[gtkada] Missing Gdk_Pixbuf_New_From_Data

[Dmitry A. Kazakov]

On Tue, 25 Sep 2007 23:40:57 +0800, you wrote:

> On Sep 25, 2007, at 11:37 PM, Dmitry A. Kazakov wrote:
> 
>> On Tue, 25 Sep 2007 22:06:38 +0800, you wrote:
>>
>>> I am looking at GtkAda source (previous version and the latest 2.10.0
>>> from Libre site). I found on the last line in gdk-pixbuf.ads that
>>> gdk_pixbuf_new_from_data is missing or is not implemented. Why?
>>
>> Because it requires flat arrays I guess.
>>
>> You can use it directly using pragma Import. For example:
>>
>>    X_Size : constant GInt := 16;
>>    Y_Size : constant GInt := 16;
>>    type Pixbuf_Image is array (Natural range 0..1023) of GUChar;
>>    pragma Convention (C, Pixbuf_Image);
>>    Pixels : constant Pixbuf_Image :=
>>    (  255,255,255,  0, 255,255,255,  0, 255,255,255,  0,  
>> 255,255,255,  0,
>>       255,255,255,  0, 255,255,255,  0, 255,255,255,  0,  
>> 255,255,255,  0,
>>       255,255,255,  0, 255,255,255,  0, 255,255,255,  0,  
>> 255,255,255,  0,
>>       255,255,255,  0, 255,255,255,  0, 255,255,255,  0,  
>> 255,255,255,  0,
>>      ...
>>    );
>>    function Get_Pixbuf
>>             (  Data       : Pixbuf_Image   := Pixels;
>>                Colorspace : Gdk_Colorspace := Colorspace_RGB;
>>                Has_Alpha  : GBoolean := 1;
>>                Bits       : Int := 8;
>>                Width      : Int := Int (X_Size);
>>                Height     : Int := Int (Y_Size);
>>                Rowstride  : Int := 64;
>>                Fn         : Address := Null_Address;
>>                Fn_Data    : Address := Null_Address
>>             )  return Gdk_Pixbuf;
>>    pragma Import (C, Get_Pixbuf, "gdk_pixbuf_new_from_data");

> I am new at manipulating JPEG and graphic formats.
> 
> How does gdk_pixbuf_read_from_data work? Say I have read a JPEG image  
> into stream. Can it recognize the "data" is JPEG?
>
> What is Rowstride?

gdk_pixbuf_new_from_data works with raw square images stored row-wise. The
parameter Rowstride determines the size of a row in octets (bytes). In the
example I gave, we have 4 bytes (RGB + alpha). The horizontal image size is
16 pixels, so Rowstride is 16*4=64. When rows occupy unaligned memory
blocks it is sometimes better to align them at the appropriate boundary. In
that case Rowstride might be greater than <pixel-size> x <columns>. As you
see it is very low-level.

If you have JPEG you could:

1. unpack JPEG in the memory and then use gdk_pixbuf_new_from_data.
2. gdk_pixbuf_new_from_file which understands JPEG.

The second option should certainly work with Ada.Streams. But if you get
JPEG from a stream, then to feed gdk_pixbuf_new_from_file you will need a
named pipe (to have a file name for it). That would make you OS-dependent.
Same if you tired to use memory-mapped files.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de