Image Router

Koa router that helps to serve responsive images in an SSR app

KoaResponsiveImageRouter.image() arguments

• target_ratio - ratio (width divided by height) of an image container.
• ratio_diff_threshold - describes how much ratio of an image can differ from target_ratio until ratio-crossed-threshold class is added to the image

Both are used to add useful classes to <img> element. See <img> Element classes

<img> Element classes

These help to avoid situations like stripes or uneven whitespace around the image and excessive clipping of images. It is done by setting object-fit values in CSS depending on these classes.

• horizontal / landscape - added when image width is greater than height
• vertical / portrait - if image height is greater
• square - when both dimensions are the same
• ratio-crossed-threshold - if image ratio differs by at least ratio_diff_threshold from target_ratio (both are KoaResponsiveImageRouter.image() optional arguments)
• ratio-above-threshold or ratio-below-threshold - added together with ratio-crossed-threshold depending if image ratio is above or below target_ratio

Image Cropping Options

The image() function accepts a crop parameter that allows you to specify cropping options for the displayed image. Use the crop parameter to control how the image is cropped or displayed within the specified dimensions, with consideration for the SmartCrop algorithm.

crop: An object representing cropping options. It can have the following properties:

• width: The target width for the cropped image. This parameter is used to specify the desired width of the cropped image when using the SmartCrop algorithm.
• height: The target height for the cropped image. This parameter is used to specify the desired height of the cropped image when using the SmartCrop algorithm.
• x (optional): The horizontal offset from the left edge of the image. This parameter is used for direct cropping to determine the starting point for cropping horizontally.
• y (optional): The vertical offset from the top edge of the image. This parameter is used for direct cropping to determine the starting point for cropping vertically..

Example Usage

Here are some examples of how to use the crop parameter in the image() function:

1. Cropping with SmartCrop Algorithm:

${await imageRouter.image({
    resolutions: [600, 1000, 1500, 2000],
    sizes_attr: "(max-width: 600) 100vw, 600px",
    path: paths.exampleSmartCropImg,
    lazy: false,
    img_style: "width: 600px; height: auto",
    crop: { width: 600, height: 600 },
})}

This example utilizes the SmartCrop algorithm to crop the image to a specific width and height.

2. Direct Cropping with Coordinates:

${await imageRouter.image({
    resolutions: [600, 1000, 1500, 2000],
    sizes_attr: "(max-width: 600px) 100vw, 600px",
    path: paths.exampleSmartCropImg,
    lazy: false,
    img_style: "width: 600px; height: auto",
    crop: { width: 2592, height: 3456, x: 2592, y: 0 },
})}

In this example, direct cropping is applied with specific x and y coordinates.

3. Default without cropping

${await imageRouter.image({
    resolutions: [600, 1000, 1500, 2000],
    sizes_attr: "(max-width: 600) 100vw, 600px",
    path: paths.exampleSmartCropImg,
    lazy: false,
    img_style: "width: 600px; height: auto",
})}

When no crop parameter is provided, the image is displayed without cropping.