The structural similarity (SSIM) index is a method for predicting the perceived quality of digital pictures and is used for measuring the similarity between two images. For better readability the value is multipled by 1000 and substracted from 1000. The lower the value the more similar are both images.
More at https://en.wikipedia.org/wiki/Structural_similarity
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Learn more about JPEG compression in my article Finally understanding JPEG.
Defines the dimensional size of the resulting image.
Removes any borders or edges of an image which did does not change in color or transparency.
The Selective Blur is used to blur a background so it can be better compressed.
Contrary to other blur algorithms, the Selective Blur doesn't act on all pixels: blur is applied only if the difference between its value and the value of the surrounding pixels is less than a defined threshold value. So, contrasts are preserved because difference is high on contrast limits.
Keeps the meta data (EXIF, IPTC, XMP) of the original image including ICC color profiles.
Choose »Grayscale« if you want to discard all color information in the resulting JPEG. Results in smaller file size.
Btw. Photoshop creates RGB JPEGs from grayscale images.
More at https://en.wikipedia.org/wiki/Color_space
Allows to ensure the compatibility with Adobe Photoshop that cannot read the generated JPEGs. Results in a slighly larger file size.
Useful for photographers who need to open the JPEGs in Photoshop at a later time.
Defines the quality for the brightness channel. Low quality results in more blocking artifacts.
Defines the quality for the color channels. Low quality results in wrong colors. Most of the time you can choose a value lower than the value of the brightness quality.
Color subsampling is the practice of encoding images by implementing less resolution for color information than for brightness information, taking advantage of the human visual system‘s lower acuity for color differences than for brightness. (from Wikipedia)
More at https://en.wikipedia.org/wiki/Chroma_subsampling
Allows to have two different quality settings for different parts of the image.
Keeps transparency. This is only possible if you use this image in an HTML environment because you have to add javascript code. You only need Javascript code if your source image contains transparency so it is safe to have "Yes" as default value.
Defines the background color if the transparency of the JPEG is not kept. Set the color in a hexadecimal format as you would do in e.g. in CSS.
Very useful if you have a transparent product image and want to flatten it on a defined background color.
Trellis quantization is an adaptive quantization technique that select the set of levels in a transform block that minimises a rate-distortion metric.
More at http://en.wikipedia.org/wiki/Trellis_quantization
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