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Mac上でjpeg/png/gifの変換をする方法

ImageMagickを使うとコマンドで簡単にフォーマット変換ができます。

インストール

brew install imagemagick

使い方

以下のコマンドで変換は完了です。
画像の種類は拡張子を見てうまく判断してくれるようです。

convert test.jpeg test.png

対応フォーマット

コマンドで確認したらかなりの種類がサポートされていました。

$ identify -list format

   Format  Module    Mode  Description
-------------------------------------------------------------------------------
      3FR  DNG       r--   Hasselblad CFV/H3D39II
      AAI* AAI       rw+   AAI Dune image
       AI  PDF       rw-   Adobe Illustrator CS2
      ART* ART       rw-   PFS: 1st Publisher Clip Art
      ARW  DNG       r--   Sony Alpha Raw Image Format
      AVI  MPEG      r--   Microsoft Audio/Visual Interleaved
      AVS* AVS       rw+   AVS X image
      BGR* BGR       rw+   Raw blue, green, and red samples
     BGRA* BGR       rw+   Raw blue, green, red, and alpha samples
     BGRO* BGR       rw+   Raw blue, green, red, and opacity samples
      BMP* BMP       rw-   Microsoft Windows bitmap image
     BMP2* BMP       -w-   Microsoft Windows bitmap image (V2)
     BMP3* BMP       -w-   Microsoft Windows bitmap image (V3)
      BRF* BRAILLE   -w-   BRF ASCII Braille format
      CAL* CALS      rw-   Continuous Acquisition and Life-cycle Support Type 1
           Specified in MIL-R-28002 and MIL-PRF-28002
     CALS* CALS      rw-   Continuous Acquisition and Life-cycle Support Type 1
           Specified in MIL-R-28002 and MIL-PRF-28002
   CANVAS* XC        r--   Constant image uniform color
  CAPTION* CAPTION   r--   Caption
      CIN* CIN       rw-   Cineon Image File
      CIP* CIP       -w-   Cisco IP phone image format
     CLIP* CLIP      rw+   Image Clip Mask
     CMYK* CMYK      rw+   Raw cyan, magenta, yellow, and black samples
    CMYKA* CMYK      rw+   Raw cyan, magenta, yellow, black, and alpha samples
      CR2  DNG       r--   Canon Digital Camera Raw Image Format
      CRW  DNG       r--   Canon Digital Camera Raw Image Format
      CUR* ICON      rw-   Microsoft icon
      CUT* CUT       r--   DR Halo
     DATA* INLINE    rw+   Base64-encoded inline images
      DCM* DCM       r--   Digital Imaging and Communications in Medicine image
           DICOM is used by the medical community for images like X-rays.  The
           specification, "Digital Imaging and Communications in Medicine
           (DICOM)", is available at http://medical.nema.org/.  In particular,
           see part 5 which describes the image encoding (RLE, JPEG, JPEG-LS),
           and supplement 61 which adds JPEG-2000 encoding.
      DCR  DNG       r--   Kodak Digital Camera Raw Image File
      DCX* PCX       rw+   ZSoft IBM PC multi-page Paintbrush
      DDS* DDS       rw+   Microsoft DirectDraw Surface
    DFONT* TTF       r--   Multi-face font package (Freetype 2.6.0)
      DNG  DNG       r--   Digital Negative
      DOT  DOT       ---   Graphviz
      DPX* DPX       rw-   SMPTE 268M-2003 (DPX 2.0)
           Digital Moving Picture Exchange Bitmap, Version 2.0.
           See SMPTE 268M-2003 specification at http://www.smtpe.org

