linear

Apply linear transformation to pixel values using the formula: output = (a × input) + b. Provides precise, reproducible control over image brightness and contrast with exact numerical values.

Usage

Terminal

$ mediaproc image linear <input> [options]

Basic Syntax

# Apply linear formula
mediaproc image linear photo.jpg -a 1.5 -b 10

# Increase contrast
mediaproc image linear photo.jpg -a 1.3

# Increase brightness
mediaproc image linear photo.jpg -b 50

Options

Option	Alias	Type	Default	Description
`--a`	`-a`	`number`	`1`	Multiplier (contrast control)
`--b`	`-b`	`number`	`0`	Offset (brightness control)
`--output`	`-o`	`string`	`[input]-linear.[ext]`	Output file path
`--quality`	`-q`	`number`	`90`	Output quality (1-100)
`--dry-run`		`boolean`	`false`	Preview operation without saving
`--verbose`	`-v`	`boolean`	`false`	Show detailed processing information

Formula

output = (a × input) + b

Where:
- input: Original pixel value (0-255)
- a: Multiplier (contrast adjustment)
- b: Offset (brightness adjustment)
- output: Result pixel value (clamped to 0-255)

Parameter Effects

Multiplier (a):

a > 1: Increase contrast
a = 1: No contrast change
a < 1: Decrease contrast
a = 0: Solid gray (128)
a < 0: Invert contrast

Offset (b):

b > 0: Increase brightness
b = 0: No brightness change
b < 0: Decrease brightness

Examples

1. Brightness Adjustment

# Increase brightness by 30
mediaproc image linear dark.jpg -a 1 -b 30

# Increase brightness by 50
mediaproc image linear photo.jpg -b 50

# Decrease brightness by 30
mediaproc image linear bright.jpg -b -30

# Maximum brightness increase
mediaproc image linear very-dark.jpg -b 80

2. Contrast Adjustment

# Increase contrast by 30%
mediaproc image linear flat.jpg -a 1.3

# Increase contrast by 50%
mediaproc image linear low-contrast.jpg -a 1.5

# Decrease contrast by 20%
mediaproc image linear harsh.jpg -a 0.8

# Dramatic contrast increase
mediaproc image linear photo.jpg -a 2.0

3. Combined Brightness and Contrast

# Brighten and increase contrast
mediaproc image linear photo.jpg -a 1.2 -b 20

# Darken and increase contrast
mediaproc image linear photo.jpg -a 1.3 -b -20

# Brighten and decrease contrast
mediaproc image linear photo.jpg -a 0.8 -b 30

# Balanced adjustment
mediaproc image linear photo.jpg -a 1.1 -b 10

4. Special Effects

# Invert (negative)
mediaproc image linear photo.jpg -a -1 -b 255

# High-key effect (bright, low contrast)
mediaproc image linear photo.jpg -a 0.5 -b 128

# Low-key effect (dark, high contrast)
mediaproc image linear photo.jpg -a 1.5 -b -40

# Posterization effect
mediaproc image linear photo.jpg -a 2 -b 0

5. Exposure Correction

# Underexposed (+1 stop equivalent)
mediaproc image linear underexposed.jpg -a 2 -b 0

# Underexposed (+0.5 stop)
mediaproc image linear dark.jpg -a 1.4 -b 10

# Overexposed (-0.5 stop)
mediaproc image linear bright.jpg -a 0.7 -b 0

# Severe underexposure correction
mediaproc image linear very-dark.jpg -a 2.5 -b 20

6. Batch Processing with Consistent Values

# Apply same adjustment to all images
for img in *.jpg; do
  mediaproc image linear "$img" -a 1.2 -b 15 -o "adjusted/$(basename "$img")"
done

# Brighten dark photos
for img in dark-photos/*.jpg; do
  mediaproc image linear "$img" -a 1.3 -b 30 -o "corrected/$(basename "$img")"
done

# Reduce contrast in harsh photos
for img in harsh/*.jpg; do
  mediaproc image linear "$img" -a 0.8 -b 10 -o "softened/$(basename "$img")"
done

7. Scientific Image Processing

# Normalize to specific range
mediaproc image linear data.jpg -a 0.5 -b 64

# Apply calibration curve
mediaproc image linear measurement.jpg -a 1.15 -b 5

# Standardize batch of scientific images
for img in samples/*.jpg; do
  mediaproc image linear "$img" -a 1.1 -b 0 -o "calibrated/$(basename "$img")"
done

