, widely supported by image tools) - Auto-detect "native palette" images: if a BMP has ≤4 palette entries and all luminances map within ±21 of the display's native gray levels (0, 85, 170, 255), skip dithering entirely and direct-map pixels - Clarify pixel processing strategy with three distinct paths: error-diffusion dithering, simple quantization, or direct mapping - Add scripts/generate_test_bmps.py for generating test images across all supported BMP formats ## Additional Context * The e-ink display has 4 native gray levels. When a BMP already uses exactly those levels, dithering adds noise to what should be clean output. The native palette detection uses a ±21 tolerance (~10%) to handle slight rounding from color space conversions in image tools. Users can now create a 4-color grayscale BMP with (imagemagic example): ``` convert input.png -colorspace Gray -colors 4 -depth ``` --- ### 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**_
This commit is contained in:
jpirnay
@@ -4,10 +4,12 @@
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#include <cstring>
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// ============================================================================
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// IMAGE PROCESSING OPTIONS - Toggle these to test different configurations
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// IMAGE PROCESSING OPTIONS
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// ============================================================================
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// Note: For cover images, dithering is done in JpegToBmpConverter.cpp
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// This file handles BMP reading - use simple quantization to avoid double-dithering
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// Dithering is applied when converting high-color BMPs to the display's native
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// 2-bit (4-level) grayscale. Images whose palette entries all map to native
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// gray levels (0, 85, 170, 255 ±21) are mapped directly without dithering.
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// For cover images, dithering is done in JpegToBmpConverter.cpp instead.
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constexpr bool USE_ATKINSON = true; // Use Atkinson dithering instead of Floyd-Steinberg
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// ============================================================================
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@@ -57,7 +59,7 @@ const char* Bitmap::errorToString(BmpReaderError err) {
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case BmpReaderError::BadPlanes:
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return "BadPlanes (!= 1)";
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case BmpReaderError::UnsupportedBpp:
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return "UnsupportedBpp (expected 1, 2, 8, 24, or 32)";
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return "UnsupportedBpp (expected 1, 2, 4, 8, 24, or 32)";
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case BmpReaderError::UnsupportedCompression:
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return "UnsupportedCompression (expected BI_RGB or BI_BITFIELDS for 32bpp)";
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case BmpReaderError::BadDimensions:
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@@ -103,7 +105,7 @@ BmpReaderError Bitmap::parseHeaders() {
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const uint16_t planes = readLE16(file);
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bpp = readLE16(file);
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const uint32_t comp = readLE32(file);
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const bool validBpp = bpp == 1 || bpp == 2 || bpp == 8 || bpp == 24 || bpp == 32;
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const bool validBpp = bpp == 1 || bpp == 2 || bpp == 4 || bpp == 8 || bpp == 24 || bpp == 32;
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if (planes != 1) return BmpReaderError::BadPlanes;
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if (!validBpp) return BmpReaderError::UnsupportedBpp;
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@@ -111,7 +113,9 @@ BmpReaderError Bitmap::parseHeaders() {
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if (!(comp == 0 || (bpp == 32 && comp == 3))) return BmpReaderError::UnsupportedCompression;
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file.seekCur(12); // biSizeImage, biXPelsPerMeter, biYPelsPerMeter
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const uint32_t colorsUsed = readLE32(file);
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colorsUsed = readLE32(file);
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// BMP spec: colorsUsed==0 means default (2^bpp for paletted formats)
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if (colorsUsed == 0 && bpp <= 8) colorsUsed = 1u << bpp;
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if (colorsUsed > 256u) return BmpReaderError::PaletteTooLarge;
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file.seekCur(4); // biClrImportant
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@@ -140,9 +144,29 @@ BmpReaderError Bitmap::parseHeaders() {
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return BmpReaderError::SeekPixelDataFailed;
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}
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// Create ditherer if enabled (only for 2-bit output)
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// Use OUTPUT dimensions for dithering (after prescaling)
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if (bpp > 2 && dithering) {
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// Check if palette luminances map cleanly to the display's 4 native gray levels.
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// Native levels are 0, 85, 170, 255 — i.e. values where (lum >> 6) is lossless.
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// If all palette entries are near a native level, we can skip dithering entirely.
