Thanks again Fred and Anthony,
> You were never clear what you were trying to achieve. that is what
> image to draw over.
I think my understanding of the command sequence was
reasonably correct except for what I still think is an
`anomaly' or `inconsistency'. I could make it `work'
but I am more interested in the `why' on this one.
When an operator such as -draw is placed inside parentheses,
it only affects the image(s) inside the parentheses.
However, I have discovered further, that if the operator is
outside the parentheses, (but following the parentheses),
it affects the image inside the parentheses in exactly the
same way as if it were inside the parentheses. So the
parentheses only protects what is outside from what is inside
the parentheses but not the inside from what is outside the
parentheses. Maybe there are some command sequences for which
this is very useful but it also lends itself to potential
ambiguity and mistakes.
I thought the paradigm of layers in order of application
was conceptually useful and is similar to layered drawing systems.
However with drawing systems one can add any drawing object
(images, text, lines ..) to any layer and know that anything on
an upper layer will cover anything on a lower layer with all
coordinates being ultimately referenced to the edges of the canvas.
The final `merge' brings it all together - but still allowing
like IM, for intermediate merges if needed. With a drawing system,
an intermediate merge may be applied to simplify the process
by reducing the number of layers that need to be considered.
Setting the four images produced by the commands below
side-by-side, to me, the third just looks out of place, and if
that was what you wanted, you would use two -draw operators,
one for the canvas and one for the overlay:
a) To be confident of the result
b) In a `real' situation, it is most unlikely that the
coordinates of an object within the canvas would exactly
match the `independent' coordinates of an object within
the overlay parentheses.
Having to use an extra -flatten in the fourth command
below to achieve the result I really want feels `redundant',
`spurious', `anomalous' etc.
convert -size 200x200 xc:white \
-stroke black -strokewidth 5 \
-draw 'line 0,100 200,100' \
\( -size 150x150 xc:lightblue -repage 0x0+25+25 \) \
-flatten A.gif
convert -size 200x200 xc:white \
-stroke black -strokewidth 5 \
\( -size 150x150 xc:lightblue -repage 0x0+25+25 \
-draw 'line 0,100 200,100' \) \
-flatten B.gif
convert -size 200x200 xc:white \
-stroke black -strokewidth 5 \
\( -size 150x150 xc:lightblue -repage 0x0+25+25 \) \
-draw 'line 0,100 200,100' \
-flatten C.gif
convert -size 200x200 xc:white \
-stroke black -strokewidth 5 \
\( -size 150x150 xc:lightblue -repage 0x0+25+25 \) \
-flatten \
-draw 'line 0,100 200,100' \
-flatten D.gif
Regards
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