How to change image's alpha value without completely reloading every pixel each frame/ OPTIMIZATION - c++

Question

so I'm trying to bring an image to visibility in sfml 1.6 by changing it's alpha value every frame. unfortunately there isn't an overall alpha value for the image, so i have to go through each pixel, one by one and change it's alpha value.
This is extremely slow however, so I wondering how I could possibly optimize my simple code, or if there was another sfml specific way to handle this.
anyway here's the code:
Each new frame I Recolor a sprite with a added alpha value of 1.7.
// #Return Ptr: a pointer to the stack allocated image so the
// user can deallocate it later
sf::Image* RecolorSprite(sf::Sprite& sprite, sf::Color filter, bool subtract){
// the image has to survive so it's put ont he stack
sf::Image* image = new sf::Image;
*image = *sprite.GetImage();
RecolorImage(*image, filter, subtract);
sprite.SetImage(*image);
return image;
}
void RecolorImage(sf::Image& image, sf::Color filter, bool subtract){
for( int x= 0; x< image.GetWidth(); x++){
for(int y= 0; y< image.GetHeight(); y++){
if(subtract){
sf::Color pixel = image.GetPixel(x, y);
SubtractColor(pixel, filter);
image.SetPixel(x, y, pixel);
}
else
image.SetPixel(x, y, image.GetPixel(x, y) + filter);
}
}
}
// int used to stop illegal operations on unsigned chars
void SubtractColor(sf::Color& col1, sf::Color& col2){
int diff = ((int)col1.r) - ((int)col2.r);
if(diff >= 0)
col1.r -= col2.r;
else
col1.r = 0;
diff = ((int)col1.g) - ((int)col2.g);
if(diff >= 0)
col1.g -= col2.g;
else
col1.g = 0;
diff = ((int)col1.b) - ((int)col2.b);
if(diff >= 0)
col1.b -= col2.b;
else
col1.b = 0;
diff = ((int)col1.a) - ((int)col2.a);
if(diff >= 0)
col1.a -= col2.a;
else
col1.a = 0;
}

Answer 1

Unless I'm misunderstanding your question, you should be able to use sf::Drawable::SetColor for this, giving white as the argument but with a differing alpha value. For instance, to set sprite's alpha to 50%, you could do the following:
sprite.SetColor(sf::Color(255, 255, 255, 128));