On 8/31/21 11:05 AM, Isaac Raway wrote:
> I am trying to capture part of a drawn canvas to an image so I can save some
> expensive drawing operations.
>
[...]
G'day Isaac,
This is a little left-field, and the resource trade-off may not be what
you want, but here's a suggestion anyway...
You can draw an image in a canvas, and then save it in some appropriate
format.
If the output format is PDF, then GhostScript can render it to a Netpbm
format (assuming netpbm was included at build time, which is likely).
Netpbm provides portable pixel formats (PBM, PGM and PPM). PBM is
black/white; PGM is greyscale; PPM is colour.
GhostScript on my system shows the following output devices in the Netpbm
family:
pbm pbmraw
pgm pgmraw pgnm pgnmraw
png16 png16m png256 png48 pngalpha
pnggray pngmono pngmonod
pnm pnmraw ppm ppmraw
The manual page for "netpbm" notes that there are over 220 separate commands
in the netpbm suite.
In particular, the program "pamcut" lets you cut a rectangle out of an
image, and produce another Netpbm image.
So, from this point there are at least two ways to proceed:
0: (Draw the expensive things in their own canvas.)
--
1. Use external programs:
1.1 Save the canvas as a temporary PDF;
1.2 Discard/delete/destroy the temporary canvas;
1.3 Use GhostScript command (gs) to render the PDF to a suitable Netpbm
format;
1.4 If required, use pamcut to extract the portions of the image that
you want to keep, into a second Netpbm temporary file;
1.5 Import the desired image as an IM image, and then render it into
your main application interface; and
1.6 Clean up temporary files.
--
* The second way is identical to the first, except that you link the
netpbm library into your application:
2.1 The initial separate-canvas-and-save-as-a-CD-supported-format
file will perhaps remain;
2.2 As before, if the output is PDF or PostScript, then GhostScript
may need to be run externally;
2.2 Once the rendered image is read in via IM, The remaining steps (e.g.
pamcut) could be done in memory;
2.3 Of course, clean up temporary files as you go;
2.4 ?? I haven't checked, but perhaps some of the image operations
(especially cropping the image) could be done by IM, before pasting
the desired result into a suitable area in the GUI.
--
As a final note, I've found that "qpdf" is a handy tool for decompressing
PDFs back into a text format, so you can see the drawing primitives in
plain text (as PostScript commands). This may be a useful intermediate
debugging step.
--
Hope this helps,
s-b etc etc etc
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