Method determineOutputFormat

processing/processing.go:284–328

determineOutputFormat determines the output image format based on the processing options and image properties. It modifies the ProcessingOptions in place to set the output format.

(
	img *vips.Image,
	imgdata imagedata.ImageData,
	po ProcessingOptions,
	animated bool,
)

//
// It modifies the ProcessingOptions in place to set the output format.
func (p *Processor) determineOutputFormat(
	img *vips.Image,
	imgdata imagedata.ImageData,
	po ProcessingOptions,
	animated bool,
) (imagetype.Type, error) {
	// Check if the image may have transparency
	expectTransparency := !po.ShouldFlatten() &&
		(img.HasAlpha() || po.PaddingEnabled() || po.ExtendEnabled())

	format := po.Format()

	switch {
	case format == imagetype.SVG:
		// At this point we can't allow requested format to be SVG as we can't save SVGs
		return imagetype.Unknown, newSaveFormatError(format)
	case format == imagetype.Unknown:
		switch {
		case po.PreferJxl() && !animated:
			format = imagetype.JXL
		case po.PreferAvif() && !animated:
			format = imagetype.AVIF
		case po.PreferWebP():
			format = imagetype.WEBP
		case p.isImageTypePreferred(imgdata.Format()):
			format = imgdata.Format()
		default:
			format = p.findPreferredFormat(animated, expectTransparency)
		}
	case po.EnforceJxl() && !animated:
		format = imagetype.JXL
	case po.EnforceAvif() && !animated:
		format = imagetype.AVIF
	case po.EnforceWebP():
		format = imagetype.WEBP
	}

	po.SetFormat(format)

	if !vips.SupportsSave(format) {
		return format, newSaveFormatError(format)
	}

	return format, nil
}

// isImageTypePreferred checks if the given image type is in the list of preferred formats.
func (p *Processor) isImageTypePreferred(imgtype imagetype.Type) bool {