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nativePalette = bpp <= 2; // 1-bit and 2-bit are always native
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if (!nativePalette && colorsUsed > 0) {
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nativePalette = true;
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for (uint32_t i = 0; i < colorsUsed; i++) {
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const uint8_t lum = paletteLum[i];
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const uint8_t level = lum >> 6; // quantize to 0-3
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const uint8_t reconstructed = level * 85; // back to 0, 85, 170, 255
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if (lum > reconstructed + 21 || lum + 21 < reconstructed) {
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nativePalette = false; // luminance is too far from any native level
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break;
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}
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}
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}
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// Decide pixel processing strategy:
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// - Native palette → direct mapping, no processing needed
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// - High-color + dithering enabled → error-diffusion dithering (Atkinson or Floyd-Steinberg)
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// - High-color + dithering disabled → simple quantization (no error diffusion)
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const bool highColor = !nativePalette;
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if (highColor && dithering) {
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if (USE_ATKINSON) {
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atkinsonDitherer = new AtkinsonDitherer(width);
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} else {
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@@ -173,12 +197,12 @@ BmpReaderError Bitmap::readNextRow(uint8_t* data, uint8_t* rowBuffer) const {
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} else if (fsDitherer) {
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color = fsDitherer->processPixel(adjustPixel(lum), currentX);
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} else {
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if (bpp > 2) {
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// Simple quantization or noise dithering
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color = quantize(adjustPixel(lum), currentX, prevRowY);
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if (nativePalette) {
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// Palette matches native gray levels: direct mapping (still apply brightness/contrast/gamma)
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color = static_cast<uint8_t>(adjustPixel(lum) >> 6);
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} else {
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// do not quantize 2bpp image
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color = static_cast<uint8_t>(lum >> 6);
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// Non-native palette with dithering disabled: simple quantization
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color = quantize(adjustPixel(lum), currentX, prevRowY);
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}
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}
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currentOutByte |= (color << bitShift);
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@@ -219,6 +243,13 @@ BmpReaderError Bitmap::readNextRow(uint8_t* data, uint8_t* rowBuffer) const {
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}
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break;
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}
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case 4: {
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for (int x = 0; x < width; x++) {
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const uint8_t nibble = (x & 1) ? (rowBuffer[x >> 1] & 0x0F) : (rowBuffer[x >> 1] >> 4);
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packPixel(paletteLum[nibble]);
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}
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break;
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}
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case 2: {
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for (int x = 0; x < width; x++) {
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lum = paletteLum[(rowBuffer[x >> 2] >> (6 - ((x & 3) * 2))) & 0x03];
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@@ -56,10 +56,12 @@ class Bitmap {
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bool topDown = false;
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uint32_t bfOffBits = 0;
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uint16_t bpp = 0;
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uint32_t colorsUsed = 0;
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bool nativePalette = false; // true if all palette entries map to native gray levels
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int rowBytes = 0;
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uint8_t paletteLum[256] = {};
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// Floyd-Steinberg dithering state (mutable for const methods)
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// Dithering state (mutable for const methods)
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mutable int16_t* errorCurRow = nullptr;
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mutable int16_t* errorNextRow = nullptr;
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mutable int prevRowY = -1; // Track row progression for error propagation
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329
scripts/generate_test_bmps.py
Normal file
329
scripts/generate_test_bmps.py
Normal file
@@ -0,0 +1,329 @@
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#!/usr/bin/env python3
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"""
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Generate test BMP images for verifying Bitmap.cpp format support.
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Creates BMP files at 480x800 (CrossPoint display in portrait orientation).
