cPicture

cPicture is updated to version 4.10

• added new JPEG10 Lossless Exposure & Contrast Adjustment
  
  
  Exposure compensation from -2EV to +2EV:

  
  Contrast from -1CV to +1CV:

  

  See this blog entry
  
  

• Updated format plugin version to 4.10
• Maintenance update and minor changes

cPicture is a portable application consisting of a single .exe file and optional plugin files.

Download the MSI file to copy the app to a folder or use the 'Check for updates' button in the 'Other' category.

Download German 64bit Installer MSI
Download English 64bit Installer MSI
More languages are available from the cPicture site.
Alternative download from github

See the Download Instructions for more details.

For questions, please mail to: cpicture

Lossless Tonal Adjustments in JPEG's DCT Domain: Exposure Compensation and Multi-Band Contrast

Most JPEG workflows treat exposure (brightness) and contrast as inherently "lossy": decode pixels, apply curves, then re-encode. That approach works, but it always introduces an additional step of quantization error.

In this github fork of the IJG JPEG-10 code, I added two options to jpegtran that operate directly on quantized DCT coefficients:

• -exposure-comp EV
• -contrast DC LOW MID HIGH

Both are applied during transcoding, so they combine naturally with existing jpegtran operations such as rotation, flipping, cropping, marker copying, and progressive conversion.

https://github.com/jurgen178/jpeg10
Download Windows x64 binary: jpegtran.zip

Quick Usage

jpegtran [standard options] [-exposure-comp EV] [-contrast DC LOW MID HIGH] input.jpg output.jpg

Examples:

# Brighten by 1 stop
jpegtran -copy all -exposure-comp 1 input.jpg output.jpg

# Darken by 0.5 stops
jpegtran -copy all -exposure-comp -0.5 input.jpg output.jpg

# Contrast (uniform: DC=LOW=MID=HIGH)
jpegtran -copy all -contrast -1   -1   -1   -1   input.jpg out-contrast-u-1.jpg
jpegtran -copy all -contrast -0.5 -0.5 -0.5 -0.5 input.jpg out-contrast-u-0.5.jpg
jpegtran -copy all -contrast  0.5  0.5  0.5  0.5 input.jpg out-contrast-u+0.5.jpg
jpegtran -copy all -contrast  1    1    1    1   input.jpg out-contrast-u+1.jpg

# Contrast (band-specific examples)
jpegtran -copy all -contrast 0 0 0.6 0   input.jpg out-contrast-mid+0.6.jpg
jpegtran -copy all -contrast 0 0 0 0.4   input.jpg out-contrast-high+0.4.jpg
jpegtran -copy all -contrast 0 0.4 0 0   input.jpg out-contrast-low+0.4.jpg

# Combine: rotate 90°, brighten 0.5 EV, and add uniform contrast +0.5
jpegtran -copy all -rot 90 -exposure-comp 0.5 -contrast 0.5 0.5 0.5 0.5 input.jpg output.jpg

Both switches accept fractional values. Practical ranges:

Integrated into cPicture with live preview:

Background: DCT Coefficient Basics

A JPEG image is encoded as a grid of DCT blocks (with 8×8 Elements in size). Each block has one DC coefficient and 63 AC coefficients. But each MCU might have more than one block depending on the color subsampling.

• DC[0] represents the (level-shifted) average sample value of the block. The relationship to pixel mean is:

  $$\mu = \frac{DC_\text{unquant}}{N} + \text{center}$$

  where $N$ is the DCT block size of 8 and $\text{center} = 2^{\text{precision}-1}$ (e.g. 128 for 8‑bit).

• AC[1..N²−1] represent spatial frequency components (texture, edges, contrast).

Both DC and AC are stored quantized: the actual stored integer is $\text{round}(\text{value} / Q_k)$, where $Q_k$ is the quantization step for coefficient $k$.