8. Quality Control

# Maximum quality output
mediaproc image linear photo.jpg -a 1.2 -b 20 -q 100 -o high-quality.jpg

# Web-optimized output
mediaproc image linear photo.jpg -a 1.1 -b 10 -q 85 -o web-optimized.jpg

# Balanced quality
mediaproc image linear photo.jpg -a 1.3 -b 15 -q 90 -o balanced.jpg

9. Preview Operations

# Preview transformation
mediaproc image linear photo.jpg -a 1.5 -b 20 --dry-run

# Detailed preview
mediaproc image linear photo.jpg -a 1.2 -b 10 --dry-run --verbose

# Test different values
mediaproc image linear photo.jpg -a 1.3 -b 0 --dry-run
mediaproc image linear photo.jpg -a 1 -b 30 --dry-run
mediaproc image linear photo.jpg -a 1.2 -b 15 --dry-run

10. Color Grading

# Slight lift (film look)
mediaproc image linear photo.jpg -a 0.9 -b 20

# Crushed blacks effect
mediaproc image linear photo.jpg -a 1.2 -b -15

# Lifted shadows, compressed highlights
mediaproc image linear photo.jpg -a 0.85 -b 30

# Cinematic look
mediaproc image linear photo.jpg -a 1.1 -b 5

11. Histogram Manipulation

# Expand dynamic range
mediaproc image linear compressed.jpg -a 1.5 -b -30

# Compress dynamic range
mediaproc image linear wide-range.jpg -a 0.7 -b 40

# Shift histogram right (brighter)
mediaproc image linear photo.jpg -a 1 -b 40

# Shift histogram left (darker)
mediaproc image linear photo.jpg -a 1 -b -40

12. Photography Workflow

# RAW-style adjustment
mediaproc image linear raw-export.jpg -a 1.15 -b 8

# Portrait brightening
mediaproc image linear portrait.jpg -a 1.05 -b 15

# Landscape contrast boost
mediaproc image linear landscape.jpg -a 1.25 -b -5

# Product photo adjustment
mediaproc image linear product.jpg -a 1.1 -b 20

13. Document Enhancement

# Improve legibility
mediaproc image linear scan.jpg -a 1.3 -b 10

# Enhance faded document
mediaproc image linear old-document.jpg -a 1.5 -b 20

# Darken washed-out text
mediaproc image linear faded.jpg -a 1.4 -b -10

14. Creative Effects

# Dreamy effect
mediaproc image linear photo.jpg -a 0.7 -b 60

# Gritty effect
mediaproc image linear photo.jpg -a 1.6 -b -20

# Washed-out vintage
mediaproc image linear photo.jpg -a 0.6 -b 80

# Moody dark
mediaproc image linear photo.jpg -a 1.3 -b -30

15. Combining with Other Operations

# Auto-orient then linear adjust
mediaproc image auto-orient photo.jpg -o temp.jpg
mediaproc image linear temp.jpg -a 1.2 -b 15 -o final.jpg
rm temp.jpg

# Linear adjust then sharpen
mediaproc image linear photo.jpg -a 1.1 -b 10 -o temp.jpg
mediaproc image sharpen temp.jpg -o final.jpg
rm temp.jpg

# Grayscale then adjust contrast
mediaproc image grayscale photo.jpg -o temp.jpg
mediaproc image linear temp.jpg -a 1.4 -b 0 -o high-contrast-bw.jpg
rm temp.jpg

Use Cases

1. Precise Exposure Correction

Reproducible Adjustments: Exact values for consistent results
Batch Processing: Same adjustment to multiple images
Scientific Work: Documented, repeatable corrections
Client Specifications: Meet exact brightness/contrast requirements

2. Photography Workflow

RAW Processing: Fine-tune RAW exports
Series Consistency: Maintain look across photo series
Style Matching: Match specific visual style
Color Grading Base: Foundation for color grading

3. Scientific and Medical Imaging

Calibration: Apply known calibration curves
Standardization: Normalize image sets
Measurement Preparation: Prepare for analysis
Quality Control: Consistent processing parameters

4. Print Preparation

Compensation: Adjust for paper/ink characteristics
Proofing: Create accurate print previews
Standards Compliance: Meet printing specifications
Batch Preparation: Process print jobs consistently

5. Web and Digital Media

Optimization: Precise web image adjustments
Consistent Branding: Maintain visual consistency
Thumbnail Generation: Standardized thumbnail brightness
User Interface: Button/icon brightness adjustment

