RCD330 Image Utilities: OVG Image Format Converter

This repository contains tools for converting OVG (proprietary image format) files to PNG and back, specifically designed for car stereo firmware modification.

Background

OVG files are a proprietary image format used in the RCD330 (and likely other) car stereo firmware. They use RLE (Run-Length Encoding) compression with RGBA color data, saved to a binary file extension.

This project was spun up out of a desire to skin more of the interface than ust the bootlogo. Much credit goes to @cr3ative for sharing his initial discoveries publicly on GitHub in the RCD 330 Image Utilities repo.

File Format Description

The converter supports two different OVG file formats:

1. RLE-Compressed OVG Format

The traditional OVG format uses RLE (Run-Length Encoding) compression:

• Command Block: 1 byte indicating compression type and pixel count
• Pixel Data: RGBA data (4 bytes per pixel)
• RLE Compression: Efficient encoding for repeated pixels

2. Raw RGBA Format

Some OVG files contain raw RGBA pixel data without compression:

• Direct RGBA: 4 bytes per pixel (R, G, B, A)
• No compression: Pixel data stored sequentially
• Square dimensions: Often forms perfect or near-perfect squares

The converter automatically detects which format is used and processes accordingly.

RLE Command Block Format (Format 1 only)

Bit 7 (MSB): Compression flag (1 = compressed, 0 = uncompressed)
Bits 6-0: Pixel count - 1 (0-127 representing 1-128 pixels)

RLE Compression Types (Format 1 only)

• Compressed (MSB = 1): Next 4 bytes (RGBA) repeated N times
• Uncompressed (MSB = 0): Next N×4 bytes are individual RGBA pixels

Format Auto-Detection

The converter automatically detects the format by analyzing:

• File size: Must be divisible by 4 for raw RGBA
• Dimensions: Raw RGBA often forms perfect squares
• Alpha patterns: Common alpha values (0, 255) indicate raw RGBA

Tools Included

1. ovg_to_png.py - OVG to PNG Decoder

Converts OVG files to PNG format with transparency preservation.

2. png_to_ovg.py - PNG to OVG Encoder

Converts PNG files back to OVG format with RLE compression.

Requirements

pip3 install --user Pillow

Usage

Decoding OVG to PNG

Basic Conversion

python3 ovg_to_png.py input.bin [output.png]

Examples:

# Convert with auto-detected dimensions and format
python3 ovg_to_png.py opt/gresfiles/img_off_clock_face_ovg.bin

# Convert with custom output name
python3 ovg_to_png.py opt/gresfiles/img_off_clock_face_ovg.bin my_clock_face.png

# The converter will automatically detect if the file is RLE-compressed or raw RGBA
# Example output:
# Detected format: raw_rgba
# Raw RGBA data contains 1521 pixels
# Auto-detected dimensions: 39x39

Specify Exact Dimensions

python3 ovg_to_png.py input.bin output.png --width 286 --height 286
python3 ovg_to_png.py input.bin --width 200  # Height calculated automatically

Interactive Size Discovery

# Use size discovery to find correct dimensions
python3 ovg_to_png.py input.bin --discover

# Customize discovery range
python3 ovg_to_png.py input.bin --discover --width-min 50 --width-max 300 --width-step 5

The discovery mode will:

• Generate test images at different sizes
• Display each size and wait for your input
• Let you jump to specific widths
• Help you find the correct dimensions visually

Available Options

• -w, --width WIDTH - Specify image width
• --height HEIGHT - Specify image height
• -d, --discover - Interactive size discovery mode
• --width-min MIN - Minimum width for discovery (default: 35)
• --width-max MAX - Maximum width for discovery (default: 400)
• --width-step STEP - Width step for discovery (default: 1)

Usage Help

python3 ovg_to_png.py

Output Files

• Clock Face: 286×286 pixels (perfect square from 81,796 pixels)
• Clock Shadow: 400×400 pixels
• Clock Spotlight: 619×619 pixels
• Clock Hands: 286×286 pixels each

Encoding PNG to OVG

Convert Specific File

python3 png_to_ovg.py input.png [output.bin]

Examples:

# Convert with custom output name
python3 png_to_ovg.py my_custom_clock.png img_off_clock_face_ovg_new.bin

# Convert with auto-generated output name (replaces .png with .bin)
python3 png_to_ovg.py clock_face_decoded.png
# Creates: clock_face_decoded.bin

Usage Help

python3 png_to_ovg.py

Shows usage information and examples.