     DXT1* DDS       rw+   Microsoft DirectDraw Surface
     DXT5* DDS       rw+   Microsoft DirectDraw Surface
     EPDF  PDF       rw-   Encapsulated Portable Document Format
      EPI  PS        rw-   Encapsulated PostScript Interchange format
      EPS  PS        rw-   Encapsulated PostScript
     EPS2* PS2       -w-   Level II Encapsulated PostScript
     EPS3* PS3       -w+   Level III Encapsulated PostScript
     EPSF  PS        rw-   Encapsulated PostScript
     EPSI  PS        rw-   Encapsulated PostScript Interchange format
      EPT  EPT       rw-   Encapsulated PostScript with TIFF preview
     EPT2  EPT       rw-   Encapsulated PostScript Level II with TIFF preview
     EPT3  EPT       rw+   Encapsulated PostScript Level III with TIFF preview
      ERF  DNG       r--   Epson Raw Format
      FAX* FAX       rw+   Group 3 FAX
           FAX machines use non-square pixels which are 1.5 times wider than
           they are tall but computer displays use square pixels, therefore
           FAX images may appear to be narrow unless they are explicitly
           resized using a geometry of "150x100%".

     FITS* FITS      rw-   Flexible Image Transport System
  FRACTAL* PLASMA    r--   Plasma fractal image
      FTS* FITS      rw-   Flexible Image Transport System
       G3* FAX       rw-   Group 3 FAX
      GIF* GIF       rw+   CompuServe graphics interchange format
    GIF87* GIF       rw-   CompuServe graphics interchange format (version 87a)
 GRADIENT* GRADIENT  r--   Gradual linear passing from one shade to another
     GRAY* GRAY      rw+   Raw gray samples
   GROUP4* TIFF      rw-   Raw CCITT Group4
       GV  DOT       ---   Graphviz
        H* MAGICK    -w-   Image expressed as a 'C/C++' char array
     HALD* HALD      r--   Identity Hald color lookup table image
      HDR* HDR       rw+   Radiance RGBE image format
HISTOGRAM* HISTOGRAM -w-   Histogram of the image
      HRZ* HRZ       rw-   Slow Scan TeleVision
      HTM* HTML      -w-   Hypertext Markup Language and a client-side image map
     HTML* HTML      -w-   Hypertext Markup Language and a client-side image map
      ICB* TGA       rw-   Truevision Targa image
      ICO* ICON      rw+   Microsoft icon
     ICON* ICON      rw-   Microsoft icon
      IIQ  DNG       r--   Phase One Raw Image Format
     INFO  INFO      -w+   The image format and characteristics
   INLINE* INLINE    rw+   Base64-encoded inline images
      IPL* IPL       rw+   IPL Image Sequence
   ISOBRL* BRAILLE   -w-   ISO/TR 11548-1 format
      JNG* PNG       rw-   JPEG Network Graphics
           See http://www.libpng.org/pub/mng/ for details about the JNG
           format.
      JNX* JNX       r--   Garmin tile format
      JPE* JPEG      rw-   Joint Photographic Experts Group JFIF format (80)
     JPEG* JPEG      rw-   Joint Photographic Experts Group JFIF format (80)
      JPG* JPEG      rw-   Joint Photographic Experts Group JFIF format (80)
      JPS* JPEG      rw-   Joint Photographic Experts Group JFIF format (80)
     JSON  JSON      -w+   The image format and characteristics
      K25  DNG       r--   Kodak Digital Camera Raw Image Format
      KDC  DNG       r--   Kodak Digital Camera Raw Image Format
    LABEL* LABEL     r--   Image label
      M2V  MPEG      rw+   MPEG Video Stream
      M4V  MPEG      rw+   Raw MPEG-4 Video
      MAC* MAC       r--   MAC Paint
   MAGICK* MAGICK    rw-   Predefined Magick Image (LOGO, ROSE, etc.); output same as 'H'
      MAP* MAP       rw-   Colormap intensities and indices
     MASK* MASK      rw+   Image Clip Mask