6. Document Processing

Scan Enhancement: Improve scanned document quality
Legibility: Enhance text readability
Archive Preparation: Standardize historical documents
OCR Preparation: Optimize for text recognition

7. Creative Effects

Film Looks: Replicate film characteristics
Mood Creation: Specific atmospheric effects
Style Development: Create signature looks
Artistic Expression: Non-realistic adjustments

Technical Details

Linear Transformation Formula

Basic formula:

output = (a × input) + b

With clamping:

output = clamp((a × input) + b, 0, 255)

Examples with Real Values

Example 1: Brighten (a=1, b=50)

Input: 100 → Output: (1 × 100) + 50 = 150
Input: 200 → Output: (1 × 200) + 50 = 250
Input: 220 → Output: (1 × 220) + 50 = 255 (clamped)

Example 2: Increase Contrast (a=1.5, b=0)

Input: 50  → Output: (1.5 × 50) + 0 = 75
Input: 128 → Output: (1.5 × 128) + 0 = 192
Input: 200 → Output: (1.5 × 200) + 0 = 255 (clamped)

Example 3: Combined (a=1.2, b=20)

Input: 50  → Output: (1.2 × 50) + 20 = 80
Input: 128 → Output: (1.2 × 128) + 20 = 174
Input: 200 → Output: (1.2 × 200) + 20 = 255 (clamped)

Visual Effects

Contrast adjustment (a parameter):

Original histogram:     After a=1.5:            After a=0.5:
[...▂▃▅▇▅▃▂...]      → [▁▂▅██▅▂▁]           → [▃▄▅▆▅▄▃]
                         (expanded)                (compressed)

Brightness adjustment (b parameter):

Original histogram:     After b=50:            After b=-50:
    [▂▃▅▇▅▃▂]       →    [▂▃▅▇▅▃▂]         → [▂▃▅▇▅▃▂]
                          (shifted right)        (shifted left)

Common Parameter Combinations

Effect	a	b	Result
Identity	1.0	0	No change
Brighten mild	1.0	30	+30 brightness
Darken mild	1.0	-30	-30 brightness
Contrast +30%	1.3	0	More contrast
Contrast -20%	0.8	0	Less contrast
Invert	-1.0	255	Negative
High-key	0.5	128	Bright, flat
Low-key	1.5	-40	Dark, contrasty
Film lift	0.9	20	Lifted shadows

Limitations and Clamping

Output values are clamped:

if (output < 0) output = 0;
if (output > 255) output = 255;

This means:

Extreme adjustments lose detail
Highlights can blow out (a too large, b too large)
Shadows can crush (a too large, b too negative)
Moderate values (a: 0.5-2.0, b: -50 to 50) are safest

Performance Characteristics

Speed: Very fast (simple arithmetic per pixel)
Memory: Efficient (in-place when possible)
Precision: Floating-point calculation, integer output
Color: Applies to all channels equally (preserves hue)
Alpha: Preserved unchanged

Common Patterns

Pattern 1: Brighten Image

# Simple brightness increase
mediaproc image linear photo.jpg -b 30

Pattern 2: Increase Contrast

# Simple contrast increase
mediaproc image linear photo.jpg -a 1.3

Pattern 3: Brighten and Boost Contrast

# Most common adjustment
mediaproc image linear photo.jpg -a 1.2 -b 20

Pattern 4: Darken with More Contrast

# Moody, dramatic look
mediaproc image linear photo.jpg -a 1.3 -b -20

Pattern 5: Invert (Negative)

# Create negative
mediaproc image linear photo.jpg -a -1 -b 255

Troubleshooting

Issue: Blown Out Highlights

Problem: Bright areas become pure white

Solutions:

# Reduce multiplier
mediaproc image linear photo.jpg -a 1.1 -b 20  # Instead of a=1.5

# Reduce offset
mediaproc image linear photo.jpg -a 1.2 -b 10  # Instead of b=30

# Use negative offset to protect highlights
mediaproc image linear photo.jpg -a 1.3 -b -10

# Preview to test
mediaproc image linear photo.jpg -a 1.5 -b 30 --dry-run

Issue: Crushed Shadows

Problem: Dark areas become pure black

Solutions:

# Reduce multiplier
mediaproc image linear photo.jpg -a 1.1 -b 20  # Instead of a=2.0

# Add positive offset
mediaproc image linear photo.jpg -a 1.2 -b 30  # Lifts shadows

# Use moderate values
mediaproc image linear photo.jpg -a 1.1 -b 15

Issue: Too Flat/Low Contrast

Problem: Image looks washed out

Solutions:

