GIMPでARGB4444に画像を変換するスクリプト。http://registry.gimp.org/taxonomy/term/1275 を改造。

1BitGrayscale.gpl

GIMP Palette
Name: 1-Bit Grayscale
Columns: 16
#
0	0	0
255	255	255

3BitGrayscale.gpl

GIMP Palette
Name: 3-Bit Grayscale
Columns: 8
#
0	0	0
36	36	36
72	72	72
109	109	109
145	145	145
182	182	182
218	218	218
255	255	255

4BitGrayscale.gpl

GIMP Palette
Name: 4-Bit Grayscale
Columns: 16
#
0	0	0
17	17	17
34	34	34
51	51	51
68	68	68
85	85	85
102	102	102
119	119	119
136	136	136
153	153	153
170	170	170
187	187	187
204	204	204
221	221	221
238	238	238
255	255	255

5BitGrayscale.gpl

GIMP Palette
Name: 5-Bit Grayscale
Columns: 16
#
0	0	0
8	8	8
16	16	16
25	25	25
33	33	33
41	41	41
49	49	49
58	58	58
66	66	66
74	74	74
82	82	82
90	90	90
99	99	99
107	107	107
115	115	115
123	123	123
132	132	132
140	140	140
148	148	148
156	156	156
165	165	165
173	173	173
181	181	181
189	189	189
197	197	197
206	206	206
214	214	214
222	222	222
230	230	230
239	239	239
247	247	247
255	255	255

6BitGrayscale.gpl

GIMP Palette
Name: 6-Bit Grayscale
Columns: 16
#
0	0	0
4	4	4
8	8	8
12	12	12
16	16	16
20	20	20
24	24	24
28	28	28
32	32	32
36	36	36
40	40	40
45	45	45
49	49	49
53	53	53
57	57	57
61	61	61
65	65	65
69	69	69
73	73	73
77	77	77
81	81	81
85	85	85
89	89	89
93	93	93
97	97	97
101	101	101
105	105	105
109	109	109
113	113	113
117	117	117
121	121	121
125	125	125
130	130	130
134	134	134
138	138	138
142	142	142
146	146	146
150	150	150
154	154	154
158	158	158
162	162	162
166	166	166
170	170	170
174	174	174
178	178	178
182	182	182
186	186	186
190	190	190
194	194	194
198	198	198
202	202	202
206	206	206
210	210	210
215	215	215
219	219	219
223	223	223
227	227	227
231	231	231
235	235	235
239	239	239
243	243	243
247	247	247
251	251	251
255	255	255

dither16bit.scm

; GIMP - The GNU Image Manipulation Program
; Copyright (C) 1995 Spencer Kimball and Peter Mattis
;
; 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 2 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, write to the Free Software
; Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
;
; Copyright (C) 2011 Xaffron Software skwong@consultant.com
;
; This script
; 1)adds alpha channel to active layer,
; 2)splits active layer into RGBA,
; 3)converts each layer to 1-, 4-, 5- or 6- bit grayscale w/ FS dither, then
; 4)recomposes it.
; The end effect is that images are "Dithered" to either:
; 12-bit color + 4-bit transparency (ARGB4444/4096colors),
; 16-bit color + no transparency (RGB565/4096colors), or
; 15-bit color + 1-bit transparency (ARGB1555/4096colors).
;
; The higher-depth dither is especially useful for mobile developers who need to pre-dither
; assets for 12- or 16-bit screens.
;
; Version 0.1 Initial Version - No frills, barely works.
; Version 0.2 Renamed to dither16bit.scm - Added RGB565 and ARGB1555.
;
; Version 0.2.1 fork dither16bit.scm - Added ARGB4444 (no dither), ARGB3333 (no dither).
;
; INSTALL:
;  Copy the dither16bit.scm file to your scripts dir.
;  Extract the separate 1Bit, 4Bit, 5Bit and 6BitGrayscale.gpl files from the zipfile
;  and copy to your palettes dir.
;  Copy the 3BitGrayscale.gpl file to your palettes dir.
;  To run, start up GIMP, go to
;Image->Mode->Dither to ARGB4444
;Image->Mode->Dither to RGB565
;Image->Mode->Dither to ARGB1555
;Image->Mode->to ARGB xxxx
;
; Please email me for any issues... I'm pretty swamped, but I'll try to address!
;

(define (script-fu-12bit-not-dither aimg adraw ckind dither dithera)
  (let* ((img (car (gimp-drawable-get-image adraw)))
         (draw (car (gimp-image-get-active-layer img)))
         (owidth (car (gimp-image-width img)))
         (oheight (car (gimp-image-height img)))
         (paltbl
          (cond
           ((= ckind 0) '("4-Bit Grayscale" "4-Bit Grayscale" "4-Bit Grayscale" "4-Bit Grayscale"))
           ((= ckind 1) '("5-Bit Grayscale" "5-Bit Grayscale" "5-Bit Grayscale" "1-Bit Grayscale"))
           ((= ckind 2) '("5-Bit Grayscale" "6-Bit Grayscale" "5-Bit Grayscale" ""))
           ((= ckind 3) '("3-Bit Grayscale" "3-Bit Grayscale" "3-Bit Grayscale" "3-Bit Grayscale"))
           ((= ckind 4) '("3-Bit Grayscale" "3-Bit Grayscale" "3-Bit Grayscale" "1-Bit Grayscale"))
           ))
         (palr (car paltbl))
         (palg (cadr paltbl))
         (palb (caddr paltbl))
         (pala (cadddr paltbl)))