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Test images use patterns designed to reveal dithering artifacts:
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- Checkerboard: sharp edges between gray levels, dithering adds noise at boundaries
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- Fine lines: thin 1px lines on contrasting background, dithering smears them
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- Mixed blocks: small rectangles of alternating grays, dithering blurs transitions
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- Gradient band: smooth transition in the middle, clean grays top/bottom
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Formats generated:
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- 1-bit: black & white (baseline, never dithered)
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- 2-bit: 4-level grayscale (non-standard CrossPoint extension, won't open on PC)
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- 4-bit: 4-color grayscale palette (standard BMP, new support)
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- 8-bit: 4-color grayscale palette (colorsUsed=4, should skip dithering)
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- 8-bit: 256-color grayscale (full palette, should be dithered)
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- 24-bit: RGB grayscale gradient (should be dithered)
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Usage:
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python generate_test_bmps.py [output_dir]
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Default output_dir: ./test_bmps/
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"""
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import struct
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import os
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import sys
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WIDTH = 480
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HEIGHT = 800
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# The 4 e-ink gray levels (luminance values)
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GRAY_LEVELS = [0, 85, 170, 255]
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def write_bmp_file_header(f, pixel_data_offset, file_size):
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f.write(b'BM')
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f.write(struct.pack('<I', file_size))
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f.write(struct.pack('<HH', 0, 0)) # reserved
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f.write(struct.pack('<I', pixel_data_offset))
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def write_bmp_dib_header(f, width, height, bpp, colors_used=0):
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f.write(struct.pack('<I', 40)) # DIB header size (BITMAPINFOHEADER)
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f.write(struct.pack('<i', width))
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f.write(struct.pack('<i', -height)) # negative = top-down
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f.write(struct.pack('<HH', 1, bpp)) # planes, bpp
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f.write(struct.pack('<I', 0)) # compression (BI_RGB)
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f.write(struct.pack('<I', 0)) # image size (can be 0 for BI_RGB)
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f.write(struct.pack('<i', 0)) # X pixels per meter
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f.write(struct.pack('<i', 0)) # Y pixels per meter
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f.write(struct.pack('<I', colors_used))
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f.write(struct.pack('<I', 0)) # important colors
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def write_palette(f, entries):
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"""Write palette entries as BGRA (B, G, R, 0x00)."""
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for gray in entries:
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f.write(struct.pack('BBBB', gray, gray, gray, 0))
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def get_test_pattern_index(x, y, width, height):
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"""Return palette index (0-3) for a test pattern designed to reveal dithering.
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Layout (4 horizontal bands):
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Band 1 (top 25%): Checkerboard of all 4 gray levels in 16x16 blocks
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Band 2 (25-50%): Fine horizontal lines (1px) alternating gray levels
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Band 3 (50-75%): 4 vertical stripes, each with a nested smaller pattern
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Band 4 (bottom 25%): Diagonal gradient-like pattern using the 4 levels
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"""
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band = y * 4 // height
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if band == 0:
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# Checkerboard: 16x16 blocks cycling through all 4 gray levels
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bx = (x // 16) % 4
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by = (y // 16) % 4
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return (bx + by) % 4
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elif band == 1:
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# Fine lines: 1px horizontal lines alternating between two gray levels
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# Left half: alternates black/white, Right half: alternates dark/light gray
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if x < width // 2:
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return 0 if (y % 2 == 0) else 3 # black/white
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else:
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return 1 if (y % 2 == 0) else 2 # dark gray/light gray
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elif band == 2:
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# 4 vertical stripes, each containing a smaller checkerboard at 4x4
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stripe = min(x * 4 // width, 3)
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inner = ((x // 4) + (y // 4)) % 2
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if stripe == 0:
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return 0 if inner else 1 # black / dark gray
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elif stripe == 1:
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return 1 if inner else 2 # dark gray / light gray
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elif stripe == 2:
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return 2 if inner else 3 # light gray / white
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else:
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return 0 if inner else 3 # black / white (max contrast)
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else:
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# Diagonal bands using all 4 levels
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return ((x + y) // 20) % 4
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def get_test_pattern_lum(x, y, width, height):
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"""Return 0-255 luminance for a continuous-tone test pattern.
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Same layout as index version but uses full grayscale range
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to show how dithering handles non-native gray values.
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"""
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band = y * 4 // height
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if band == 0:
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# Checkerboard with intermediate grays (not native levels)
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bx = (x // 16) % 4
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by = (y // 16) % 4
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# Use values between native levels to force dithering decisions
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values = [0, 64, 128, 192]
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return values[(bx + by) % 4]
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elif band == 1:
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# Fine lines with intermediate values
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if x < width // 2:
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return 32 if (y % 2 == 0) else 224 # near-black / near-white
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else:
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return 96 if (y % 2 == 0) else 160 # mid-grays
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elif band == 2:
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# Smooth horizontal gradient across full width
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return (x * 255) // (width - 1)
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else:
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# Diagonal bands with smooth transitions
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return ((x + y) * 255 // (width + height))
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def generate_1bit(path):
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"""1-bit BMP: checkerboard pattern."""
