Enable native libpng support to fix truncated PNG output for large images

[changhee-somite]

Summary

• Fix truncated PNG output when processing large images with draw-xy command
• Enable native libpng support in CImg by adding cimg_use_png compile definition
• Link against system libpng library

Problem

When cimg_use_png is not defined, CImg falls back to ImageMagick's convert command for PNG encoding. ImageMagick has policy-enforced resource limits (typically 256MP area, 1GiB memory in /etc/ImageMagick-6/policy.xml) that cause silent failures when processing large images.

For example, an 11460×22360 pixel image (256.3 megapixels) exceeds the default 256MP limit, resulting in a truncated PNG file containing only header chunks (IHDR, cHRM, bKGD) but no image data (IDAT) or end marker (IEND). This causes downstream tools like GDAL to fail with ERROR 1: libpng: Read Error.

Root Cause Analysis

Traced via strace:

convert-im6.q16: cache resources exhausted
convert-im6.q16: No IDATs written into file

The CImg → ImageMagick pipeline:

1. CImg saves temporary PPM file
2. CImg calls convert -quality 100 /tmp/xxx.ppm output.png
3. ImageMagick exceeds resource limits and fails silently
4. CImg sees the (truncated) output file exists and reports success

Solution

Enable native libpng support so CImg writes PNG directly without ImageMagick:

1. find_package(PNG REQUIRED) - locate system libpng
2. add_compile_definitions(cimg_use_png) - enable CImg's native PNG code path
3. Link PNG::PNG to the executable

Test Results

Tested with a 675M line Xenium dataset producing an 11460×22360 PNG:

Metric	Before	After
PNG file size	91 bytes	98 MB
PNG valid	❌ Truncated	✅ Valid IEND
GDAL processing	❌ libpng: Read Error	✅ Success

Test plan

• Build spatula with -DUSE_LIBDEFLATE=ON
• Run spatula draw-xy on large dataset (>256 megapixels)
• Verify output PNG is valid (has IEND marker)
• Verify GDAL can process the PNG (gdal_translate)

🤖 Generated with Claude Code