Question

I just want to know
how to set a particular pixel’s colour to red?
suppose x =37 y=54 and i want to change this pixel’s colour to red.
I have no clue how to do it.
I have got the values of points around a particular object into an array of pixels using marching square algo.

Answer 1

You cannot change the pixels of an existing CGImage. You have to create a new CGImage with the pixel changed. These are the steps:

1. Create a CGBitmapContext with CGBitmapContextCreate.
2. Draw the existing CGImage into it using CGContextDrawImage.
3. Draw the pixel using CGContextSetFillColorWithColor and CGContextFillRect.
4. Create a new CGImage using CGBitmapContextCreateImage.

Instead of using CGContextSetFillColorWithColor and CGContextFillRect, you could tweak the bitmap data directly after retrieving a pointer to it with CGBitmapContextGetData. That would be faster if you’re going to do it a lot.

Also, if you’re going to do it a lot, you will want to create the bitmap context and draw the original image into it just once, and keep the bitmap context around for diddling. But creating the new CGImage from the bitmap context may be a bottleneck.