Instance Method createARGBBitmapContextFromImage not found. Trying to get a pixel of an image

Question

I am trying a few approaches to get pixel color or alpha of an image. So far every attempt has brought me long time process.

For now, I am trying the following method, but I can't go through as xcode is telling me that Ïnstance Method createARGBBitmapContextFromImage not found when trying to get context (CGContextRef cgctx ).

Any ideas why this is happening?

- (BOOL) isWallPixel:(CGPoint)point
{

BOOL result = FALSE;
UIColor* color = nil;

CGImageRef inImage;

UIImage* imagem = [UIImage imageNamed:@"mask.png"];

inImage = imagem.CGImage;


// Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpa, Red, Green, Blue
CGContextRef cgctx = [self createARGBBitmapContextFromImage:inImage];
if (cgctx == NULL) { return nil; /* error */ }

size_t w = CGImageGetWidth(inImage);
size_t h = CGImageGetHeight(inImage);
CGRect rect = {{0,0},{w,h}};


// Draw the image to the bitmap context. Once we draw, the memory
// allocated for the context for rendering will then contain the
// raw image data in the specified color space.
CGContextDrawImage(cgctx, rect, inImage);

// Now we can get a pointer to the image data associated with the bitmap
// context.
unsigned char* data = CGBitmapContextGetData (cgctx);
if (data != NULL) {
    //offset locates the pixel in the data from x,y.
    //4 for 4 bytes of data per pixel, w is width of one row of data.
    int offset = 4*((w*round(point.y))+round(point.x));
    int alpha =  data[offset];
    int red = data[offset+1];
    int green = data[offset+2];
    int blue = data[offset+3];
    color = [UIColor colorWithRed:(red/255.0f) green:(green/255.0f) blue:(blue/255.0f) alpha:(alpha/255.0f)];
}

// When finished, release the context
//CGContextRelease(cgctx);
// Free image data memory for the context
if (data) { free(data); }
if(alpha==0)
  result = TRUE;

return result;
}

Answer 1

Here is the code you're looking for, which I found in this related question. You need to add this to your .m file and also add the method declaration to your .h file.

- (CGContextRef)createARGBBitmapContextFromImage:(CGImageRef)inImage
{
    CGContextRef context = NULL;
    CGColorSpaceRef colorSpace;
    void *bitmapData;
    int bitmapByteCount;
    int bitmapBytesPerRow;

    // Get image width, height. We'll use the entire image.
    size_t pixelsWide = CGImageGetWidth(inImage);
    size_t pixelsHigh = CGImageGetHeight(inImage);

    // Declare the number of bytes per row. Each pixel in the bitmap in this
    // example is represented by 4 bytes; 8 bits each of red, green, blue, and
    // alpha.
    bitmapBytesPerRow   = (pixelsWide * 4);
    bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);

    // Use the generic RGB color space.
    colorSpace = CGColorSpaceCreateDeviceRGB();

    if (colorSpace == NULL)
    {
        fprintf(stderr,"Error allocating color space\n");
        return NULL;
    }

    // Allocate memory for image data. This is the destination in memory
    // where any drawing to the bitmap context will be rendered.
    bitmapData = malloc(bitmapByteCount);
    if (bitmapData == NULL)
    {
        fprintf(stderr,"Memory not allocated!");
        CGColorSpaceRelease(colorSpace);
        return NULL;
    }

    // Create the bitmap context. We want pre-multiplied ARGB, 8-bits
    // per component. Regardless of what the source image format is
    // (CMYK, Grayscale, and so on) it will be converted over to the format
    // specified here by CGBitmapContextCreate.
    context = CGBitmapContextCreate(bitmapData,
                                    pixelsWide,
                                    pixelsHigh,
                                    8,   // bits per component
                                    bitmapBytesPerRow,
                                    colorSpace,
                                    kCGImageAlphaPremultipliedFirst);
    if (context == NULL)
    {
        free(bitmapData);
        fprintf(stderr,"Context not created!");
    }

    // Make sure and release colorspace before returning
    CGColorSpaceRelease(colorSpace);

    return context;
}