# Increase multiplier
mediaproc image linear photo.jpg -a 1.4 -b 0

# Reduce offset if too bright
mediaproc image linear photo.jpg -a 1.5 -b -10

# Dramatic contrast boost
mediaproc image linear photo.jpg -a 2.0 -b 0

Issue: Image Too Dark Overall

Problem: Entire image is too dark

Solutions:

# Increase brightness offset
mediaproc image linear photo.jpg -a 1 -b 50

# Or combine with slight contrast increase
mediaproc image linear photo.jpg -a 1.1 -b 40

# Large brightness boost
mediaproc image linear photo.jpg -a 1.2 -b 60

Issue: Colors Look Wrong

Problem: Colors appear off after adjustment

Solutions:

# Linear applies to all channels equally
# This is correct behavior - preserves color relationships

# If colors need separate adjustment, use:
# - modulate (HSL adjustments)
# - recomb (channel mixing)
# - curves (per-channel curves)

# Linear is best for luminance-only adjustments
mediaproc image linear photo.jpg -a 1.2 -b 15

Issue: Hard to Find Right Values

Problem: Trial and error to find good parameters

Solutions:

# Start with conservative values
# a: 1.0-1.3 (contrast)
# b: 0-30 (brightness)

# Preview quickly
mediaproc image linear photo.jpg -a 1.2 -b 20 --dry-run
mediaproc image linear photo.jpg -a 1.3 -b 15 --dry-run
mediaproc image linear photo.jpg -a 1.1 -b 25 --dry-run

# For brightness only, use b
# For contrast only, use a
# For both, combine moderately

Best Practices

Start with Moderate Values
- a: 0.8-1.5 (safe contrast range)
- b: -30 to 30 (safe brightness range)
- Test with --dry-run first
- Increase gradually
Use for Reproducibility
- Document exact values used
- Apply same values to similar images
- Create processing scripts with fixed values
- Maintain consistency across projects
Preserve Originals
- Keep original files
- Use descriptive output names
- Document adjustments
- Allow for re-processing
Combine Parameters Carefully
- High a + high b = blown highlights
- High a + negative b = crushed shadows
- Moderate both for balanced results
- Preview combinations
Quality Management
- Use high quality (90-100) for important images
- PNG for lossless adjustments
- JPEG for final output
- Consider re-compression impact
Scientific Work
- Document formula and parameters
- Apply same transformation to all samples
- Maintain metadata
- Use lossless formats (PNG, TIFF)
Creative Work
- Experiment with extreme values
- Create lookup tables for consistent looks
- Combine with other operations
- Test on representative images

Integration Examples

With Other Image Commands

# Grayscale then adjust contrast
mediaproc image grayscale photo.jpg -o temp.jpg
mediaproc image linear temp.jpg -a 1.4 -b 0 -o high-contrast-bw.jpg

# Adjust then sharpen
mediaproc image linear photo.jpg -a 1.2 -b 15 -o temp.jpg
mediaproc image sharpen temp.jpg -o final.jpg

# Resize then adjust
mediaproc image resize photo.jpg -w 1920 -o temp.jpg
mediaproc image linear temp.jpg -a 1.1 -b 10 -o final.jpg

# Auto-enhance then fine-tune
mediaproc image auto-enhance photo.jpg -o temp.jpg
mediaproc image linear temp.jpg -a 1.05 -b 5 -o final.jpg

In Scripts

#!/bin/bash
# Batch process with consistent linear adjustment

input_dir="originals"
output_dir="adjusted"

# Parameters
a_value=1.2
b_value=15

mkdir -p "$output_dir"

for img in "$input_dir"/*.jpg; do
  filename=$(basename "$img")
  echo "Processing: $filename"

  mediaproc image linear "$img" -a "$a_value" -b "$b_value" \
    -o "$output_dir/$filename"
done

echo "Processed $(ls "$input_dir"/*.jpg | wc -l) images"
echo "Used formula: ($a_value × input) + $b_value"

Creating Custom LUTs

#!/bin/bash
# Create consistent look for photo series

series_dir="photo-series"
output_dir="styled-series"

mkdir -p "$output_dir"

# Define style parameters
style_a=1.15
style_b=10

for img in "$series_dir"/*.jpg; do
  filename=$(basename "$img" .jpg)

  # Apply linear transformation
  mediaproc image linear "$img" -a "$style_a" -b "$style_b" \
    -q 95 -o "$output_dir/${filename}-styled.jpg"
done

echo "Style applied: a=$style_a, b=$style_b"