ImageMagick: Possible to remove middle in picture?

ImageMagick: Removing the Middle Section of an Image

Learn how to precisely remove a middle section from an image using ImageMagick, a powerful command-line tool. This guide covers various techniques for cropping and rejoining image parts.

Image manipulation often requires precise control over image regions. A common task is to remove a specific middle section of an image, effectively 'stitching' the remaining parts together. While graphical tools like GIMP or Inkscape offer visual interfaces for this, ImageMagick provides a robust command-line solution perfect for scripting and automation. This article will guide you through the process, explaining different approaches and their applications.

Understanding the Challenge: Removing a Middle Strip

Removing a middle section isn't a single 'cut' operation. Instead, it involves two primary steps: first, cropping the image into two or more pieces, and second, rejoining the desired pieces. The complexity lies in accurately defining the crop regions and then seamlessly merging the remaining parts. We'll focus on removing a horizontal strip, but the principles can be adapted for vertical removal.

flowchart TD
    A[Original Image] --> B{Define Crop Regions}
    B --> C1[Top/Left Crop]
    B --> C2[Bottom/Right Crop]
    C1 --> D[Cropped Part 1]
    C2 --> E[Cropped Part 2]
    D & E --> F[Rejoin Parts]
    F --> G[Resulting Image (Middle Removed)]

Workflow for removing a middle section from an image.

Method 1: Cropping and Concatenating Two Halves

This method is suitable when you want to remove a horizontal strip from the middle. You'll crop the image into a top part and a bottom part, excluding the middle, and then stack them vertically. This requires knowing the exact coordinates or dimensions of the section to be removed.

# Define original image dimensions (example)
ORIG_WIDTH=1000
ORIG_HEIGHT=800

# Define the section to remove (e.g., 100px tall, starting at Y=300)
REMOVE_Y=300
REMOVE_HEIGHT=100

# Calculate top part dimensions
TOP_HEIGHT=$REMOVE_Y

# Calculate bottom part dimensions
BOTTOM_Y=$((REMOVE_Y + REMOVE_HEIGHT))
BOTTOM_HEIGHT=$((ORIG_HEIGHT - BOTTOM_Y))

# 1. Crop the top part
convert input.png -crop "${ORIG_WIDTH}x${TOP_HEIGHT}+0+0" top_part.png

# 2. Crop the bottom part
convert input.png -crop "${ORIG_WIDTH}x${BOTTOM_HEIGHT}+0+${BOTTOM_Y}" bottom_part.png

# 3. Concatenate the two parts vertically
convert top_part.png bottom_part.png -append output.png

# Clean up temporary files (optional)
rm top_part.png bottom_part.png

Bash script to remove a horizontal middle section by cropping and appending.

Method 2: Using Virtual Pixels and Offsets for More Complex Scenarios

For more advanced scenarios, or if you prefer a single convert command, you can use virtual pixels and offsets. This method can be less intuitive but offers powerful control. It involves creating a canvas and then placing the desired parts onto it, effectively skipping the middle.

# Assuming input.png is 1000x800
# Remove a 100px tall strip starting at Y=300

convert input.png \
  -crop 1000x300+0+0 +repage \
  null: \
  input.png \
  -crop 1000x400+0+400 +repage \
  -gravity North -background none -append output_single_command.png

Removing a middle section using a single ImageMagick command with null: and append.

Let's break down the single command:

1. input.png -crop 1000x300+0+0 +repage: This crops the top 300 pixels of the image.
2. null:: This acts as a separator, indicating that the next image operation should start a new image sequence.
3. input.png -crop 1000x400+0+400 +repage: This crops the bottom 400 pixels (from Y=400 to Y=800) of the original image.
4. -gravity North -background none -append: This appends the two cropped parts vertically, aligning them to the top and ensuring transparency if applicable.

Alternative: Using GIMP or Inkscape for Visual Editing

While ImageMagick is excellent for automation, for one-off tasks or if you prefer a visual approach, GIMP (for raster images) or Inkscape (for vector images, though it can handle raster too) are great alternatives.

GIMP Steps:

1. Open your image in GIMP.
2. Use the Rectangle Select tool to select the top part you want to keep.
3. Copy (Ctrl+C) and Paste as New Layer (Ctrl+Shift+V).
4. Repeat for the bottom part.
5. Align the two new layers to remove the gap.
6. Merge visible layers and export.

Inkscape Steps (for raster images):

1. Import your raster image.
2. Draw two rectangles over the parts you want to keep.
3. Select the image and one rectangle, then go to Object > Clip > Set.
4. Repeat for the second part. You'll have two clipped images.
5. Align and group them, then export as a raster image.

1. Determine Crop Coordinates

Identify the exact pixel coordinates (x, y, width, height) of the section you want to remove. This is crucial for accurate cropping.

2. Crop Top/Left Section

Use convert input.png -crop WxH+X+Y top_part.png to extract the portion of the image above or to the left of the section to be removed.

3. Crop Bottom/Right Section

Use convert input.png -crop WxH+X+Y bottom_part.png to extract the portion of the image below or to the right of the section to be removed.

4. Concatenate Parts

Combine the cropped parts using convert part1.png part2.png -append output.png for vertical stacking or +append for horizontal stacking.

5. Verify and Clean Up

Check the output.png to ensure the middle section is correctly removed. Delete any temporary files created during the process.