    (gimp-context-push)
    (gimp-image-undo-group-start img)
    (if (equal? pala "") #f (gimp-layer-add-alpha draw))

    (let* ((layers (if (equal? pala "")
                     (plug-in-decompose 1 img draw "RGB" 0)
                     (plug-in-decompose 1 img draw "RGBA" 0)))
           (red (car layers))
           (green (cadr layers))
           (blue (caddr layers))
           (alpha (if (equal? pala "") #f (cadddr layers))))

      (gimp-image-convert-rgb red)
      (gimp-image-convert-indexed red dither 4 4096 FALSE FALSE palr)
      (gimp-image-convert-grayscale red)
      (gimp-image-convert-rgb green)
      (gimp-image-convert-indexed green dither 4 4096 FALSE FALSE palg)
      (gimp-image-convert-grayscale green)
      (gimp-image-convert-rgb blue)
      (gimp-image-convert-indexed blue dither 4 4096 FALSE FALSE palb)
      (gimp-image-convert-grayscale blue)
      (if (equal? pala "")
        (gimp-display-new (car (plug-in-compose 1 red 0 green blue 0 "RGB")))
        (begin
          (gimp-image-convert-rgb alpha)
          (gimp-image-convert-indexed alpha dithera 4 4096 FALSE FALSE pala)
          (gimp-image-convert-grayscale alpha)
          (gimp-display-new (car (plug-in-compose 1 red 0 green blue alpha "RGBA")))))
      (gimp-image-delete red)
      (gimp-image-delete green)
      (gimp-image-delete blue)
      (if (equal? pala "") #f (gimp-image-delete alpha)))

    (gimp-selection-none img)
    (gimp-image-undo-group-end img)
    (gimp-displays-flush)
    (gimp-context-pop)
    ))

(define (script-fu-12bit-dither aimg adraw)
  (script-fu-12bit-not-dither aimg adraw 0 1 1))

(define (script-fu-argb1555-dither aimg adraw)
  (script-fu-12bit-not-dither aimg adraw 1 1 1))

(define (script-fu-rgb565-dither aimg adraw)
  (script-fu-12bit-not-dither aimg adraw 2 1 1))

(script-fu-register
 "script-fu-12bit-dither"
 _"Dither to ARGB_4444..."
 _"Dither to ARGB4444 (4096) colors.  Useful for mobile applications."
 "Sunny Kwong <skwong@consultant.com>"
 "Sunny Kwong"
 "4/27/2011"
 "*"
 SF-IMAGE       "Input image" 0
 SF-DRAWABLE    "Input drawable" 0
 )

(script-fu-register
 "script-fu-rgb565-dither"
 _"Dither to RGB_565..."
 _"Dither to RGB565 (4096) colors.  Useful for mobile applications."
 "Sunny Kwong <skwong@consultant.com>"
 "Sunny Kwong"
 "10/23/2012"
 "*"
 SF-IMAGE       "Input image" 0
 SF-DRAWABLE    "Input drawable" 0
 )

(script-fu-register
 "script-fu-argb1555-dither"
 _"Dither to ARGB_1555..."
 _"Dither to ARGB1555 (4096) colors.  Useful for mobile applications."
 "Sunny Kwong <skwong@consultant.com>"
 "Sunny Kwong"
 "10/23/2012"
 "*"
 SF-IMAGE       "Input image" 0
 SF-DRAWABLE    "Input drawable" 0
 )

(script-fu-register
 "script-fu-12bit-not-dither"
 _"to ARGB_xxxx..."
 _"to ARGB4444 (4096) or ARGB3333 (512) colors."
 "Sunny Kwong <skwong@consultant.com>, mieki256"
 "Sunny Kwong, mieki256"
 "03/03/2015"
 "*"
 SF-IMAGE       "Input image" 0
 SF-DRAWABLE    "Input drawable" 0
 SF-OPTION      "Color"  '("ARGB4444" "ARGB1555" "RGB565" "ARGB3333" "ARGB1333")
 SF-OPTION      "RGB Dither"  '("none" "fs-dither" "fslowbleed-dither" "fixed-dither")
 SF-OPTION      "Alpha Dither"  '("none" "fs-dither" "fslowbleed-dither" "fixed-dither")
 )

(script-fu-menu-register "script-fu-12bit-dither" "<Image>/Image/Mode")
(script-fu-menu-register "script-fu-rgb565-dither" "<Image>/Image/Mode")
(script-fu-menu-register "script-fu-argb1555-dither" "<Image>/Image/Mode")
(script-fu-menu-register "script-fu-12bit-not-dither" "<Image>/Image/Mode")