-exposure-comp EV — Exposure Compensation

Exposure compensation from -2EV to +2EV:

Concept

A photographic EV step corresponds to doubling (or halving) the amount of light. Applied in linear light:

$$\text{gain} = 2^{EV}$$

Because JPEG samples are gamma-coded (sRGB), pixel values cannot be multiplied directly. Instead:

1. Estimate a representative level from the DC blocks.
2. Compute the equivalent additive pixel-domain offset by applying the gain in linear light at that reference level.
3. Translate the offset into a quantized DC delta.
4. Add the delta to every DC coefficient.

Only DC is modified. AC coefficients are not modified, so local contrast and texture are preserved.

Reference Level — Log-Average

A geometric mean (log-average) of all block mean levels is used as the exposure reference:

$$\bar{L} = \exp\!\left(\frac{1}{B}\sum_{i=1}^{B} \ln(L_i + 1)\right) - 1$$

where $L_i$ is the intensity mean of block $i$ (clamped to $[0, \text{MAX}]$) and $B$ is the total number of blocks.

sRGB Linearisation

The gain is applied in linear light:

$$u_\text{ref} = \frac{\bar{L}}{\text{MAX}}$$

$$u_\text{ref,lin} = f_\text{lin}(u_\text{ref})$$

$$u_\text{new,lin} = \min(u_\text{ref,lin} \cdot \text{gain},\; 1.0)$$

$$u_\text{new} = f_\text{sRGB}(u_\text{new,lin})$$

The sRGB transfer functions used:

$$f_\text{lin}(u) = \begin{cases} u / 12.92 & u \le 0.04045 \\ \left(\dfrac{u + 0.055}{1.055}\right)^{2.4} & u > 0.04045 \end{cases}$$

$$f_\text{sRGB}(u) = \begin{cases} 12.92\,u & u \le 0.0031308 \\ 1.055\,u^{1/2.4} - 0.055 & u > 0.0031308 \end{cases}$$

Pixel-Domain Offset → Quantized DC Delta

$$\Delta_\text{samples} = (u_\text{new} - u_\text{ref}) \cdot \text{MAX}$$

Clamped to available headroom/shadow room to limit clipping, then converted to a quantized DC delta:

$$\Delta_{DC} = \text{round}\!\left(\frac{\Delta_\text{samples} \cdot N}{Q_0}\right)$$

where $N$ is the DCT block size and $Q_0$ is the DC quantization step.

Component Policy

For CMYK and YCCK the delta is computed in an inverted intensity domain ($I = \text{MAX} - \text{sample}$) so that +EV brightens and −EV darkens.

-contrast DC LOW MID HIGH — Contrast Adjustment

Contrast from -1CV to +1CV:

Concept

This option provides four separate controls (all in stops):

• DC controls the DC coefficient (block mean)
• LOW, MID, HIGH control the AC coefficients in frequency order

All controls are interpreted as log2 gains (stops). For a value $x$, the gain is:

$$g(x) = 2^{x}$$

So +1 doubles, -1 halves.

DC

DC is scaled by:

$$g_\mathrm{DC} = 2^{DC}$$

and applied as:

$$DC' = \mathrm{clamp}(\mathrm{round}(g_\mathrm{DC} \cdot DC))$$

AC (low/mid/high weighting)

AC coefficients are processed in zigzag order (the JPEG natural order). Let $z$ be the AC position with $z = 1 \ldots A$, where $A$ is the number of AC coefficients.

