Re: Writing image to blob PNG vs TIFF

I thought maybe using blob::update() like this might work (although
wasteful) but no luck:

  // Create a gm blob

  char nullbyte = '\0';

  Blob * pBlob = GetPBlob();

  if(strcmp(GetGMType(), "TIFF") == 0)
pBlob->update((const void *)&nullbyte, (size_t)1);Â //
force an update

  gmimage.write(pBlob, GetGMType());

-------- Forwarded Message --------

Subject:

Re: [GM-help] Writing image to blob PNG vs TIFF

Date:
Mon, 18 Dec 2017 12:43:09 -0500

From:
Robert H. Olsen <[email protected]>

Reply-To:

Requests for help with GraphicsMagick
<[email protected]>

To:
Requests for help with GraphicsMagick
<[email protected]>

Hello Bob

I appreciate the quick reply!

I think I'm probably misunderstanding what you mean by "multiple image
frames" though?

I'm only interested in dealing with a single image at a time - not, for
example, making a multiple-image TIFF. I want to capture an image,
process it, package it, send it out over the network and then wait for
another. And this works fine if I tell Image.write that it's a PNG. Is
there some way of saying "this is a new, distinct image" when writing
TIFF to a blob, other than changing its size? Would toggling between two
different pBlob memory locations trick it?

Thanks again.

On 12/18/17 12:13, Bob Friesenhahn wrote:
> On Mon, 18 Dec 2017, Robert H. Olsen wrote:
>>
>> // Create a formatted gm blob
>> Blob * pBlob = GetPBlob(); // points to space allocated elsewhere
>> gmimage.write(pBlob, GetGMType()); // GetGMType() returns either
>> "PNG" or "TIFF"
>> // gmimage.write(pBlob, "TIFF");
>> // gmimage.write(pBlob, "PNG");
>>
>> This works well for "PNG" but if the type is "TIFF" then the image at
>> pBlob is correct only the first time through after starting up or
>> changing from PNG mode, or if I change the number of times minify()
>> is run on the image, otherwise the image data is not changed though
>> it should have been updated when the new trigger was received. If I
>> switch back to PNG everything is fine again. Do TIFF and PNG deal
>> with previously allocated space differently, or something like that?
>
> Due to my decision to use STL containers to support multiple image
> frames, the Image read/write functions only support a single image frame.
>
> You must use the STL-based interfaces readImages() and writeImages()
> from Magick++/STL.h to deal with multiple frames.
>
> Please see http://www.graphicsmagick.org/Magick++/STL.html for
> documentation on how to use the STL-based interfaces.
>
> Unfortunately, this means that some of your code will need to be
> re-written.
>
> At the C language level, a sequence of image frames is represented by
> a linked list. For the C++ interface, these sequences (based on
> previous/next pointers) are broken (previous/next pointers set to
> NULL) and the STL container functionality is used to remember the
> relationship between frames. When the C++ interface calls into the C
> interface it must temporarily re-create a linked list of image frames
> (which it again destroys afterward). For this reason, you should not
> use multiple threads on the same STL container.
>
> Bob
>
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