mikeash.com: just this guy, you know?

Posted at 2012-08-31 14:52 | RSS feed (Full text feed) | Blog Index
Next article: Friday Q&A 2012-09-14: Implementing a Flood Fill
Previous article: Friday Q&A 2012-08-24: Things You Never Wanted To Know About C
Tags: cocoa fridayqa image
Friday Q&A 2012-08-31: Obtaining and Interpreting Image Data
by Mike Ash  
This article is also available in Russian (translation by Science for Everyone), Belarusian (translation by Science for Everyone), Lithuanian (translation by Erelis Steponavicius), Ukrainian (translation by Sandi Wolfe), Czech (translation by Andrijana Savicević), Portuguese (translation by Artur Weber & Adelina Domingos), and Norwegian (translation by Rune Sk).

Cocoa provides some great abstractions for working with images. NSImage lets you treat an image as an opaque blob that you can just draw where you want it. Core Image wraps a lot of image processing in an easy-to-use API that frees you from worrying about how individual pixels are represented. However, sometimes you really just want to get at the raw pixel data in code. Scott Luther suggested today's topic: fetching and manipulating that raw pixel data.

The simplest image representation is a plain bitmap. This is an array of bits, one per pixel, indicating whether it's black or white. The array contains rows of pixels one after another, so that the total number of bits is equal to the width of the image multiplied by the height. Here's an example bitmap of a smiley face:

    0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0
    0 0 1 0 0 1 0 0
    0 0 0 0 0 0 0 0
    0 0 0 0 0 0 0 0
    0 1 0 0 0 0 1 0
    0 0 1 1 1 1 0 0
    0 0 0 0 0 0 0 0

Pure black and white is not a very expressive medium, of course, and accessing individual bits in an array is a bit of a hassle. Let's move a step up to using one byte per pixel, which allows grayscale (we can have zero be black, 255 be white, and numbers in between be different shades of gray) and makes it easier to access the elements as well.

Once again, we'll use an array of bytes with sequential rows. Here's some example code to allocate memory for the image:

    uint8_t *AllocateImage(int width, int height)
        return malloc(width * height);

To get to a particular pixel at (x, y), we have to move down y rows, then across that row by x pixels. Since the rows are laid out sequentially, we move down y rows by moving through the array by y * width bytes. The index for a particular pixel is then x + y * width. Based on this, here are two functions for getting and setting a grayscale pixel at a particular coordinate:

    uint8_t ReadPixel(uint8_t *image, int width, int x, int y)
        int index = x + y * width;
        return image[index];

    void SetPixel(uint8_t *image, int width, int x, int y, uint8_t value)
        int index = x + y * width;
        image[index] = value;

Grayscale is still not all that interesting in many cases, and we want to be able to represent color. The typical way to represent colored pixels is with a combination of three values for red, green, and blue components. All zeroes results in black, with other values mixing the three colors together to form whatever color is needed. It's typical to use 8 bits per color, which results in 24 bits per pixel. Sometimes these are packed together, and sometimes they're padded with an extra 8 bits of emptiness to give 32 bits per pixel, which is nicer to work with since computers are usually good at manipulating 32-bit values.

Transparency, or alpha, can also be handy to represent in an image. 8 bits of transparency fits nicely into the 8 bits of padding in a 32 bit pixel, and using 32 bit pixels holding red, green, blue, and alpha is probably the most common pixel format currently in use.

There are two ways to pack these pixels together. The common way is to just run them all together in sequence, so you'd have one byte of red, one byte of green, one byte of blue, and one byte of alpha all next to each other. Then you'd have red, green, blue, and alpha for the next pixel, and so forth. Each pixel occupies four bytes of contiguous memory.

It's also possible to store each color in a separate chunk of memory. Each chunk is called a plane, and this format is called "planar". In this case, you essentially have three or four (depending on whether alpha is present) regions of memory, each of which is laid out exactly like the pixels from the grayscale example from above. The pixel's color is a combination of the values from all of the planes. This can sometimes be more convenient to work with, but is often slower, due to bad locality of reference, and often more complex to work with, so it's a much less common format.