Test Roundtrip Conversion

# Test with default file
python3 png_to_ovg.py test

# Test with specific file
python3 png_to_ovg.py test opt/gresfiles/img_off_clock_face_ovg.bin

This will:

1. Decode the original OVG to PNG
2. Encode the PNG back to OVG
3. Compare file sizes and report compression efficiency
4. Clean up temporary files automatically

Complete Workflow for Clock Customization

Step 1: Extract Original Images

# Extract specific files with custom names
python3 ovg_to_png.py opt/gresfiles/img_off_clock_face_ovg.bin my_clock_face.png
python3 ovg_to_png.py opt/gresfiles/img_off_clock_shadow_ovg.bin my_shadow.png
python3 ovg_to_png.py opt/gresfiles/img_off_clock_spotlight_ovg.bin my_spotlight.png

# Or extract with auto-generated names
python3 ovg_to_png.py opt/gresfiles/img_off_clock_face_ovg.bin
# Creates: img_off_clock_face_ovg_decoded.png

Step 2: Edit Images

• Open your extracted PNG files in your image editor
• Modify the clock face design as desired
• Edit other components (shadow, spotlight, hands) if needed
• Save as PNG with transparency preserved

Step 3: Convert Back to OVG

# Convert specific files with custom output names
python3 png_to_ovg.py my_clock_face.png img_off_clock_face_ovg_new.bin
python3 png_to_ovg.py my_shadow.png img_off_clock_shadow_ovg_new.bin

# Or convert with auto-generated names
python3 png_to_ovg.py img_off_clock_face_ovg_decoded.png
# Creates: img_off_clock_face_ovg_decoded.bin

Step 4: Replace in Firmware

• Backup original files first!
• Replace original .bin files with corresponding *_new.bin files
• Update firmware with modified files

Key Files

Input Files (Original OVG)

• opt/gresfiles/img_off_clock_face_ovg.bin - Main clock face
• opt/gresfiles/img_off_clock_shadow_ovg.bin - Clock shadow
• opt/gresfiles/img_off_clock_spotlight_ovg.bin - Clock spotlight
• opt/gresfiles/img_off_clock_hour_XX_ovg.bin - Hour hand positions
• opt/gresfiles/img_off_clock_minute_XX_ovg.bin - Minute hand positions
• opt/gresfiles/img_off_clock_second_XX_ovg.bin - Second hand positions

Output Files (Editable PNG)

• clock_face_decoded.png - Main clock face (286×286)
• img_off_clock_*_decoded.png - Individual components

Generated Files (New OVG)

• img_off_clock_face_ovg_new.bin - Modified clock face
• img_off_clock_*_ovg_new.bin - Modified components

Technical Details

Compression Performance

Typical compression ratios achieved:

• Clock Face: ~2.8:1 compression
• Clock Shadow: ~10.8:1 compression
• Clock Hands: ~33-41:1 compression
• Spotlight: ~11.3:1 compression

Image Specifications

• Format: RGBA (32-bit with alpha channel)
• Typical Size: 286×286 pixels for main components
• Color Depth: 8 bits per channel (R, G, B, A)
• Byte Order: Standard RGBA pixel ordering

Troubleshooting

PIL/Pillow Not Found

pip3 install --user Pillow

File Not Found Errors

Ensure the opt/gresfiles/ directory exists with original OVG files.

Size Mismatch

The converter automatically calculates optimal dimensions and detects file format. If dimensions appear incorrect:

1. Check format detection: The converter will show "Detected format: raw_rgba" or "Detected format: rle_ovg"
2. Raw RGBA files: Should auto-detect to perfect or near-perfect squares
3. RLE OVG files: May require manual dimension specification with --width and --height
4. Use discovery mode: --discover to find correct dimensions visually

Compression Issues

If the generated OVG file is significantly larger than the original:

1. Check for unnecessary transparency in your PNG
2. Ensure solid color areas are truly solid (no noise/gradients)
3. The RLE compression works best with areas of repeated pixels

Development Notes

Discovery Process

1. Initial attempts tried standard image formats (BMP, RGB, etc.)
2. Analyzed byte patterns showing 0xFF padding and RLE-like structures
3. Found reference to NXP AN4339 PDF describing similar RLE format
4. Reverse-engineered the exact command block structure
5. Implemented both decoder and encoder with roundtrip testing

Format Insights

• Files start with 0xFF padding that should be skipped
• Command blocks use 7-bit pixel counts (1-128 pixels per command)
• RLE compression is very effective for clock graphics with large solid areas
• Alpha channel is preserved and properly handled

References

• NXP AN4339 Application Note - Describes similar RLE routine
• Reverse Engineering Stack Exchange - Initial format identification

License

This project is provided as-is for educational and personal use. Always backup original firmware before making modifications.