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bpp = 1
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palette = [0, 255]
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row_bytes = (WIDTH * bpp + 31) // 32 * 4
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palette_size = len(palette) * 4
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pixel_offset = 14 + 40 + palette_size
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file_size = pixel_offset + row_bytes * HEIGHT
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with open(path, 'wb') as f:
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write_bmp_file_header(f, pixel_offset, file_size)
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write_bmp_dib_header(f, WIDTH, HEIGHT, bpp, len(palette))
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write_palette(f, palette)
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for y in range(HEIGHT):
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row = bytearray(row_bytes)
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for x in range(WIDTH):
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# 16x16 checkerboard
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val = ((x // 16) + (y // 16)) % 2
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if val:
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row[x >> 3] |= (0x80 >> (x & 7))
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f.write(row)
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print(f" Created: {path} ({bpp}-bit, {len(palette)} colors)")
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def generate_2bit(path):
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"""2-bit BMP: 4-level grayscale test pattern (non-standard, CrossPoint extension)."""
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bpp = 2
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palette = GRAY_LEVELS
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row_bytes = (WIDTH * bpp + 31) // 32 * 4
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palette_size = len(palette) * 4
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pixel_offset = 14 + 40 + palette_size
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file_size = pixel_offset + row_bytes * HEIGHT
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with open(path, 'wb') as f:
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write_bmp_file_header(f, pixel_offset, file_size)
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write_bmp_dib_header(f, WIDTH, HEIGHT, bpp, len(palette))
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write_palette(f, palette)
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for y in range(HEIGHT):
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row = bytearray(row_bytes)
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for x in range(WIDTH):
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idx = get_test_pattern_index(x, y, WIDTH, HEIGHT)
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byte_pos = x >> 2
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bit_shift = 6 - ((x & 3) * 2)
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row[byte_pos] |= (idx << bit_shift)
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f.write(row)
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print(f" Created: {path} ({bpp}-bit, {len(palette)} colors)")
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def generate_4bit(path):
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"""4-bit BMP: 4-color grayscale test pattern (standard, should skip dithering)."""
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bpp = 4
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palette = GRAY_LEVELS
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row_bytes = (WIDTH * bpp + 31) // 32 * 4
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palette_size = len(palette) * 4
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pixel_offset = 14 + 40 + palette_size
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file_size = pixel_offset + row_bytes * HEIGHT
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with open(path, 'wb') as f:
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write_bmp_file_header(f, pixel_offset, file_size)
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write_bmp_dib_header(f, WIDTH, HEIGHT, bpp, len(palette))
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write_palette(f, palette)
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for y in range(HEIGHT):
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row = bytearray(row_bytes)
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for x in range(WIDTH):
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idx = get_test_pattern_index(x, y, WIDTH, HEIGHT)
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byte_pos = x >> 1
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if x & 1:
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row[byte_pos] |= idx
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else:
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row[byte_pos] |= (idx << 4)
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f.write(row)
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print(f" Created: {path} ({bpp}-bit, {len(palette)} colors)")
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def generate_8bit_4colors(path):
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"""8-bit BMP with only 4 palette entries (colorsUsed=4, should skip dithering)."""
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bpp = 8
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palette = GRAY_LEVELS
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row_bytes = (WIDTH * bpp + 31) // 32 * 4
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palette_size = len(palette) * 4
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pixel_offset = 14 + 40 + palette_size
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file_size = pixel_offset + row_bytes * HEIGHT
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with open(path, 'wb') as f:
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write_bmp_file_header(f, pixel_offset, file_size)
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write_bmp_dib_header(f, WIDTH, HEIGHT, bpp, len(palette))
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write_palette(f, palette)
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for y in range(HEIGHT):
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row = bytearray(row_bytes)
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for x in range(WIDTH):
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row[x] = get_test_pattern_index(x, y, WIDTH, HEIGHT)
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f.write(row)
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print(f" Created: {path} ({bpp}-bit, {len(palette)} colors)")
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def generate_8bit_256colors(path):
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"""8-bit BMP with full 256 palette (should be dithered normally)."""
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bpp = 8
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palette = list(range(256))
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row_bytes = (WIDTH * bpp + 31) // 32 * 4
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palette_size = len(palette) * 4
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pixel_offset = 14 + 40 + palette_size
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file_size = pixel_offset + row_bytes * HEIGHT
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with open(path, 'wb') as f:
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write_bmp_file_header(f, pixel_offset, file_size)
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write_bmp_dib_header(f, WIDTH, HEIGHT, bpp, len(palette))
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write_palette(f, palette)
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for y in range(HEIGHT):
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row = bytearray(row_bytes)
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for x in range(WIDTH):
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row[x] = get_test_pattern_lum(x, y, WIDTH, HEIGHT)
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f.write(row)
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print(f" Created: {path} ({bpp}-bit, {len(palette)} colors)")
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def generate_24bit(path):
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"""24-bit BMP: RGB grayscale test pattern (should be dithered normally)."""
