Legacy ImageMagick Discussions Archive

RMagick creates a new image by caling AcquireImage, then calls SetImageBackgroundColor to set the background color. In 6.4.8 and earlier, if the background color is "none" then the alpha channel is activated. I notice that in the 6.4.9-0 beta the alpha channel is not activated and I have to activate it explicitly. Is this the expected behavior going forward? Is there a better way for me to do this?

RMagick converts "none" to a PixelPacket using QueryColorDatabase. In both 6.4.8 and 6.4.9-0 the returned pixel has the values {red=0, green=0, blue=0, opacity=65535}. Then it stores the pixel packet in the ImageInfo background_color field before calling AcquireImage and subsequently SetImageBackgroundColor.

I hope my written description is sufficient. I'm having a hard time reproducing the problem using convert. Please let me know if you need more info and I'll work up a reproduction in C. Thanks for your help.

Here's the identify output when I don't explicitly activate the alpha channel:

Code: Select all

Image: rmnone-6.4.9.miff
  Format: MIFF (Magick Image File Format)
  Class: DirectClass
  Geometry: 200x200+0+0
  Resolution: 72x72
  Print size: 2.77778x2.77778
  Units: Undefined
  Type: Bilevel
  Base type: Bilevel
  Endianess: Undefined
  Colorspace: RGB
  Depth: 16/1-bit
  Channel depth:
    gray: 1-bit
  Channel statistics:
    gray:
      min: 0 (0)
      max: 0 (0)
      mean: 0 (0)
      standard deviation: 0 (0)
      kurtosis: 0
      skewness: 0
  Histogram:
     40000: (    0,    0,    0) #000000000000 black
  Rendering intent: Undefined
  Interlace: None
  Background color: white
  Border color: rgb(223,223,223)
  Matte color: grey74
  Transparent color: black
  Page geometry: 200x200+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: None
  Orientation: Undefined
  Properties:
    create-date: 2009-01-31T21:35:15+00:00
    modify-date: 2009-01-31T21:33:54+00:00
    signature: dd046ccaee01de364ee5306b48d1dde4b8d06ba484eb66c2123f92f1211c30ad
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 235kb
  Number pixels: 39.1kb
  Version: ImageMagick 6.4.8-9 2009-01-24 Q16 http://www.imagemagick.org

Same program, except I activated the alpha channel before writing the image:

Code: Select all

Image: rmalpha.miff
  Format: MIFF (Magick Image File Format)
  Class: DirectClass
  Geometry: 200x200+0+0
  Resolution: 72x72
  Print size: 2.77778x2.77778
  Units: Undefined
  Type: Bilevel
  Base type: Bilevel
  Endianess: Undefined
  Colorspace: RGB
  Depth: 16/1-bit
  Channel depth:
    gray: 1-bit
    alpha: 1-bit
  Channel statistics:
    gray:
      min: 0 (0)
      max: 0 (0)
      mean: 0 (0)
      standard deviation: 0 (0)
      kurtosis: 0
      skewness: 0
    alpha:
      min: 0 (0)
      max: 0 (0)
      mean: 0 (0)
      standard deviation: 0 (0)
      kurtosis: 0
      skewness: 0
  Alpha: none   #0000000000000000
  Histogram:
     40000: (    0,    0,    0,    0) #0000000000000000 none
  Rendering intent: Undefined
  Interlace: None
  Background color: white
  Border color: rgba(223,223,223,1)
  Matte color: grey74
  Transparent color: none
  Page geometry: 200x200+0+0
  Dispose: Undefined
  Iterations: 0
  Compression: None
  Orientation: Undefined
  Properties:
    create-date: 2009-01-31T21:42:15+00:00
    modify-date: 2009-01-31T21:42:15+00:00
    signature: ac2878215ad33205d1732578c89f25ededaad1b676911c540825bcf4baa42d9b
  Artifacts:
    verbose: true
  Tainted: False
  Filesize: 313kb
  Number pixels: 39.1kb
  Version: ImageMagick 6.4.8-9 2009-01-24 Q16 http://www.imagemagick.org

