Manipulations

A manipulation is a list of transformations that are chained together using the character /. There is no limit to the number of transformations you can chain, save for the limit in size of a URL as enforced by your browser. No matter how complex the manipulation, TwicPics will optimize it for speed and accuracy on the fly.

Transformations all have the same structure:

<name>=<parameters>

where:

  • <name> is the name of the transformation
  • <parameters> is an expression specifying the parameters for the transformation

For instance:

  • resize=400 will resize the image to 400 pixels in width while conserving the source image aspect ratio
  • resize=640x480 will resize the image to exactly 640 pixels in width per 480 pixels in height, potentially altering its aspect ratio

Types of parameters

The TwicPics API strives to be as consistent as possible and, as such, transformations will use the same format for parameters that represent the same underlying concept.

number

TwicPics numbers can be JSON-encoded number literals or expressions that, when computed, result in an actual number. Expressions are embedded in parenthesis. Operators +, -, * and / are supported for additions, subtractions, multiplications and divisions respectively. Classic algebraic precedence is respected and parenthesis can be used to circumvent it.

For instance, 50, 7.2, (1/3), (5*(7+2)/3) are all valid numbers.

length

TwicPics lengths are equivalent to CSS length values. They consist of a number eventually followed by a unit specifier:

  • when no unit is specified, the length is in pixels, for instance 50 is a pixel length that can be read as "50 pixels"
  • with the unit specifier "s", the length is a scale, for instance (1/3)s is a scale length that can be read as "one third"
  • with the unit specifier "p", the length is a percentage, for instance 4.5p is a percentage length representing "4.5%"

coordinates

Coordinates represent a point in an image, specified as a couple of positive lengths separated by the character x:

  • the first length is the coordinate along the x-axis (following the width of the image)
  • the second length is the coordinate along the y-axis (following the height of the image)

TwicPics uses the same coordinate system as CSS: zero-based, left-to-right and top-to-bottom.

If we take the example of an image that is 640 pixel-wide and 480 pixel-high:

  • 0x0 points to the top-left corner pixel
  • 639x479 points to the bottom-right corner pixel.

It is perfectly fine to mix lengths of different units in the same coordinates. For instance, in the context of yet another 640 per 480 sample image, coordinates 100x50p actually translate to 100x240.

size

A size represents a 2D area, specified as a couple of strictly positive lengths separated by the character x:

  • the first length is the width of the area
  • the second length is the height of the area

For instance 640x480 is 640 pixel-wide per 480 pixel-high.

It is possible to omit one of the dimensions using the character -. In that case, TwicPics will automatically compute the missing dimension so that the size respects the aspect ratio of the source image. For instance, if the source image is 640 pixel-wide per 480 pixel-high, then sizes 320x- and -x240 are both equivalent to 320x240.

As a shortcut, it is possible to omit the height by specifying just a width. For instance, the size 320 is equivalent to 320x-.

As for coordinates, it is totally fine to mix lengths of different units in the same size. For instance, 10px150 is a perfectly valid size.

crop size

A crop size is a size where omitted dimensions are assumed to be the same as the input image.

For instance:

  • 320 and 320x- are equivalent to 320x1s
  • -x240 is equivalent to 1sx240

ratio

A ratio represents the proportional relationship between a width and a height. It is specified as a couple of strictly positive numbers separated by the character ::

  • the first number is the number of length units contained in the width
  • the second number is the number of length units contained in the height

For instance, the ratio 9:3 indicates that the width is 9-units long while the height is 3-units long. In terms of proportions, the width is (9/3=)3 times longer than the height.

Focus point

Transformations behave differently depending on which point in the image is the main focus. TwicPics will do its best to keep this focus point as central as possible within the transformed image.

By default, the focus point is in the middle of the image but you can change its coordinates by using the focus transformation.

Focus window

The biggest extractable area of the image that has the focus point as its center is called the focus window. By default, since the focus point is right in the middle of the image, the focus window is actually the image itself.

Here is, delimited in red, the focus window of a 300 pixel-wide per 188 pixel-high cat face with the focus point set at 85x85:

Cat focus window

Chaining transformations

When adding a transformation to the chain, the parameters given are interpreted as if previous transformations had already been performed (ie. as if the source image was the result of the previous transformations).

For instance:

  • resize=340/resize=50p will result in an image that is 170 pixel-wide
  • resize=50p/focus=20x10 will put the focus point at coordinates 40x20 of the source image

Since TwicPics will optimize the manipulation, be aware that a transformation may shadow what came before it. For instance resize=50p/resize=340 will result in an image that is 340 pixel-wide: TwicPics will simply ignore the first resize.