Photo Correction Steps

Tutorial on Photo Correction

> Nowdays digital cameras have become very popular: every year we take thousands of shots, most of which need correction and improving.

> We all experienced that digital cameras and scanners often produce noisy pictures with wrong exposure and colors and weak sharpness

> Another reason to improve photos digitally is human mistake: for example if we miss using a flash, significant details may be lost in the shadows.

> With Focus Photoeditor you can correct all these problems very precisely and without needing to be a professional or expert.

> In our software you can use both automatic and manual corrections

> Automatic corrections:

> Focus Photoeditor's automatic algorithms don't need input from the user, don't lower the quality of the picture and produce very good results. Use them when you are unsure how to start the correction or to experiment before manually adjust the picture.

> Manual corrections:

> Sometimes automatic correction are just the first step. To have a really perfect photo, you may also need to act yourself and use manual corrections to solve some problems more precisely.

> Remember always that manual corrections are affected by the screen, because they rely on what your eyes see, so be sure that your monitor is well calibrated. Nowdays monitors are getting better and there is not really need for a true calibration, just make sure that the black, gray, and white points have the correct intensity and that there is no color cast present. An other monitor property is the Gamma (a measure of the brightness response of the monitor). All monitors for pc have a gamma value of about 2.2, but Macintosh monitors have a gamma of 1.6 (they are much brighter): so a picture that looks ok on a pc can result too much bright on a mac. Gamma is not really a problem for printers because they have a gamma of 1.8 (more near to pc screen) and most of them have their internal algorithm to perform gamma adjustments automatically.

> Steps for digital photo correction:

> Correcting a photo should follow a logical sequence, in order to preserve as much as possible the original pixels information. For example you should know that HLS and HVS, if needed, are to be used only as last step , because working in these color spaces could loose a few information in the picture.

> Below we provide an ideal sequence of steps; of course depending from the kind of picture it's possible to change the order or skip some step. In case your picture has more than one problem try always to correct the most evident first.

> #### 1st Step Recovering Details:

> Sometimes some details in a photo are hidden or not clearly visible: they can be lost in the darkness or in the light. If you want to recover these details and bring them to life, you should use an intelligent correction which acts only in the dark or bright areas.

> - Smart Flash and Reduce Highlights:

> Focus Photoeditor uses Smart Flash to automatically detect details lost in the darkness and bring them to light.

> When to use it? When some parts of the picture are too dark but the remaining part has a fair exposure. This special correction acts like a lamp that brings light only where it's needed.

> Let's look to the following example:

> To the left is the original picture, to the right how it looks after applying the smart flash. You may notice that only those parts hidden in the shadow were brought to light, while the central part of the picture retained all the details.

> Focus Photoeditor uses Reduce Hilights to automatically detect and make visible details lost in the lights.

> To the picture on the left was applied Reduce Hilights to recover the buildings lost in the strong light.

> #### 2nd Step Contrast

> When a picture is foggy and colors are like washed out you need to increase the contrast:

> The best way to improve the contrast is to do it automatically, choosing from the main menu Auto > AutoContrast.

> You can as well choose manual corrections: Exposure > Histogram Stretch. Both these functions adjust contrast using a linear formula.

> Exposure > Exposure / Contrast and Exposure > Levels / Contrast perform a non linear correction: this can be used as further step. It is also very useful when you want to decrease the contrast without loosing any image information.

> If you want to learn more about Contrast or you want to be more precise take also a look to the Histogram Stretch Operation.

> #### 3rd Step Color Stretch

> This step is needed when one or more color channels (R,G or B) do not cover the whole histogram range. The histogram range goes from 0 (number of pixels that have 0 intensity in that color channel) to 255 (number of pixels that have maximum intensity). A stretched histogram usually improves both the contrast and the colors in a picture.

> You can perform a full histogram stretch automatically choosing from main menu Auto > AutoStretch or manually using the Histogram Stretch Operation.

> Let's take a look at this example:

> On the left you can see how the lacking of stretch of red channel in the shadows and of blue channel in the hilights creates a green-foggy picture. On the right the problem was solved applying an AutoStretch command.

> In 70% of cases pictures will benefit from a full Histogram stretch (Color Stretch), but there are cases when this operation shouldn't be applied, especially in indoor pictures in presence of artificial light. In these cases better to use the Auto > AutoContrast that will stretch only the luminance channel producing results closer to the real scene.

> 4th Step Color Cast elimination

> Sometimes a photo can appear too yellowish, or too bluish or to have any other color cast. Some digital cameras infact are more sensitive to some colors and this can result in a moderate or strong color cast visible in your pictures.

> The best way to remove it is to detect and correct it, using a reference color.

> From Main menu Color > Calibrate colors, you can access a special tool that lets you select a reference color from the picture and change it into a color that is supposed to be known. It's recommended to select those points that should be almost white (white point) or neuter (gray point).

> Let's Look at the example below:

> Selecting the color on the snow as reference color we can turn it into gray and all the picture will be adjusted.

> The Autocolors correction can in many cases give good results as well: to use it select from the main menu Auto > AutoColors.

> 5th Step: Reduce Noise

> Digital cameras as well other digital instruments have to deal with noise. Noise in a picture is a local perturbation of the expected colors. For example if you have a picture with a blue sea and there are some dots that tend to green or to red too much, you are probably facing a noise. A small amount of noise is always present in a digital picture: small errors are impossible to avoid in any instrument.

> Normally you can encounter a few kind of noises:

> - Noise from camera sensor: this noise appears when the camera is operating in conditions of light that are beyond the capacity of its sensor. For example if you take a picture when there is a low illumination, darkest pixels can be the best candidates for containing this noise. The noise can be observed as soon as you increase the visibility of these pixels. While it's not possible to guess how the noisy pixel should have been originally we can at least replace it with an average of all the surrounding pixels, in order to get a better aesthetical result.

> The Reduce Chroma Noise gets rid of color noise from the sensor.

> The Reduce Luminance Noise gets rid of gray noise from the sensor.

> - Artifacts from Jpeg Compression: As you probably know jpeg is a lossy format and high compression rates shouldn't be used when saving to this format. A heavily compressed Jpeg can present small and big artefacts.

> Use Remove RGB Noise and other Noise Reduction algorithm to get rid of this problem.

> Remove RGB Noise is also very useful to clean skin in people portraits.

> 6th Step: Correct Sharpness

> The easiest way to improve Sharpness is to use from the main menu Auto > Autosharpen. Sharpen operations enhance edges in the picture, thus making them look more sharp.

> Often sharpen filters can emphasize unwanted noise: to avoid this you can use a better sharpening algorithm called Unsharp Mask. You can find it in the main menu > Filters > Sharpen > Unsharp Mask.

> Another Command that enhances edges making them more dark or bright is the Main menu > Auto > Clarify command. This increases contrast just near the edges.