I didn't see a quick and easy answer in these threads, but check them out yourself

http://www.google.com/#hl=en&q=%22magic+pink%22+sprite+blend+site%3Aallegro.cc&aq=f&oq=&aqi=&fp=aazfVjAAjGM

How about one which goes though every .bmp file in a directory and converts every almost-magic-pink pixel to magic-pink? It's not perfect, but it should save you some time. With the image you provided, I found giving it a tolerance value of 100 converted most of the effected pixels, without converting any of your asteroid.

Good luck with it! I'm pretty sure it won't blow anything up.

I forgot to say, make a copy of the bitmaps before you run it, as it will overwrite the originals (though you could easily modify the code to create new files).

That just happened! I changed the tolerance to 200 and lost all my original bmps!! I'm glad I made a backup before executing! hahah

I guess that's a magic function to make magic pink! Setting 150 tolerance gave me a pretty impressive result.

{"name":"598962","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/e\/8\/e84eb1aa7fd58592e3f17c0ec86ff8ac.jpg","w":632,"h":440,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/e\/8\/e84eb1aa7fd58592e3f17c0ec86ff8ac"}

For 160 tolerance I lost some pixels. Can you explain the logic behind this condition ?

if ((r > 255 - tol) && (g < tol) && (b > 255 - tol))
putpixel(bmp, x, y, magic_pink);

You can see in the picture that there are still some pixels that need to be replaced. I think for these pixels, we could set that for tolerances between 150 and 200 put black instead of leaving it the same color.

Thank you so much!

Method in GIMP without layers:

• Layer->add alpha channel

• Magic-wand-select the pink

• Delete

• Select->select none

• Scale

• Layer->transparency->threshold alpha(127)

• Layer->transparency->remove alpha channel(Make sure your background colour is magic pink)

Addition: Pink select for sprites with troublesome myriad of holes, but no colours resembling magic pink(uses layers)

• Layer->transparency->colour to alpha(magic pink)

• Layer->duplicate layer

• @bottom layer:
  Layer->transparency->threshold alpha
  (Channels:) Turn of alpha channel editing (i.e. not alpha channel view)
  edit->fill with bg colour(magic pink)
  (Channels:) Turn on alpha channel editing (i.e. not alpha channel view)

• @top layer:
  merge down

• scale

• threshold apha

• remove alpha channel (magic pink as bg colour)

Yes, my method totally fails. At least the thing with placing magic oink underneath.

I think the main problem is that a one bit transparency mask (like magic oink) should only be used with 8 bit depth, where you have 255 colours and one transparent "colour". If you want high colour sprites (24 bit), your transparency should use 8 bits (32 bits alltogether) to match the smoothness of the colours. Magic oink is something inbetween. You have high colour sprites, 16.7 M colours and one of these colours is your transparent colour.

Came to think of it, I have done what I tried to describe as the step 5 in my earlier post. But instead of painting the underlying layer with magic oink, I painted it with a solid colour as equal as possible the colour of the background in my game scene, where the sprite most likely would appear. Then I merged the two layers (still in Gimp), then I used the magic wand to mark the background. Here I could use a threshold near or equal to zero. Then I changed the marked background area to magic oink, ensuring that the marked area didn't have an antialiased edge.

If your asteroid sprite were to appear on a light blue sky, the sprite would now have a bluish glow around the edge, before the magic oink begins. It would look good when using draw_sprite() on the light blue background, but it would look terrible against a black deep skye. To be able to use same sprites on any background, the edge of the sprite must be very jagged, which only look somewhat good in 8 bit graphics. Either that or you should use 32 bit sprites with 8 bit alpha transparency.