We'll need to see a program to illustrate the problem. Our program shows the matte channel is activated for a background color of "none":

Code: Select all

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <magick/MagickCore.h>

int main(int argc,char **argv)
{
  ExceptionInfo *exception;
  Image *image;
  ImageInfo *info;

  MagickCoreGenesis(argv[0],MagickFalse);
  puts(GetMagickVersion(NULL));
  exception = AcquireExceptionInfo();
  info = AcquireImageInfo();
  image= AcquireImage(info);
  (void) QueryColorDatabase("none",&image->background_color,exception);
  (void) SetImageBackgroundColor(image);
  (void) printf("matte is %d\n",image->matte);
  MagickCoreTerminus();
}

Your reproduction looks like what RMagick is doing. I'll get you something asap. Thanks!

My description was incomplete. Here's a reproduction. At 6.4.9 the following program prints:

Code: Select all

$ ./none
ImageMagick 6.4.9-0 2009-01-31 Q16 http://www.imagemagick.org
name is #000000000000
matte is 0

At 6.4.8 it prints

Code: Select all

$ ./none
ImageMagick 6.4.8-9 2009-01-24 Q16 http://www.imagemagick.org
name is #000000000000
matte is 1

I suspect some coding error on my part. I notice that my code to convert a PixelPacket to a MagickPixelPacket is based on SetMagickPixelPacket in mogrify-private.h in 6.4.8.

Thanks for your advice.

Code: Select all

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <magick/MagickCore.h>

int main(int argc,char **argv)
{
  ExceptionInfo *exception;
  Image *image;
  ImageInfo *info;
  PixelPacket pp;
  MagickPixelPacket mpp;
  char name[MaxTextExtent];


  MagickCoreGenesis(argv[0],MagickFalse);
  puts(GetMagickVersion(NULL));
  exception = AcquireExceptionInfo();
  info = AcquireImageInfo();


  (void) QueryColorDatabase("none",&pp,exception);

  GetMagickPixelPacket(NULL, &mpp);
  mpp.red     = (MagickRealType) pp.red;
  mpp.green   = (MagickRealType) pp.green;
  mpp.blue    = (MagickRealType) pp.blue;
  mpp.opacity = (MagickRealType) (mpp.matte ? pp.opacity : OpaqueOpacity);
  mpp.index   = (MagickRealType) 0;
  (void) GetColorTuple(&mpp, MagickTrue, name);

  SetImageOption(info, "background", name);
  info->background_color = pp;
  printf("name is %s\n", name);

  image= AcquireImage(info);
  (void) SetImageBackgroundColor(image);
  (void) printf("matte is %d\n",image->matte);
  MagickCoreTerminus();
}

The problem is related to your call to SetImageOption(). 6.4.9-0 options override the ImageInfo structure members. Initially, AcquireImage() sets the background color to info->background_color but its modified by the "background" option. Since your background option sets an opaque color (i.e. #000000000000) the image matte channel is off. If you remove the SetImageOption() call, it behaves as expected. Alternatively, set the "background" option to a transparent color such as rgba(0,0,0,0). Another option is to set the mpp.matte to true:

mpp.matte = MagickTrue;

Thanks! I'll fix it.

Another good reason to try the betas!

Legacy ImageMagick Discussions Archive

Change in SetImageBackgroundColor in 6.4.9-0 beta

Change in SetImageBackgroundColor in 6.4.9-0 beta

Re: Change in SetImageBackgroundColor in 6.4.9-0 beta

Re: Change in SetImageBackgroundColor in 6.4.9-0 beta

Re: Change in SetImageBackgroundColor in 6.4.9-0 beta

Re: Change in SetImageBackgroundColor in 6.4.9-0 beta

Re: Change in SetImageBackgroundColor in 6.4.9-0 beta