View transform for images

The image returned to the client in response to a req=img request is derived from the composite image by considering the following values: wid=, hei=, fit=, scl=, rgn=, attribute::DefaultPix, attribute::MaxPix, and the size of the composite image.

If wid= and hei= are specified, and scl= is not, composite image is scaled so that it fits fully within the view rect defined by wid= and hei=. If the aspect ratio of the view rect is different than that of the composite image, then the scaled composite image is aligned within the view rect using the align= value, if specified, or it is centered otherwise. Any space not covered by image data is filled with bgc= or, if not specified, with attribute::BkgColor.

If scl= is specified, the composite image is scaled by that scale factor. If wid= and/or hei= is specified as well, the scaled image is then cropped to wid= and/or hei= or extra space is added, as needed. align= specifies where the image is cropped or extra space is added, and any extra space is filled with bgc= or attribute::BkgColor.

If neither wid=, hei= nor scl= are specified, and if either width or height of the composite image exceeds attribute::DefaultPix, then the composite image is scaled to not exceed attribute::DefaultPix. Otherwise, the composite image is used without scaling.

To guarantee that the view image is returned without any further scaling, specify scl=1.

If rgn= is specified, the reply image is then cropped accordingly to arrive at the final reply image size. This size is compared with attribute::MaxPix (if defined), and an error is generated if the reply image is larger in either dimension.

If fmt= specifies data without alpha, any transparent areas in the reply image are filled with bgc= or attribute::BkgColor.

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