Question

How can I replace specific color(RGB value) in CGBitmapContext that has already drawn?

Is there any easy way?

Thanks in advance.

Answer 1

You’ll want to get a pointer to the pixels and information about their format by doing something like this:

// This assumes the data is RGBA format, 8-bits per channel. 
// You'll need to verify that by calling CGBitmapContextGetBitsPerPixel (), etc.
typedef struct RGBA8 {
    UInt8 red;
    UInt8 green;
    UInt8 blue;
    UInt8 alpha;
} RGBA8;

RGBA8* pixels = CGBitmapContextGetData (context);
UInt32 height = CGBitmapContextGetHeight (context);
UInt32 width = CGBitmapContextGetWidth (context);
UInt32 rowBytes = CGBitmapContextGetBytesPerRow (context);
UInt32 x, y;
for (y = 0; y < height; y++)
{
    RGBA8* currentRow = (RGBA8*)((UInt8*)pixels + y * rowBytes);
    for (x = 0; x < width; x++)
    {
        if ((currentRow->red == replaceRed) && (currentRow->green == replaceGreen) &&
            (currentRow->blue == replaceBlue) && (currentRow->alpha == replaceAlpha))
        {
            currentRow->red = newRed;
            currentRow->green = newGreen;
            currentRow->blue = newBlue;
            currentRow->alpha = newAlpha;
        }
        currentRow++;
    }
}