World: r3wp

Check out help.gif, it has an alpha channel, which survives after 
saving and loading.
	print mold help.gif/alpha
	save/png %image.png help.gif
	view layout [image (load %image.png)]

Now process the image, replacing a particular colour with transparent:
	img: copy help.gif
	colour: pick img 12x12

 repeat i length? img [if colour = pick img i [poke img i 0.0.0.255]]
	view layout [box red at 10x10 image (img)]

but that alpha is only a one bit plane no?

No, 8-bit alpha. Observe:
	img: copy help.gif
	colour: pick img 12x12

 repeat i length? img [if colour = val: pick img i [val/4: mod i 256 
 poke img i val]]
	save/png %image.png img

 view layout [box 300x300 red at 20x20 image (load %image.png) (img/size 
 * 8)]

that's really strange.   its possible that converting from bmp to 
png, RT realised the need to save the potential green mask.  so they 
probably store the image properly inside, but when creating one from 
scratch.. the channel is not.

and this leads me to... how in hell this you stumble accross this? 
 :-)

The image! datatype has an optional alpha channel. If it does not 
exist, it will be created when you set a pixel with alpha.

ok, I read back a bit and I understand more the origin of the discussion. 
 I had assumed brock had an image with an alpha and was not able 
to save it out.  his real question basically... how do I CREATE the 
alpha channel from View gfx.

and you have answered that.

sorry for stickin my head in my ass, I should have read above.

Is there any way to disable the exit button in the upper right corner 
of a window (the button with the X in it) or else reprogram it to 
exit according to my own rebol code?

the window's feel is different and accepts 'close events (in feel/detect).

it not very easy to deal with this because when you call 'view on 
a window, its feel is replaced automatically by view.  so you must 
explicitely replace the feel AFTER calling view on a layout or a 
face.


alternatively, you can also use an input even handler, which is probably 
a better solution:

this is one way to handle the close for all windows: 
   http://www.rebol.com/docs/view-system.html#section-5.13

Anton, great example... I was able to get this to work with a sample 
image that I created.  Thanks so much.

damn anti-aliasing!!  now I have fringing or a halo effect around 
my image due to anti-aliasing.  I'll see what I can do about this. 
 Thanks again.

(I'm trying to create custom rounded bullets)

ajusting gamma in the draw block may help, some artifacts may depend 
specifically on the colour being layered...

My example uses some very contrasting colours.  I'm guessing from 
your statement that you are suggesting making the original image 
background less contrasting and the effect will be minimized.

I'll also play with eh gamma as you suggested as well.

bad colors to mix are green and pink, for example.  the effects are 
usually worse when they are side by side and edges overlap.

a good trick is to set the gamma to one end, do the drawing and then 
compensate gamma back with its complement, to get original colors 
back.

yes, I had a green background, changing to a dark grey (near black) 
works nicely for my purpose, atleast on a larger image.

this is an old trick to fix the flickering stars effect you get when 
you try to move around small dots of white on a screen which has 
aliasing.

(and bg is black)

but there are many possible edge artifacts... and its hard to say, 
what will be the best solution without looking at it directly.

(plus I'm not an AGG expert specifically)

I'm not using AGG in this instance.

I found that the dark grey 'border' worked okay with a dark background 
and not so as suitable for a light background, atleast for the smaller 
image.

I'll play a little more before trying your gamma trick.

dark haloes are sometimes due to the contrast of actual pixels when 
two colors are placed side-by side.

consider this to be 2 pixels

[b g r]  [b g r]

if you had these two colors side by side 
(- meening that pixel channel is not lit):

[b - - ] [- - r]

then I guess you can understand that our eye will notice that large 
4 channel gap in between pixels.

the ordering of channels also means that these side-effects only 
occur in one direction:

if you placed  [ - - r]  [b - - ]  there would be no edge between 
red and blue (but will be from blue to next pixel)

MS's new font aliasing engine uses this technique to use sub channels 
so that edges actually hit pixel channels instead of complete pixels... 
 whic is why some letters look colored on edges... but when you see 
it, there is very nice increase in overall quality... its like increasing 
the resolution of your screen by a factor of 3 !

Maxim, thanks. I'll study that doc.

brock, why don't you just use an alpha channel?

I was able to get Anton's example to work in the scenario I had indicated 
above.  I'm open to looking into your suggestion, do you have an 
example that I can use.  I checked some of the rebol/view docs and 
the image datatype doc and don't see anything that guides me in the 
direction you are suggesting.

By the way Anton, thanks for your example, works well except for 
the fringing around the ani-aliased part of the image... still maintains 
some of the colour of the original background colour.

if i understand correctly, you need to use draw to produce a transparent 
image. is that correct?

so my question is, why do you need to make one color transparent, 
instead of using the alpha channel?

img: make image! 50x50
img/alpha: 255 ; make bg transparent
draw img [pen black line-width 2 circle 25x25 20]
save/png %test.png img

he is not using draw. AFAIK

Maxim, yes, I'm using draw.

doh... sorry.

I'm creating the image in a block, then using draw to display the 
final image.

can you give us a real equivalent example?

there are many possible edge defects... seeing them will help us 
cure the booboo :-)

right now we're like swatting flies in the dark... with you having 
the infra red goggles and saying: 
LEFT ! LEFT! .. oh no  RIGHT! RIGHT
;-)

hilight-colour: 255.0.0 ;0.112.99	;request-color


font-style: make face/font [style: 'bold  name: "font-sans-serif" 
 size: 26]

hilights: [
	transform 0 1 1 0x0 0x0
	fill-pen snow pen snow	;white circle
	circle 10x10 9
	fill-pen black pen black	;black circle
	circle 13x13 9
	fill-pen hilight-colour	;0.112.99
	pen hilight-colour	;serve dark green - 0.112.99
	circle 11x11 9
	fill-pen snow pen snow
	font font-style
	text 1 6x0 "*"
]

view center-face out: layout [
	origin 0
	bx: box 23x23 50.50.50 effect [draw hilights effect []][ 
		file: request-file/only/save/file %hilight.png
		if not file [exit]
		if not find file ".png" [append file ".png"]
		save/png file to-image bx
		img: load file
		colour: pick img 1x1

  repeat i length? img [if colour = pick img i [poke img i 0.0.0.255]]
		save/png file img
	]
]

this is a stripped down version (this time one that works without 
error)

clicking on the image brings up the save requestor

Using Gabriel's method creates a clean image without the fringing....

hilight-colour: 255.0.0 ;0.112.99	;request-color


font-style: make face/font [style: 'bold  name: "font-sans-serif" 
 size: 26]

img: make image! 23x23
img/alpha: 255	;make bg transparent


draw img [
	transform 0 1 1 0x0 0x0
	fill-pen snow pen snow	;white circle
	circle 10x10 9
	fill-pen black pen black	;black circle
	circle 13x13 9
	fill-pen hilight-colour	;0.112.99
	pen hilight-colour	;serve dark green - 0.112.99
	circle 11x11 9
	fill-pen snow pen snow
	font font-style
	text 1 6x0 "*"
]

file: request-file/only/save/file %hilight.png
if not file [exit]
if not find file ".png" [append file ".png"]
save/png file img