rendering a red object white

[~Spaceaholic]

Hi Guys,
I want to make a cool invincibility power up effect for a game I am working on, It is all 2d, rendered with opengl, using sprite sheets....
the main character for the game is a bright red crab, and I would like to make him blink white when the user is using the invincibility power up... the sprite sheet is quite large already so having a secondary sprite sheet with the crab white is not really an option.... I have played a bit with setting the color in opengl via gl.glColor4f(1,1,1,1); and was wondering is there a color combination that would allow me to render the crab white(perhaps if the rgb value of the crab was already known?)? or is there another way I should go about implementing this?
Thanks

[Riven]

As you're using OpenGL, you should use a fragment shader, where you can manipulate all colors. In your case you'd simply add RGB(0,1,1) to your crab.

[davedes]

A fixed function approach:

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//set environment mode to ADD      glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_ADD);       glColor4f(1f, 1f, 1f, 1f);       //draw the sprite; the colors will be white but the alpha will be in tact      image.draw(50, 50);       //reset mode to default...      glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

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[theagentd]

Turn the problem around! Make the original crab image white and use glColor() to make it red! Definitely the easiest approach.

[Riven]

Turn the problem around! Make the original crab image white and use glColor() to make it red! Definitely the easiest approach.

Yeah, that was my first idea too, then I thought the crab was probably not 100% RGB(*,0,0)

[theagentd]

Turn the problem around! Make the original crab image white and use glColor() to make it red! Definitely the easiest approach.

Yeah, that was my first idea too, then I thought the crab was probably not 100% RGB(*,0,0)

Why does that matter?

[Riven]

Because then you cannot make the crab white (well, greyscale) as you'd be losing information, that cannot be added back later.

[~Spaceaholic]

Thanks for all the info, I ended up using davedes's approach, and it works great... I can make the crab blink any color now!