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bpp = 24
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row_bytes = (WIDTH * bpp + 31) // 32 * 4
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pixel_offset = 14 + 40
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file_size = pixel_offset + row_bytes * HEIGHT
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with open(path, 'wb') as f:
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write_bmp_file_header(f, pixel_offset, file_size)
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write_bmp_dib_header(f, WIDTH, HEIGHT, bpp, 0)
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for y in range(HEIGHT):
|
||||
row = bytearray(row_bytes)
|
||||
for x in range(WIDTH):
|
||||
gray = get_test_pattern_lum(x, y, WIDTH, HEIGHT)
|
||||
offset = x * 3
|
||||
row[offset] = gray # B
|
||||
row[offset + 1] = gray # G
|
||||
row[offset + 2] = gray # R
|
||||
f.write(row)
|
||||
|
||||
print(f" Created: {path} ({bpp}-bit)")
|
||||
|
||||
|
||||
def main():
|
||||
output_dir = sys.argv[1] if len(sys.argv) > 1 else './test_bmps'
|
||||
os.makedirs(output_dir, exist_ok=True)
|
||||
|
||||
print(f"Generating test BMPs in {output_dir}/")
|
||||
print(f"Resolution: {WIDTH}x{HEIGHT}")
|
||||
print()
|
||||
|
||||
generate_1bit(os.path.join(output_dir, 'test_1bit_bw.bmp'))
|
||||
generate_2bit(os.path.join(output_dir, 'test_2bit_4gray.bmp'))
|
||||
generate_4bit(os.path.join(output_dir, 'test_4bit_4gray.bmp'))
|
||||
generate_8bit_4colors(os.path.join(output_dir, 'test_8bit_4colors.bmp'))
|
||||
generate_8bit_256colors(os.path.join(output_dir, 'test_8bit_256gray_gradient.bmp'))
|
||||
generate_24bit(os.path.join(output_dir, 'test_24bit_gradient.bmp'))
|
||||
|
||||
print()
|
||||
print("Test pattern layout (4 horizontal bands):")
|
||||
print(" Band 1 (top): 16x16 checkerboard cycling all 4 gray levels")
|
||||
print(" Band 2: Fine 1px horizontal lines alternating grays")
|
||||
print(" Band 3: 4 stripes with nested 4x4 checkerboard detail")
|
||||
print(" Band 4 (bottom): Diagonal bands cycling all 4 levels")
|
||||
print()
|
||||
print("What to look for:")
|
||||
print(" Direct mapping: Sharp, clean edges between gray blocks")
|
||||
print(" Dithering: Noisy/speckled boundaries, smeared fine lines")
|
||||
print()
|
||||
print("Expected results on device:")
|
||||
print(" 1-bit: Clean B&W checkerboard, no dithering")
|
||||
print(" 2-bit: Clean 4-gray pattern, no dithering (non-standard BMP, won't open on PC)")
|
||||
print(" 4-bit: Clean 4-gray pattern, no dithering (standard BMP, viewable on PC)")
|
||||
print(" 8-bit (4 colors): Clean 4-gray pattern, no dithering (standard BMP, viewable on PC)")
|
||||
print(" 8-bit (256 colors): Same layout but with intermediate grays, WITH dithering")
|
||||
print(" 24-bit: Same layout but with intermediate grays, WITH dithering")
|
||||
print()
|
||||
print("Note: 2-bit BMP is a non-standard CrossPoint extension. Standard image viewers")
|
||||
print("will not open it. Use the 4-bit BMP instead for images created with standard tools")
|
||||
print("(e.g. ImageMagick: convert input.png -colorspace Gray -colors 4 -depth 4 BMP3:output.bmp)")
|
||||
print()
|
||||
print("Copy files to /sleep/ folder on SD card to test as custom sleep screen images.")
|
||||
|
||||
|
||||
if __name__ == '__main__':
|
||||
main()
|
||||
Reference in New Issue
Block a user