Fix indexed image detection to prevent PDF size inflation

[Copilot]

Fix indexed image detection to prevent PDF size inflation

Fixes #1289

Problem

Indexed images (GIFs and PNGs with indexed colors) were incorrectly detected as RGB images, causing significant PDF size inflation (100%+ in some cases as reported in the issue).

Root Cause

When loading GIF/PNG via ImageIO, BufferedImage with IndexColorModel was converted to RGB using PixelGrabber, which:

• Lost the palette information
• Expanded 1-byte indexed pixels to 3-byte RGB pixels
• Significantly inflated the resulting PDF file size

Solution Implemented

Modified Image.getInstance(java.awt.Image, java.awt.Color, boolean) to:

1. Detect BufferedImage with IndexColorModel before the PixelGrabber conversion
2. Extract palette data (RGB values) directly from the IndexColorModel
3. Extract pixel indices directly from the raster without conversion
4. Create an indexed image with the original palette stored in PDF format: [/Indexed /DeviceRGB maxIndex palette]
5. Normalize bitsPerPixel to valid PDF values (1, 2, 4, or 8)

Changes Made

• Image.java: Added IndexColorModel detection and handling before the existing PixelGrabber code path
• ImageTest.java: Added 2 comprehensive tests:
  • shouldDetectIndexedColorGif: Tests loading indexed GIF from file
  • shouldDetectIndexedColorFromBufferedImage: Tests programmatically created indexed BufferedImage

Testing Results

• ✅ All 11 ImageTest tests pass
• ✅ All 2039 existing openpdf-core tests pass with no failures or errors
• ✅ Verified indexed GIF files are correctly detected with colorspace=1
• ✅ Verified additional dictionary with indexed colorspace array is set
• ✅ No security vulnerabilities detected by CodeQL

Verification

Tested with H.gif (indexed GIF from test resources):

• Image correctly detected as indexed (colorspace=1)
• Additional dictionary properly set with indexed colorspace
• Raw data size minimal (1459 bytes for 171x250 image)
• All existing tests continue to pass

This fix ensures that indexed images maintain their compact representation in PDFs, preventing the size inflation reported in issue #1289.

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud