Image rendering engine

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The official documentation is at: http://docs.alfresco.com



3.3


Overview


The Image rendering engine is designed to support renditions of source nodes whose content is an image MIME type. It was introduced in version 3.3 as one of the first rendering engines available to the core rendition service. This rendering engine has a name of 'imageRenderingEngine' and is represented by the class org.alfresco.repo.rendition.executer.ImageRenderingEngine.


3.3


Design


Like all rendering engines, the Image Rendering Engine generates one rendition node from a given source node. Based on the various parameters provided to the engine, it can perform image-related changes on an image source node producing a different image rendition node. Simple examples of such changes include cropping and rescaling. The parameters accepted by the Image Rendering Engine are as follows

Resize parameters


  • PARAM_RESIZE_WIDTH: (int or float) this optional parameter specifies the width of the image after resizing. This may be expressed as pixels or it may represent a percentage of the original image width, depending on the value of the PARAM_IS_PERCENT_RESIZE parameter. If no value is specified for this parameter then the width of the image will be unchanged. If an image is being cropped and resized then the cropping happens first, followed by resizing of the cropped image.
  • PARAM_RESIZE_HEIGHT: (int or float) this optional parameter specifies the height of the image after resizing. This may be expressed as pixels or it may represent a percentage of the original image height, depending on the value of the PARAM_IS_PERCENT_RESIZE parameter. If no value is specified for this parameter then the height of the image will be unchanged. If an image is being cropped and resized then the cropping happens first, followed by resizing of the cropped image.
  • PARAM_IS_PERCENT_RESIZE: (boolean) this optional flag parameter specifies how the PARAM_RESIZE_HEIGHT and PARAM_RESIZE_WIDTH parameters are interpreted. If this parameter is set to true then the rendition height and width are represented as a percentage of the original image height and width. If this parameter is set to false then the rendition height and width are represented as pixels. This parameter defaults to false.
  • PARAM_MAINTAIN_ASPECT_RATIO: (boolean) this optional flag parameter determines whether the rendered image maintains its original aspect ratio or is stretched to fit the specified height and width. If this parameter is true then the rendered image will always maintain its aspect ratio and will be resized to best fit within the given width and height. For example if an image starts at 100x200 pixels and it is resized to 50x50 pixels then the rendered image will actually be 25x50 pixels. If this parameter is false then the image will be stretched or compressed to fit the given height and width, regardless of the original aspect ratio. This parameter defaults to true
  • PARAM_RESIZE_TO_THUMBNAIL: (boolean) this optional flag parameter specifies a mode for dramatically shrinking large images in a performant way. If set to true the rendering process will be more performant for large images but the rendered image will be of lower quality. If set to false the rendering process will take longer but the resulting image will usually be of better quality.

Crop parameters


  • PARAM_CROP_WIDTH: (int or float) this optional parameter specifies the width of the image after cropping. This may be expressed as pixels or it may represent a percentage of the original image width, depending on the value of the PARAM_IS_PERCENT_CROP parameter. If no value is specified for this parameter then the width of the image will be unchanged. If an image is being cropped and resized then the cropping happens first, followed by resizing of the cropped image.
  • PARAM_CROP_HEIGHT: (int or float) this optional parameter specifies the height of the image after cropping. This may be expressed as pixels or it may represent a percentage of the original image height, depending on the value of the PARAM_IS_PERCENT_CROP parameter. If no value is specified for this parameter then the height of the image will be unchanged. If an image is being cropped and resized then the cropping happens first, followed by resizing of the cropped image.
  • PARAM_CROP_X_OFFSET: (int) this optional parameter specifies the horizontal position of the start point of the area to be cropped. By default this parameter sets the distance, in pixels, from the left-hand edge of the image to the start position of the crop area. By default a positive value will shift the start-position to the right, while a negative value will shift the start position to the left. Setting the PARAM_CROP_GRAVITY parameter may change this, however. If this parameter is not set it is assumed to be 0.
  • PARAM_CROP_Y_OFFSET: (int) this optional parameter specifies the vertical position of the start point of the area to be cropped. By default this parameter sets the distance, in pixels, from the top edge of the image to the start position of the crop area. By default a positive value will shift the start-position downwards, while a negative value will shift the start position upwards. Setting the PARAM_CROP_GRAVITY parameter may change this, however. If this parameter is not set it is assumed to be 0.
  • PARAM_CROP_GRAVITY: (String) this optional parameter determines the 'zero' position from which offsets are measured and also determines the direction of offsets. The allowed values of gravity are the four cardinal points (North, East, etc.), the four ordinal points (NorhtWest, SouthEast, etc) and Center. By default NorthWest gravity is used. If an ordinal gravity is set then the point from which offsets originate will be the appropriate corner. For example, NorthWest gravity would originate at teh top-left corner while SouthWest origin would originate at the bottom-left corner. Cardinal gravity sets the origin at the center of the appropriate edge. Center origin sets the origin at the center of the image. Gravity also affects the direction of offsets and how the offset position relates to the cropped image. For example, NorthWest gravity sets positive horizontal offset direction to right, positive vertical direction to down and sets the cropped image origin to the top-left corner. Northerly gavities set the positive vertical direction to down. Southerly gavities set teh positive vertical direction to up. Easterly gavities set teh positive horizontal positive direction to left. Westerly gavities set teh positive horizontal positive direction to right. Some gravity values do not specify a horizontal or a vertical direction explicitly. For example North does not specify a horizontal direction, while Center does not specify either horizontal or vertical direction. In thse cases the positive horizontal offset direction is always right and the positive vertical offset direction is always down. The gravity also affects how the cropped image relates to the offset position. For example, NorthWest gravity causes the top-left corner of the cropped area to be the offset position, while NorthEast gravity would set the top-right corner of the cropped are to the offset position. When a direction is not explicitly specified then the center of the cropped area is placed at the offset position. For example, with North gravity the horizontal position is unspecified so the cropped area would be horizontally centered on the offset position, but the top edge of the cropped area would be at the offset position. For Center gravity the cropped area will be centered over the offset position both horizontally and vertically.
  • PARAM_IS_PERCENT_CROP: (boolean) this optional flag parameter specifies how the PARAM_CROP_HEIGHT and PARAM_CROP_WIDTH parameters are interpreted. If this parameter is set to true then the cropped image height and width are represented as a percentage of the original image height and width. If this parameter is set to false then the rendition height and width are represented as pixels. This parameter defaults to false.




  • PARAM_COMMAND_OPTIONS: (String) this optional parameter specifies any additional ImageMagick commands that the user wishes to add. These commands are appended after the various crop and resize options.

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