What is the best way to change png color pixel by pixel on Android?

Question

I need to change a particular color of a PNG stored in the SDCard. I can't use tintcolor over a bitmap or over any other object because this will tint all the image not a particular pixel color.

Why do I need to do this?
I'm trying to develop an avatar application and I want to be able to change the hair of the avatar with any color I select from a palette. This Hair have two colors, one for the border and the other for the rest of the hair. I just want to change the air color and keep the border one.

This a simple case but there could be more than one color in the image.

I was looking up for a solution. This is the only thing I found (meaby there could be more but I am not lucky):

Android Bitmap: Convert transparent pixels to a color

And this is what it is exposed there:

Bitmap b = ...;
for(int x = 0; x<b.getWidth(); x++){
    for(int y = 0; y<b.getHeight(); y++){
        if(b.getPixel(x, y) == Color.TRANSPARENT){
            b.setPixel(x, y, Color.WHITE);
        }
     }
}

I want to know if there is a better way to do this. Something like a:

bipmapImage.changeColor(originalColor, newColor);

I don't know if using a loop to check pixel a pixel is a good performance. Imagine a 1080 x 1080 image.

Thanks in advance.

Answer 1

You can get better performance compared to getPixel() and setPixel() by calling copyPixelsToBuffer() to copy the pixels to a buffer, then modifying the pixel values in the buffer, and finally calling copyPixelsFromBuffer() to copy from the buffer back to the bitmap:

boolean changeColor(Bitmap bitmap, int originalColor, int newColor)
{
    // bitmap must be mutable and 32 bits per pixel:
    if((bitmap.getConfig() != Bitmap.Config.ARGB_8888) || !bitmap.isMutable())
        return false;

    int pixelCount = bitmap.getWidth() * bitmap.getHeight();
    IntBuffer buffer = IntBuffer.allocate(pixelCount);
    bitmap.copyPixelsToBuffer(buffer);
    int[] array = buffer.array();
    for(int i = 0; i < pixelCount; i++)
    {
        if(array[i] == originalColor)
            array[i] = newColor;
    }
    bitmap.copyPixelsFromBuffer(buffer);

    return true;    
}

However, there are extra complications: the bitmap must use the ARGB_8888 pixel format (if it's different you will need to write extra code to handle that) and you need to catch out of memory exceptions that can occur when allocating the IntBuffer. You should first profile the code using setPixels() and see if the speed is unacceptable.

This is also likely not the fastest solution, which will probably be to use a native function call in a library. But it will still be faster than setPixel() and you don't need to add a library to your project.

Answer 2

You'd better use OpenCV Matrix APIs especially for performance (and compactness).

Check the OpenCV Android tutorial here.

Assume you've installed the OpenCV features, you can change the color of some particular areas in your image.

(You should understand the Mat feature first.)

Actually I haven't used OpenCV with Android.
Here's some example codes to change your hair color to red in C++ :

// 1. Loading
Mat src = imread("yourImagePath/yourOriginalImage.jpg");  // This code will automatically loads image to Mat with 3-channel(BGR) format
Mat mask = imread("yourImagePath/yourHairMaskImage.png", CV_GRAYSCALE);  // This code will automatically loads mask image to Mat with 1-channel(grayscale) format

// 2. Splitting RGB channel into 3 separate channels
vector<Mat> channels;   // C++ version of ArrayList<Mat>
split(src, channels);   // Automatically splits channels and adds them to channels. The size of channels = 3

// 3-1. Adjusting red color
Mat adjustRed = channels[0]*1.5;

// 3-2. Copy the adjusted pixels into org Mat with mask
channels[2] = adjustRed & mask + channels[0] & ~mask;

// 4. Merging channels
Mat dst;
merge(channels, dst);   // dst is re-created with 3-channel(BGR).
// Note that OpenCV applies BGR by default if your array size is 3,
// even if actual order is different. In this case this makes sense.

// 5. Saving
imwrite("yourImagePath/yourDstImage.jpg", dst);

The Android version code will be not that different I think.