Clash Royale CLAN TAG#URR8PPP

Get pixels and colours from NSImage

I have created an NSImage object, and ideally would like to determine how many of each pixels colour it contains. Is this possible?

6 Answers
6

I suggest creating your own bitmap context, wrapping it in a graphics context and setting that as the current context, telling the image to draw itself, and then accessing the pixel data behind the bitmap context directly.

This will be more code, but will save you both a trip through a TIFF representation and the creation of thousands or millions of NSColor objects. If you're working with images of any appreciable size, these expenses will add up quickly.

Get an NSBitmapImageRep from your NSImage. Then you can get access to the pixels.

NSBitmapImageRep

NSImage

NSImage* img = ...;
NSBitmapImageRep* raw_img = [NSBitmapImageRep imageRepWithData:[img TIFFRepresentation]];
NSColor* color = [raw_img colorAtX:0 y:0];


colorAtX:y:

NSColor

NSColor colorAtX:y

This code renders the NSImage into a CGBitmapContext:

NSImage

CGBitmapContext

- (void)updateImageData

if (!_image)
return;

// Dimensions - source image determines context size

NSSize imageSize = _image.size;
NSRect imageRect = NSMakeRect(0, 0, imageSize.width, imageSize.height);

// Create a context to hold the image data

CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);

CGContextRef ctx = CGBitmapContextCreate(NULL,
imageSize.width,
imageSize.height,
8,
0,
colorSpace,
kCGImageAlphaPremultipliedLast);

// Wrap graphics context

NSGraphicsContext* gctx = [NSGraphicsContext graphicsContextWithCGContext:ctx flipped:NO];

// Make our bitmap context current and render the NSImage into it

[NSGraphicsContext setCurrentContext:gctx];
[_image drawInRect:imageRect];

// Calculate the histogram

[self computeHistogramFromBitmap:ctx];

// Clean up

[NSGraphicsContext setCurrentContext:nil];
CGContextRelease(ctx);
CGColorSpaceRelease(colorSpace);



Given a bitmap context, we can access the raw image data directly, and compute the histograms for each colour channel:

- (void)computeHistogramFromBitmap:(CGContextRef)bitmap

// NB: Assumes RGBA 8bpp

size_t width = CGBitmapContextGetWidth(bitmap);
size_t height = CGBitmapContextGetHeight(bitmap);

uint32_t* pixel = (uint32_t*)CGBitmapContextGetData(bitmap);

for (unsigned y = 0; y < height; y++)

for (unsigned x = 0; x < width; x++)

uint32_t rgba = *pixel;

// Extract colour components
uint8_t red = (rgba & 0x000000ff) >> 0;
uint8_t green = (rgba & 0x0000ff00) >> 8;
uint8_t blue = (rgba & 0x00ff0000) >> 16;

// Accumulate each colour
_histogram[kRedChannel][red]++;
_histogram[kGreenChannel][green]++;
_histogram[kBlueChannel][blue]++;

// Next pixel!
pixel++;




@end


I've published a complete project, a Cocoa sample app, which includes the above.

Look for "histogram" in the Core Image documentation.

Using colorAtX with NSBitmapImageRep does not always lead to the exact correct color.

colorAtX

NSBitmapImageRep

I managed to get the correct color with this simple code:

[yourImage lockFocus]; // yourImage is just your NSImage variable
NSColor *pixelColor = NSReadPixel(NSMakePoint(1, 1)); // Or another point
[yourImage unlockFocus];


This maybe a more streamlined approach for some and reduce complexity of dropping into memory management.

https://github.com/koher/EasyImagy

Code sample
https://github.com/koher/EasyImagyCameraSample

import EasyImagy

let image = Image<RGBA<UInt8>>(nsImage: "test.png") // N.B. init with nsImage

print(image[x, y])
image[x, y] = RGBA(red: 255, green: 0, blue: 0, alpha: 127)
image[x, y] = RGBA(0xFF00007F) // red: 255, green: 0, blue: 0, alpha: 127

// Iterates over all pixels
for pixel in image
// ...




//// Gets a pixel by subscripts Gets a pixel by
let pixel = image[x, y]
// Sets a pixel by subscripts
image[x, y] = RGBA(0xFF0000FF)
image[x, y].alpha = 127
// Safe get for a pixel
if let pixel = image.pixelAt(x: x, y: y)
print(pixel.red)
print(pixel.green)
print(pixel.blue)
print(pixel.alpha)

print(pixel.gray) // (red + green + blue) / 3
print(pixel) // formatted like "#FF0000FF"
else
// `pixel` is safe: `nil` is returned when out of bounds
print("Out of bounds")



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