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Crop Node ​

Purpose & Use Cases ​

The crop node extracts rectangular regions from images with support for both normalized coordinates (0-1) and pixel coordinates. It enables precise region selection for focus areas, object extraction, and image composition.

Real-World Applications:

  • Object Extraction: Extract specific objects or regions of interest
  • Face Cropping: Extract detected faces from photographs
  • Document Processing: Extract text regions or signatures from scanned documents
  • Product Photography: Extract product images from larger scenes
  • Art Composition: Extract interesting portions of artwork or photography

/flows/node-red-contrib-rosepetal-image-tools/assets/nodes/transform/crop-demo.gif

Input/Output Specification ​

Inputs ​

  • Single Image: Standard image object format
  • Image Array: Array of image objects for batch cropping
  • Dynamic Coordinates: Crop parameters can be provided via message properties

Outputs ​

  • Cropped Image: Extracted rectangular region as image object
  • Format Options: Raw image object or encoded file formats (JPEG/PNG/WebP)

Configuration Options ​

Input/Output Paths ​

  • Input From: msg.payload (default), flow.*, global.*
  • Output To: msg.payload (default), flow.*, global.*

Coordinate System ​

  • Normalized Coordinates: Toggle between 0-1 range and pixel coordinates
  • Normalized Mode: Values from 0.0 to 1.0 (relative to image dimensions)
  • Pixel Mode: Absolute pixel coordinates

Crop Parameters ​

X Position (cropX) ​

  • Normalized: 0.0 (left edge) to 1.0 (right edge)
  • Pixel: 0 to image width
  • Sources: Number, msg.*, flow.*, global.*

Y Position (cropY) ​

  • Normalized: 0.0 (top edge) to 1.0 (bottom edge)
  • Pixel: 0 to image height
  • Sources: Number, msg.*, flow.*, global.*

Width ​

  • Normalized: 0.0 to 1.0 (proportion of image width)
  • Pixel: Absolute width in pixels
  • Sources: Number, msg.*, flow.*, global.*

Height ​

  • Normalized: 0.0 to 1.0 (proportion of image height)
  • Pixel: Absolute height in pixels
  • Sources: Number, msg.*, flow.*, global.*

Output Format Options ​

  • Raw: Standard image object (fastest)
  • JPEG: Compressed with quality control (1-100)
  • PNG: Lossless compression with optional optimization
  • WebP: Modern format with quality control

Performance Notes ​

C++ Backend Processing ​

  • High-Speed Extraction: Optimized OpenCV-based region extraction
  • Memory Efficient: Direct memory copy without full image processing
  • Bounds Checking: Automatic coordinate validation and clipping
  • Parallel Processing: Array inputs processed concurrently

Coordinate Validation ​

  • Bounds Clipping: Coordinates automatically clipped to image boundaries
  • Error Handling: Invalid regions logged with graceful fallback
  • Status Display: Processing time and result dimensions shown

Real-World Examples ​

Face Extraction (Normalized) ​

[Face Detection] → [Set msg.faceBox] → [Crop: X=msg.x, Y=msg.y, W=msg.w, H=msg.h] → [Face Image]

Extract detected faces using normalized coordinates from AI detection.

Center Crop ​

[Image-In] → [Crop: X=0.25, Y=0.25, W=0.5, H=0.5] → [Center Square]

Extract the center 50% of any image.

Document Region Extraction (Pixel) ​

[Scanned Document] → [Crop: X=100, Y=150, W=800, H=600] → [Text Region]

Extract specific document regions using pixel coordinates.

Batch Product Extraction ​

[Product Photos] → [Array-Out] → [Crop: Normalized] → [Product Cutouts]

Extract product regions from multiple photos.

Dynamic ROI Extraction ​

[Analysis] → [Set ROI Coords] → [Crop: Dynamic] → [Region of Interest]

Extract regions based on image analysis results.

Common Issues & Troubleshooting ​

Coordinate System Confusion ​

  • Issue: Unexpected crop results
  • Solution: Verify normalized vs pixel mode matches your coordinate system
  • Normalized: Use 0-1 values for relative positioning
  • Pixel: Use absolute coordinates

Out of Bounds Cropping ​

  • Issue: Crop region extends beyond image boundaries
  • Result: Automatic clipping to image bounds with warning
  • Prevention: Validate coordinates before processing

Empty Crop Results ​

  • Issue: Crop width or height is zero or negative
  • Result: Warning logged, original image passed through
  • Solution: Ensure width and height are positive values

Dynamic Coordinate Errors ​

  • Issue: Message properties don't contain valid coordinates
  • Solution: Use debug nodes to verify coordinate values
  • Validation: Check that dynamic sources provide numbers