Image Filter Program

A command-line image processing tool written in C that applies various filters to images. The program supports multiple image formats (converted to BMP internally) and can download images from URLs.

Features

Image Filters

• Grayscale (-g): Converts color images to grayscale
• Sepia (-s): Applies a warm, vintage sepia tone effect
• Blur (-b): Creates a blur effect using box blur algorithm
• Reflection (-r): Horizontally mirrors the image
• Edge Detection (-e): Detects edges using Sobel operators
• Inversion (-i): Inverts all colors (negative effect)

Additional Features

• Web Image Download (-w): Downloads images directly from URLs
• Format Conversion: Automatically converts non-BMP images to BMP format
• Multiple Format Support: Works with various image formats through stb_image library

Dependencies

Required Libraries

• curl - for downloading images from URLs
• stb_image.h - for reading various image formats
• stb_image_write.h - for writing image files
• Standard C libraries (stdio.h, stdlib.h, math.h, getopt.h)

Installation of Dependencies

Ubuntu/Debian

sudo apt-get update
sudo apt-get install libcurl4-openssl-dev

macOS (using Homebrew)

brew install curl

Windows

Download and install curl development libraries from the official curl website.

Compilation

gcc -o filter filter.c -lcurl -lm

Make sure you have the stb_image.h and stb_image_write.h header files in your project directory.

Usage

Basic Syntax

./filter [flag] input_file output_file

Download Image from Web

./filter -w [URL]

Filter Examples

Apply Grayscale Filter

./filter -g input.jpg output.bmp

Apply Sepia Filter

./filter -s photo.png sepia_photo.bmp

Apply Blur Filter

./filter -b image.bmp blurred_image.bmp

Reflect Image Horizontally

./filter -r photo.jpg reflected_photo.bmp

Apply Edge Detection

./filter -e landscape.png edges.bmp

Invert Colors

./filter -i portrait.jpg inverted_portrait.bmp

Download Image from URL

./filter -w https://example.com/image.jpg

Directory Structure

The program expects/creates the following directory structure:

./
├── filter (executable)
├── stb_image.h
├── stb_image_write.h
└── images/
    ├── temp.bmp (temporary conversion file)
    └── downloaded_image.* (downloaded images)

Make sure to create the images/ directory before running the program.

How It Works

Image Processing Pipeline

1. Input Validation: Checks command-line arguments and filter flags
2. File Reading: Reads the input image file
3. Format Conversion: If the input isn't a 24-bit BMP, converts it using stb_image
4. Filter Application: Applies the selected filter algorithm
5. Output Writing: Saves the processed image as a BMP file

Filter Algorithms

Grayscale

Converts RGB values to grayscale by averaging the red, green, and blue components.

Sepia

Applies sepia tone using the standard sepia transformation matrix:

• Red = 0.393R + 0.769G + 0.189*B
• Green = 0.349R + 0.686G + 0.168*B
• Blue = 0.272R + 0.534G + 0.131*B

Blur

Uses a box blur algorithm that averages each pixel with its surrounding neighbors.

Edge Detection

Implements Sobel edge detection using horizontal and vertical convolution kernels.

Reflection

Swaps pixels horizontally to create a mirror effect.

Inversion

Subtracts each color component from 255 to create a negative effect.

Error Handling

The program includes comprehensive error handling for:

• Invalid command-line arguments
• File I/O errors
• Memory allocation failures
• Network download errors
• Unsupported image formats

Limitations

• Output is always in BMP format
• Requires 24-bit color depth for direct BMP processing
• Web downloads require internet connectivity
• Memory usage scales with image size

Exit Codes

• 0: Success
• 1: Invalid filter flag
• 2: Multiple filters specified
• 3: Incorrect number of arguments
• 4: Cannot open input file
• 5: Cannot create output file
• 7: Insufficient memory

Examples

Process a JPEG image with blur filter

./filter -b vacation.jpg blurred_vacation.bmp

Download and process an image

# First download
./filter -w https://example.com/photo.jpg

# Then process the downloaded image
./filter -g ./images/downloaded_image.jpg gray_photo.bmp