libgd-gis

map.image — Accessing the Rendered Image

After rendering a map, libgd-gis exposes the underlying GD::Image instance through map.image. This allows you to post-process the rendered map using the full power of ruby-libgd.

This design cleanly separates:

• Geographic rendering (libgd-gis)
• Image manipulation (ruby-libgd)

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Basic usage

map.render
img = map.image

img is a GD::Image object and supports all ruby-libgd operations.

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What you can do with map.image

Once you have access to the image, you can:

• Draw text and labels
• Add custom overlays (rectangles, lines, circles)
• Apply colors, alpha blending, and transparency
• Composite images or icons
• Save in multiple formats (PNG, JPEG, GIF)

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Example: adding a label after render

img = map.image

font = "../fonts/DejaVuSans-Bold.ttf"

img.filled_rectangle(24, 24, 264, 88, [0, 0, 0])

img.text(
  "TOKYO",
  x: 48,
  y: 68,
  size: 32,
  color: [255, 255, 255],
  font: font
)

img.save("tokyo.png")

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Why post-render manipulation matters

• Keeps GIS rendering deterministic
• Avoids polluting the map pipeline with presentation logic
• Enables advanced compositions and branding
• Allows reuse of ruby-libgd knowledge and tools

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Relationship with overlays

map.image complements overlay methods such as:

• add_points
• add_lines
• add_polygons

Use overlays for geographic data, and map.image for pure image manipulation.

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Summary

• map.image returns a GD::Image
• Full ruby-libgd API is available
• Ideal for labels, branding, and post-processing
• Clean separation between GIS and image layers

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