从 NSImage 获取像素和颜色

Question

我创建了一个 NSImage 对象，理想情况下想确定它包含多少个像素颜色。这可能吗？

Answer 1

此代码将 NSImage 渲染为 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);
}

给定位图上下文，我们可以直接访问原始图像数据，并计算每个颜色通道的直方图：

- (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

我已经发布一个完整的项目，一个Cocoa示例应用程序，其中包括上述内容。

• https://github.com/gavinb/CocoaImageHistogram.git

Answer 2

我建议创建您自己的位图上下文，将其包装在图形上下文并将其设置为当前上下文，告诉图像自行绘制，然后直接访问位图上下文后面的像素数据。

这将需要更多代码，但可以节省您的 TIFF 表示和创建数千或数百万个 NSColor 对象的麻烦。如果您处理任何可观尺寸的图像，这些费用将会迅速增加。

Answer 3

获取 NSBitmapImageRep< /code>来自您的 NSImage。然后你就可以访问像素了。

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

Answer 4

在 Core Image 文档中查找“直方图”。

Answer 5

将 colorAtX 与 NSBitmapImageRep 一起使用并不总是会产生完全正确的颜色。

我设法用这个简单的代码获得正确的颜色：

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

Answer 6

对于某些人来说，这可能是一种更简化的方法，并降低了内存管理的复杂性。

https://github.com/koher/EasyImagy

代码示例
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")
}