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.

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

Progressive

Image information will be stored from less details to more details. So a big image will be instantly visible but in low quality (default).

Baseline

Pixel information will be stored sequentially in the JPEG file. So a big image will get visible step by step in full resolution.

JPEG < 10 kB

Baseline (75% chance of smaller image)

JPEG > 10 kB

Progressive (94% chance of smaller image)

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)

2x2 (4:2:0)
» smaller file size

Breaks the image into 2x2 pixel blocks and only stores the average color information for each 2x2 pixel group (default).

1x1 (4:4:4)
» bigger file size

The color information of every pixel will be stored.

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.

The best quantization table depends on the contents of your input file.

JPEG Annex

The default tables from the JPEG specification (default).

Flat

Better details, but worse for areas with slight gradients.

Custom, tuned for MS-SSIM

Generates better results for Multi-Scale SSIM. Slightly better gradients but hides details.

ImageMagick table by N. Robidoux

JPEG quantization table created by Dr. Nicolas Robidoux for use with 2x2 Chroma subsampling and quality level around 75 (based on Table from paper by Klein, Silverstein and Carney).

Custom, tuned for PSNR-HVS

Generates better results for Multi-Scale SSIM. Slightly better gradients but hides details.

Table from paper by Klein, Silverstein and Carney

Slightly better gradients.

Trellis quantization is an adaptive quantization technique that select the set of levels in a transform block that minimises a rate-distortion metric.

Yes

Softer image, smaller file size (default).

No

Sharper image, bigger file size.

More at http://en.wikipedia.org/wiki/Trellis_quantization