Add support for image optimization

[petrsvihlik]

Kentico Kontent now supports image optimization. It gives developers the option of transforming images using URL parameters. The goal of this task is to implement a helper class that will make generation of image URLs easy for the developers.

The reference code can be observed in the .NET SDK.

The full specification follows:

Example

Scale
image.jpg?mode=scale&width=300
image.jpg?mode=scale&height=150
image.jpg?mode=fit&height=150&width=300

DPR
image.jpg?mode=scale&width=300&dpr=2.0

Specification

If a developer provides a parameter with invalid value or omits a parameter that is required for desired optimization, the Asset API will either ignore this parameter, or return the original asset without any optimizations.

Also, to keep our sanity, we will transform each image optimization parameter independently from others. Yes, this approach might sometimes produce surprising results. However, both Fastly and imgix are designed to handle a set of parameters that do not make sense and we are not making the situation worse.

Regarding requests to the Asset API, the idea is to go through all parameters, transform the supported ones and get rid of everything else.

To get a grasp of the image optimization You can experiment with two identical images:

• https://kentico.imgix.net/src/image.jpg
• https://www.fastly.io/image.jpg

rect=x,y,w,h (incompatible with crop)

-	-
x is not float	Nothing
x < 0.0	Nothing
y is not float	Nothing
y < 0.0	Nothing
w is not float	Nothing
w <= 0.0	Nothing
h is not float	Nothing
h <= 0.0	Nothing
h <= 0.0	Nothing
otherwise	crop={w},{h},x{x},y{y}

If the rect parameter is malformed, just ignore it.

fit=crop&crop=focalpoint&fp-x=x&fp-y=y&fp-z=z (incompatible with rect)

-	-
x is not float	set x as 0.5
y is not float	set y as 0.5
z is not float	Nothing
z <= 1.0	Nothing
1 / z < 1.0	Nothing
otherwise	crop={1 / z},{1 / z},offset-x{(x * z - 0.5) / (z - 1)},offset-y{(y * z - 0.5) / (z - 1) * 100}

Both offset-x and offset-y must be clamped into range from 0 to 100.

imgix supports crop by both rectangle and focal point. Unfortunately, it is not so easy to calculate Fastly parameters. Therefore, let's declare the rect parameter more important. So, if both crop by rectangle and focal point are specified, choose the first.

fm=x

-	-
x = gif	format=gif
x = png	format=png
x = png8	format=png8
x = jpg	format=jpg
x = pjpg	format=pjpg
x = webp	format=webp
otherwise	Nothing

fm=webp&lossless=x

-	-
x = 1	format=webpll
x = true	format=webpll
x = 0	format=webply
x = false	format=webply
otherwise	format=webp

q=x

-	-
x is not float	Nothing
otherwise	quality={x}\

auto=x

-	-
x = format	auto=webp
otherwise	Nothing

Local variables and related functions will be required to transform parameters.