      MAT  MAT       rw+   MATLAB level 5 image format
    MATTE* MATTE     -w+   MATTE format
      MEF  DNG       r--   Mamiya Raw Image File
     MIFF* MIFF      rw+   Magick Image File Format
      MKV  MPEG      rw+   Multimedia Container
      MNG* PNG       rw+   Multiple-image Network Graphics (libpng 1.6.18)
           See http://www.libpng.org/pub/mng/ for details about the MNG
           format.
     MONO* MONO      rw-   Raw bi-level bitmap
      MOV  MPEG      rw+   MPEG Video Stream
      MP4  MPEG      rw+   MPEG-4 Video Stream
      MPC* MPC       rw+   Magick Persistent Cache image format
     MPEG  MPEG      rw+   MPEG Video Stream
      MPG  MPEG      rw+   MPEG Video Stream
      MRW  DNG       r--   Sony (Minolta) Raw Image File
      MSL* MSL       rw+   Magick Scripting Language
     MSVG  SVG       rw+   ImageMagick's own SVG internal renderer
      MTV* MTV       rw+   MTV Raytracing image format
      MVG* MVG       rw-   Magick Vector Graphics
      NEF  DNG       r--   Nikon Digital SLR Camera Raw Image File
      NRW  DNG       r--   Nikon Digital SLR Camera Raw Image File
     NULL* NULL      rw-   Constant image of uniform color
      ORF  DNG       r--   Olympus Digital Camera Raw Image File
      OTB* OTB       rw-   On-the-air bitmap
      OTF* TTF       r--   Open Type font (Freetype 2.6.0)
      PAL* UYVY      rw-   16bit/pixel interleaved YUV
     PALM* PALM      rw+   Palm pixmap
      PAM* PNM       rw+   Common 2-dimensional bitmap format
    PANGO* PANGO     ---   Pango Markup Language
  PATTERN* PATTERN   r--   Predefined pattern
      PBM* PNM       rw+   Portable bitmap format (black and white)
      PCD* PCD       rw-   Photo CD
     PCDS* PCD       rw-   Photo CD
      PCL  PCL       rw+   Printer Control Language
      PCT* PICT      rw-   Apple Macintosh QuickDraw/PICT
      PCX* PCX       rw-   ZSoft IBM PC Paintbrush
      PDB* PDB       rw+   Palm Database ImageViewer Format
      PDF  PDF       rw+   Portable Document Format
     PDFA  PDF       rw+   Portable Document Archive Format
      PEF  DNG       r--   Pentax Electronic File
      PES* PES       r--   Embrid Embroidery Format
      PFA* TTF       r--   Postscript Type 1 font (ASCII) (Freetype 2.6.0)
      PFB* TTF       r--   Postscript Type 1 font (binary) (Freetype 2.6.0)
      PFM* PFM       rw+   Portable float format
      PGM* PNM       rw+   Portable graymap format (gray scale)
    PICON* XPM       rw-   Personal Icon
     PICT* PICT      rw-   Apple Macintosh QuickDraw/PICT
      PIX* PIX       r--   Alias/Wavefront RLE image format
    PJPEG* JPEG      rw-   Joint Photographic Experts Group JFIF format (80)
   PLASMA* PLASMA    r--   Plasma fractal image
      PNG* PNG       rw-   Portable Network Graphics (libpng 1.6.18)
           See http://www.libpng.org/ for details about the PNG format.
    PNG00* PNG       rw-   PNG inheriting bit-depth, color-type from original if possible
    PNG24* PNG       rw-   opaque or binary transparent 24-bit RGB (zlib 1.2.5)
    PNG32* PNG       rw-   opaque or transparent 32-bit RGBA
    PNG48* PNG       rw-   opaque or binary transparent 48-bit RGB
    PNG64* PNG       rw-   opaque or transparent 64-bit RGBA
     PNG8* PNG       rw-   8-bit indexed with optional binary transparency
      PNM* PNM       rw+   Portable anymap
      PPM* PNM       rw+   Portable pixmap format (color)
  PREVIEW* PREVIEW   -w-   Show a preview an image enhancement, effect, or f/x
       PS  PS        rw+   PostScript
      PS2* PS2       -w+   Level II PostScript