The only other thing to figure out is how the colors are ordered. RGBA (red, green, blue, then alpha) ordering is the most common on the Mac, but orders like ARGB and BGRA show up occasionally as well. There's no particular reason to choose one over another, other than compatibility or speed. To avoid expensive format conversions, it's best to match the format used by whatever you'll be drawing to, saving to, or loading from, when possible.

Obtaining Pixel Data
The Cocoa class which contains and provides pixel data is NSBitmapImageRep. This is a subclass of NSImageRep, which is an abstract class for a single "representation" of an image. NSImage is a container for one or more NSImageRep instances. In the case where there's more than one representation, they may represent different sizes, resolutions, color spaces, etc., and NSImage will choose the best one for the current context when drawing.

Given that, it seems like it should be pretty easy to get the image data from an NSImage: find an NSBitmapImageRep in its representations, then ask that representation for its pixel data.

There are two problems with this. First, the image may not have an NSBitmapImageRep at all. There are representation types that aren't bitmaps. For example, an NSImage representing a PDF will contain vector data, not bitmap data, and use a different type of image representation. Second, even if the image does have an NSBitmapImageRep, there's no telling what the pixel format of that representation will be. It's not practical to write code to handle every possible pixel format, especially since it's going to be difficult to test most of the cases.

There's a lot of code out there that does this anyway. It gets away with it by making assumptions about the contents of the NSImage and the pixel format of the NSBitmapImageRep. This is not reliable, and should be avoided.

How do you reliably get pixel data, then? You can draw an NSImage reliably, and you can draw into an NSBitmapImageRep using the NSGraphicsContext class, and you can get pixel data from that NSBitmapImageRep. Chain it all together, and you can get pixel data.

Here's some code to handle this sequence. The first thing it does is figure out the pixel width and height of the bitmap representation. This is not necessarily obvious, as NSImage's size doesn't have to correspond to pixel dimensions. This code will use size anyway, but depending on your situation, you may want to use a different way to figure out the size:

    NSBitmapImageRep *ImageRepFromImage(NSImage *image)
        int width = [image size].width;
        int height = [image size].height;

        if(width < 1 || height < 1)
            return nil;

Next, we create the NSBitmapImageRep. This involves the use of a really long initializer method which looks kind of frightening, but I'll go through all of the parameters in detail:

        NSBitmapImageRep *rep = [[NSBitmapImageRep alloc]
                                 initWithBitmapDataPlanes: NULL
                                 pixelsWide: width
                                 pixelsHigh: height
                                 bitsPerSample: 8
                                 samplesPerPixel: 4
                                 hasAlpha: YES
                                 isPlanar: NO
                                 colorSpaceName: NSCalibratedRGBColorSpace
                                 bytesPerRow: width * 4
                                 bitsPerPixel: 32]

Let's look at these parameters one by one. The first argument, BitmapDataPlanes, allows you to specify the memory where the pixel data will be stored. Passing NULL here, as this code does, tells NSBitmapImageRep to allocate its own memory internally, which is usually the most convenient way to handle this.

Next, the code specifies the number of pixels wide and high, which it computed previously. It just passes those values in for pixelsWide and pixelsHigh.

Now we start getting into the actual pixel format. I mentioned earlier that 32-bit RGBA (where red, green, blue, and alpha each take up one byte and are laid out contiguously in memory) is a common pixel format, and that's what we're going to use. Since each sample is one byte, the code passes 8 for bitsPerSample:. The samplesPerPixel: parameter refers to the number of different components used in the image. We have four components (R, G, B, and A) and so the code passes 4 here.

The RGBA format has alpha, so we pass YES for hasAlpha. We don't want a planar format, so we pass NO for isPlanar. We want an RGB color space, so we pass NSCalibratedRGBColorSpace.

