How to reduce iterations for each pixel?

Questions & Answers
1

I am building a game in Unity and I have a RawImage with the dimensions 1024 x 1024 and it doesn’t have a texture initially. I build one at runtime. At the moment, I have a button that when pressed generates a new texture and sets it to the RawImage. Basically, I generate a random array of Vector2 that contains 4 random positions within the dimensions of the RawImage. I’m facing long processing times because I need to set each pixel of the RawImage to my desired background color and all of this for just 4 positions. In other words, I do 1024 x 1024 = 1048576 iterations when I would just need 4. Here is a graphical representation as an example:
[182102-example.png*|182102]

*In this example I used a resolution of 40x40 in order to make the points more visible.

Here is the code that I use at the moment:

public RawImage myImage;

    public void GenerateButtonPressed()
    {
        Texture2D topTexture = new Texture2D(40, 40);
        Vector2[] randomValues = new Vector2[4];
        Color pointColor = Color.black;
        Color backgroundColor = Color.cyan;

        for(int i=0; i < randomValues.Length; i++)
        {
            randomValues *= new Vector2(Random.Range(0, 40), Random.Range(0, 40));*

}

int randomCounter = 0;
for(int i = 0; i < topTexture.width; i++)
{
for(int j = 0; j < topTexture.width; j++)
{
if(i == randomValues[randomCounter].x && j == randomValues[randomCounter].y)
{
topTexture.SetPixel(i, j, pointColor);
randomCounter++;
}
else
{
topTexture.SetPixel(i, j, backgroundColor);
}

}
}
topTexture.Apply();
myImage.texture = topTexture;
}
*
As you can see, I do 2 nested for loops with a pretty big index size just to set 4 points. Is there a way to reset the background without doing all those iterations and to set just the 4 points when the button is presed? Any amount of help would be appreciated because this improvement for sure will make my game more enjoyable. Thanks in advance!

2

There are multiple things you could reconsider. Setting all pixels at once using SetPixels instead, with a prepared array (which can potentially be prepared on a background thread). Using the *32 version (SetPixel32, SetPixels32) to avoid the floating point conversions. Possibly the fastest option would be to use Texture2D.SetPixelData, which does just a dumb data copy - so make sure that your bytes are in a correct format.
Another thing - do you need to re-set all pixels every time? What if you remembered, which pixels were black, and only cleared those and set the new ones in the next image?