      PS3* PS3       -w+   Level III PostScript
      PSB* PSD       rw+   Adobe Large Document Format
      PSD* PSD       rw+   Adobe Photoshop bitmap
     PTIF* TIFF      rw+   Pyramid encoded TIFF
      PWP* PWP       r--   Seattle Film Works
RADIAL-GRADIENT* GRADIENT  r--   Gradual radial passing from one shade to another
      RAF  DNG       r--   Fuji CCD-RAW Graphic File
      RAS* SUN       rw+   SUN Rasterfile
      RAW  DNG       r--   Raw
      RGB* RGB       rw+   Raw red, green, and blue samples
     RGBA* RGB       rw+   Raw red, green, blue, and alpha samples
     RGBO* RGB       rw+   Raw red, green, blue, and opacity samples
      RGF* RGF       rw-   LEGO Mindstorms EV3 Robot Graphic Format (black and white)
      RLA* RLA       r--   Alias/Wavefront image
      RLE* RLE       r--   Utah Run length encoded image
      RMF  DNG       r--   Raw Media Format
      RW2  DNG       r--   Panasonic Lumix Raw Image
      SCR* SCR       r--   ZX-Spectrum SCREEN$
      SCT* SCT       r--   Scitex HandShake
      SFW* SFW       r--   Seattle Film Works
      SGI* SGI       rw+   Irix RGB image
    SHTML* HTML      -w-   Hypertext Markup Language and a client-side image map
      SIX* SIX       rw-   DEC SIXEL Graphics Format
    SIXEL* SIXEL     rw-   DEC SIXEL Graphics Format
SPARSE-COLOR* TXT       -w+   Sparse Color
      SR2  DNG       r--   Sony Raw Format 2
      SRF  DNG       r--   Sony Raw Format
  STEGANO* STEGANO   r--   Steganographic image
      SUN* SUN       rw+   SUN Rasterfile
      SVG  SVG       rw+   Scalable Vector Graphics (XML 2.9.2)
     SVGZ  SVG       rw+   Compressed Scalable Vector Graphics (XML 2.9.2)
     TEXT* TXT       rw+   Text
      TGA* TGA       rw-   Truevision Targa image
THUMBNAIL* THUMBNAIL -w+   EXIF Profile Thumbnail
     TIFF* TIFF      rw+   Tagged Image File Format (LIBTIFF, Version 4.0.6)
   TIFF64* TIFF      rw-   Tagged Image File Format (64-bit) (LIBTIFF, Version 4.0.6)
     TILE* TILE      r--   Tile image with a texture
      TIM* TIM       r--   PSX TIM
      TTC* TTF       r--   TrueType font collection (Freetype 2.6.0)
      TTF* TTF       r--   TrueType font (Freetype 2.6.0)
      TXT* TXT       rw+   Text
     UBRL* BRAILLE   -w-   Unicode Text format
      UIL* UIL       -w-   X-Motif UIL table
     UYVY* UYVY      rw-   16bit/pixel interleaved YUV
      VDA* TGA       rw-   Truevision Targa image
    VICAR* VICAR     rw-   VICAR rasterfile format
      VID* VID       rw+   Visual Image Directory
     VIFF* VIFF      rw+   Khoros Visualization image
     VIPS* VIPS      rw+   VIPS image
      VST* TGA       rw-   Truevision Targa image
     WBMP* WBMP      rw-   Wireless Bitmap (level 0) image
      WMV  MPEG      rw+   Windows Media Video
      WPG* WPG       r--   Word Perfect Graphics
      X3F  DNG       r--   Sigma Camera RAW Picture File
      XBM* XBM       rw-   X Windows system bitmap (black and white)
       XC* XC        r--   Constant image uniform color
      XCF* XCF       r--   GIMP image
      XPM* XPM       rw-   X Windows system pixmap (color)
      XPS  XPS       r--   Microsoft XML Paper Specification
       XV* VIFF      rw+   Khoros Visualization image
    YCbCr* YCbCr     rw+   Raw Y, Cb, and Cr samples
   YCbCrA* YCbCr     rw+   Raw Y, Cb, Cr, and alpha samples
      YUV* YUV       rw-   CCIR 601 4:1:1 or 4:2:2

* native blob support
r read support
w write support
+ support for multiple images

その他できること

ImageMagickを使うと、フォーマットの変換だけでなく画像サイズの変換やフィルターをかける事もできるようです。

http://www.envinfo.uee.kyoto-u.ac.jp/user/susaki/image/magick_process.html