Define a normalized position:

$$t = \begin{cases} \dfrac{z-1}{A-1} & A > 1 \\ 0 & A = 1 \end{cases}$$

Triangular weights:

• low weight fades out from low frequencies

$$w_\mathrm{low} = \max(0, 1 - 2t)$$

• mid weight peaks in the middle

$$w_\mathrm{mid} = 1 - |2t - 1|$$

• high weight fades in toward high frequencies

$$w_\mathrm{high} = \max(0, 2t - 1)$$

Per-coefficient exponent and gain:

$$v(z) = LOW\cdot w_\mathrm{low} + MID\cdot w_\mathrm{mid} + HIGH\cdot w_\mathrm{high}$$

$$g(z) = 2^{v(z)}$$

Applied to each AC coefficient:

$$AC'[z] = \mathrm{clamp}(\mathrm{round}(g(z)\cdot AC[z]))$$

If DC = LOW = MID = HIGH = X, then all coefficients are scaled by the same gain $2^X$ (uniform contrast adjustment).

Component Policy

Same as -exposure-comp:

• YCbCr/BG_YCC/YCCK: luma only
• RGB subtract-green: base/green only
• otherwise: all components

Ordering and Composition

Both -exposure-comp and -contrast are applied as a post step after any geometric transform (-rot, -flip, -crop, …). The tonal operations work on the final output coefficient arrays, so the order of switches on the command line does not matter.

Implementation notes

• Core implementation:
  • transupp.c: do_exposure_comp() and do_contrast()
  • transupp.h: adds new fields to jpeg_transform_info
• CLI parsing:
  • jpegtran.c
• Feature flags and parameters are stored in jpeg_transform_info in transupp.h

Summary

• -exposure-comp EV shifts brightness by changing only DC coefficients, with EV evaluated in linear light (sRGB transfer) at a log-average reference.
• -contrast DC LOW MID HIGH scales DC and AC coefficients, with AC gains varying smoothly over frequency order using low/mid/high controls.
• Both run in the DCT domain and integrate naturally into the lossless-transformation workflow of jpegtran.

Example pictures

cPicture is updated to version 4.9

• updated to JPEG10 including the new lossless negate
  

• added video cut feature (Alt-X) to cut

  • video frame
  • video length
  • audio
    
    See this blog entry for examples of the new video crop feature.
    
    

• updated plugin interface to support the new JPEG10 and video cut features

• added convert functions for mp3 and mp4

  

  
  

• Maintenance update and minor changes

cPicture is a portable application consisting of a single .exe file and optional plugin files.

Download the MSI file to copy the app to a folder or use the 'Check for updates' button in the 'Other' category.

Download German 64bit Installer MSI
Download English 64bit Installer MSI
More languages are available from the cPicture site.
Alternative download from github

See the Download Instructions for more details.

For questions, please mail to: cpicture

cPicture is updated to version 4.8

• Added distance option to index print feature
• Customization to the clipboard copy (Alt-C) options and menu selection when run to simplify data copy
• Maintenance update and minor changes
  

cPicture is a portable application consisting of a single .exe file and optional plugin files.

Download the MSI file to copy the app to a folder or use the 'Check for updates' button in the 'Other' category.

Download German 64bit Installer MSI
Download English 64bit Installer MSI
More languages are available from the cPicture site.
Alternative download from github

See the Download Instructions for more details.

For questions, please mail to: cpicture

cPicture is updated to version 4.7

• Added index print feature to add EXIF data or any text to the film strip
  any picture data can be printed at the top and bottom film stripe
  
  
  
  
  Use either the index print function, or use the macro processing.
  Example macro definition to print the exposure data at the top, and the file name at the bottom:

  exposure data print
        index_print 6 "" "" "" "" "%[sh] %[fn] %[is]" "%fe" 0 0 50 50 1

  
  
  

  Example macro definition:

  flower data print
        index_print 6 "" "" "" "" "Flower %d" "%wx%h , file size=%fs" 1 2 50 50 1

• Maintenance update and minor changes
  
  
  

cPicture is a portable application consisting of a single .exe file and optional plugin files.

Download the MSI file to copy the app to a folder or use the 'Check for updates' button in the 'Other' category.

Download German 64bit Installer MSI
Download English 64bit Installer MSI
More languages are available from the cPicture site.
Alternative download from github

See the Download Instructions for more details.

For questions, please mail to: cpicture