If you would like to know, in Photoshop, when you turn Image > Modes > Indexed Mode on, It resizes pretty much like allegro does, and the borders stay well. Still, bitmaps look as ugly when software rendering resizes

Here's why you are in trouble:
Magic pink is only magic to allegro, for most other things (including photoshop, GIMP, and what have you) it is a regular color (255, 0, 255). When resizing, photoshop and some other programs apply anti-aliasing, depending on the settings, which means that for each pixel, a weighted average of the corresponding pixels in the original image is used. When one or more of these pixels are magic pink, the resulting color will also be pink to some degree, but not fully. For example, if four pixels are in the mix, one of which is black, and the other three are pink, and all equally weighted (academic example, but easier to calculate) the resulting color will be (191, 0, 191). This still looks fairly pink, but it not the exact same color as 'magic' pink, so allegro doesn't treat it as magic. A difference of 1 in only one component is enough to turn magic pink into regular pink.
To solve the issue, you need to make sure whatever program is responsible for resizing treats the transparent pixels as transparent, not as pink. Alternatively, you can choose not to use anti-aliasing, in which case the whole problem disappears since no color interpolation will happen.

Here are some options you have to do this:
a) Save your images in a format that supports actual transparency or even alpha channels (e.g. gif, png, tga), and use alpha blending or a proper loading routine in allegro. You may have to jump through a few hoops to make it work in allegro, but it's probably worth the effort (especially considering the excellent compression rates you can get with png compared to grabber and bmp).
b) Use a background in a color that matches the average background color in your game (e.g. black for a space shooter, greenish brown for a top-down army shooter, etc.); after resizing, use a flood fill with zero tolerance to turn the background color into magic pink. If your art style is cartoon-ish (that is, with black outlines), use (0, 0, 0) for the outlines and something like (1, 1, 1) for the background.
c) Turn anti-aliasing off
d) Write a little tool that you can use to batch-resize your sprites, and which keeps transparency intact. If you intend to use paletted images, you can add color conversion to the same tool, and if you make it a command-line tool, you can even include it in your build process.
e) Put the background color into a separate layer, and edit the image in a layer that has one bit of alpha (if your image editor can handle this). Keep this version for further editing; then resize and merge down.

As a general note, it is a good idea to keep the original and resized versions in separate folders, so that the risk of overwriting the wrong one is smaller. Also, use some sort of backup mechanism - either manually zip the entire image folder every now and then, or use a version control package (e.g. subversion) so you can always revert to the last "good" situation. The same goes for code, obviously.

Hey, I didn't read all the suggestions, but here are some of mine. I had a bit of trouble with this myself over the speedhack weekend.

In photoshop, if you go to preferences and set the interpolation method to nearest neighbor (under general settings), it shouldn't resize so ugly. However, this wasn't added until after version 3.0, which is what I have, so it didn't help me (nor did I get to test it obviously).

I got so sick of waisting my time during the competition, that I decided to write a hack to blend the magic pink pixels in. It's essentially LennyLen's idea, just blending (that is, reducing red and blue, increasing green by a proportionaly amount...) instead of replacing the offending pixels. It works okay, but the zooming feature lets everyone know my not-so-well-kept secret.

Another method is to do everything without the magic pink. This can be done in photoshop or paint (although better in photoshop). When your done with the graphics and have it to the size you want, make a layer behind the image and color it something that looks average to the colors you have in your graphics. For your rock picture, you could use a low-saturated brown. Then add a layer on top and paint the intended magic pink areas magic pink (with anti-aliasing turned off).

Lastly, you can make an anti-aliased image using TGA with this method. Those are the different methods I currently use depending on the situation...

I tested that on Gimp and the Paintbucket did some anti-alias kind of blending at the edges. Maybe that's avoidable with some settings. One would like to be able to use the Paintbucket with some threshold but without the soft edge/anti-aliasing thing.

Instead of directly using the Paintbucket, one can use the colour marker or wand with some threshold and without the soft edge. And after that Fill with FG color.

This is the exact point of error. You may not resize anything that has magic pink. It doesn't make any difference whether it's inside or outside. Any border between pink and any other colour (or any two colours) gets mixed when resized.

The very fundamental order is:
1. Remove pink
2. Resize
3. Put pink back