This GIMP script makes a cube with the current layer as a texture for each side

create-cube-from-layer.scm

;;     This script converts a single layer into an isometric cube.
;;     Copyright (C) 2015  Jeromy Reimer
;; 
;;     This program is free software: you can redistribute it and/or modify
;;     it under the terms of the GNU General Public License as published by
;;     the Free Software Foundation, either version 3 of the License, or
;;     (at your option) any later version.
;; 
;;     This program is distributed in the hope that it will be useful,
;;     but WITHOUT ANY WARRANTY; without even the implied warranty of
;;     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;     GNU General Public License for more details.
;; 
;;     You should have received a copy of the GNU General Public License
;;     along with this program.  If not, see <http://www.gnu.org/licenses/>.

(define (create-isometric-box-from-layer image)
	(let* (
			(activelayer 0)
			(cube-top 0)
			(cube-bottom 0)
			(cube-left 0)
			(cube-right 0)
			(cube-b-left 0)
			(cube-b-right 0)
			(background 0)
			)

		; convert image to rgb mode if it isn't already
		(if (not (equal? (car (gimp-image-base-type image)) 0))
			(gimp-image-convert-rgb image)
			)

		(set! activelayer (car (gimp-image-get-active-layer image)))
		(gimp-image-resize image 150 150 0 0)

		; creating a few duplicates of the orignal layer
		(set! cube-top (car (gimp-layer-copy activelayer 1)))
		(set! cube-left (car (gimp-layer-copy activelayer 1)))
		(gimp-image-remove-layer image activelayer)

		; paste a left side layer
		(gimp-image-insert-layer image cube-left -1 0)
		(gimp-layer-set-name cube-left "Cube Left")

		; edit the left side layer
		(set! activelayer (car (gimp-image-get-layer-by-name image "Cube Left")))
		(gimp-layer-scale-full activelayer 134 162 1 0)
		(gimp-brightness-contrast activelayer -30 0)

		; copy the left side for the right side
		(set! cube-right (car (gimp-layer-copy activelayer 1)))
		(gimp-image-insert-layer image cube-right -1 0)
		(gimp-layer-set-name cube-right "Cube Right")

		; some more edits for the left side
		(gimp-drawable-transform-shear-default activelayer 1 67 0 0)
		(gimp-layer-scale-full activelayer 67 115 1 0)

		; some more edits for the right side
		(set! activelayer (car (gimp-image-get-layer-by-name image "Cube Right")))
		(gimp-drawable-transform-shear-default activelayer 1 -67 0 0)
		(gimp-brightness-contrast activelayer -97 0)
		(gimp-layer-scale-full activelayer 67 115 1 0)
		(gimp-layer-resize activelayer 67 115 1 0)

		; paste and edit the top layer
		(gimp-image-insert-layer image cube-top -1 0)
		(gimp-layer-set-name cube-top "Cube Top")
		(set! activelayer (car (gimp-image-get-layer-by-name image "Cube Top")))
		(gimp-layer-scale-full activelayer 144 144 1 0)
		(gimp-drawable-transform-rotate activelayer 2.35619449 1 0 0 0 0 0 1 0)
		(gimp-layer-scale-full activelayer 134 67 1 0)

	;move the cube top layer to the correct position
	(let* (
			(layerOffsets (gimp-drawable-offsets (car (gimp-image-get-layer-by-name image "Cube Top"))))
			(layerOffsetsX (car layerOffsets))
			(layerOffsetsY (cadr layerOffsets))
			(activelayer 0)
			)
		(set! activelayer (car (gimp-image-get-layer-by-name image "Cube Top")))
		(gimp-layer-translate activelayer (- 8 layerOffsetsX) (- 1 layerOffsetsY))
	)

	;move the cube right layer to the correct position
	(let* (
			(layerOffsets (gimp-drawable-offsets (car (gimp-image-get-layer-by-name image "Cube Right"))))
			(layerOffsetsX (car layerOffsets))
			(layerOffsetsY (cadr layerOffsets))
			(activelayer 0)
			)
		(set! activelayer (car (gimp-image-get-layer-by-name image "Cube Right")))
		(gimp-layer-translate activelayer (- 74 layerOffsetsX) (- 33 layerOffsetsY))
	)

	;move the cube left layer to the correct position
	(let* (
			(layerOffsets (gimp-drawable-offsets (car (gimp-image-get-layer-by-name image "Cube Left"))))
			(layerOffsetsX (car layerOffsets))
			(layerOffsetsY (cadr layerOffsets))
			(activelayer 0)
			)
		(set! activelayer (car (gimp-image-get-layer-by-name image "Cube Left")))
		(gimp-layer-translate activelayer (- 9 layerOffsetsX) (- 33 layerOffsetsY))
	)

	;finishing touches. merging layers.
	(set! background (car (gimp-layer-new image 150 150 1 "background" 0 0)))
	(gimp-image-insert-layer image background 0 0)
	(set! activelayer (car (gimp-image-get-active-layer image)))
	(gimp-image-merge-down image (car (gimp-image-get-layer-by-name image "background")) 0)
	(gimp-image-merge-down image (car (gimp-image-get-layer-by-name image "Cube Top")) 0)
	(gimp-image-merge-down image (car (gimp-image-get-layer-by-name image "Cube Right")) 0)
	(set! activelayer (car (gimp-image-get-active-layer image)))
	(gimp-layer-set-name activelayer "Cube")
;; 	(gimp-convert-indexed image 0 0 200 0 0 "ignored")
	(gimp-displays-flush)
	)
)

(script-fu-register "create-isometric-box-from-layer"
		    "<Image>/Layer/Cre_at Isometric Cube"
		    "Create an isometric cube from current layer"
		    "Jeromy Reimer"
		    "Copyright (C) 2015 Jeromy Reimer"
		    "September 13, 2015 v1.0"
		    "*"
		    SF-IMAGE "Image" 0
		    )