Next, NSBitmapImageRep wants to know how many bytes make up each row of the image. This is used in case padding is desired. Sometimes an image row uses more than the strictly minimum number of bytes, usually for performance reasons, to keep things aligned nicely. We don't want to mess around with padding, so we pass the minimum number of bytes needed for one row of pixels, which is just width * 4.

Finally, it asks for the number of bits per pixel. At 8 bits per component and 4 components, this is just 32.

We now have an NSBitmapImageRep with the format we want, but how do we draw into it? The first step is to make an NSGraphicsContext with it:

        NSGraphicsContext *ctx = [NSGraphicsContext graphicsContextWithBitmapImageRep: rep];

An important note when troubleshooting: not all parameters for an NSBitmapImageRep are acceptable when creating an NSGraphicsContext. If this line complains about an unsupported format, that means that one of the parameters used to create the NSBitmapImageRep wasn't to the system's liking, so go back and double-check those.

The next step is set this context as the current graphics context. In order to make sure that we don't mess with any other graphics activity that might be going on, we first save the current graphics state, so we can restore it later:

        [NSGraphicsContext saveGraphicsState];
        [NSGraphicsContext setCurrentContext: ctx];

At this point, any drawing we do will go into our newly-minted NSBitmapImageRep. The next step is to simply draw the image.

        [image drawAtPoint: NSZeroPoint fromRect: NSZeroRect operation: NSCompositeCopy fraction: 1.0];

NSZeroRect is simply a convenient shortcut which tells NSImage to draw the entire image.

Now that the image is drawn, we flush the graphics context to ensure none of this stuff is still queued up, restore the graphics state, and return the bitmap:

        [ctx flushGraphics];
        [NSGraphicsContext restoreGraphicsState];

        return rep;

Using this technique, you can get anything that Cocoa is able to draw into a convenient 32-bit RGBA bitmap.

Interpreting Pixel Data
Now that we have the pixel data, what do we do with it? Precisely what to do with it is up to you, but let's look at how to actually get at the pixel data.

Let's start by defining a struct to represent an individual pixel:

    struct Pixel { uint8_t r, g, b, a; };

This will line up with the RGBA pixel data stored in the NSBitmapImageRep. We can grab a pointer out of it to use:

    struct Pixel *pixels = (struct Pixel *)[rep bitmapData];

Accessing a specific pixel at (x, y) works just like the previous example code for grayscale images:

    int index = x + y * width;
    NSLog(@"Pixel at %d, %d: R=%u G=%u B=%u A=%u",
          x, y

Make sure that x and y are located within the image bounds before doing this, or else hilarious results may ensue. If you're lucky, out-of-bounds coordinates will crash.

To iterate over all of the pixels in the image, a simple pair of for loops will do:

    for(int y = 0; y < height; y++)
        for(int x = 0; x < width; x++)
            int index = x + y * width;
            // Use pixels[index] here

Notice how the y loop is the outermost one, even though x first would be the natural order. This is because it's much faster to iterate over the pixels in the same order that they're stored in memory, so that adjacent pixels are accessed sequentially. Putting x on the inside does this, and the resulting code is much friendlier to the cache and memory controllers which are built to handle sequential access.

A modern compiler is likely to generate good code for the above, but in case you're paranoid and want to make sure the compiler won't generate a multiply and array index for every loop iteration, you can iterate using pointer arithmetic instead:

    struct Pixel *cursor = pixels;
    for(int y = 0; y < height; y++)
        for(int x = 0; x < width; x++)
            // Use cursor->r, cursor->g, etc.

Finally, note that this data is mutable. If you should so desire, you can actually modify r, g, b, and a, and the NSBitmapImageRep will reflect the changes.

Dealing with raw pixel data isn't something you usually have to do, but if you need it, Cocoa makes it relatively easy. The technique is a little roundabout, but by drawing into an NSBitmapImageRep with a chosen pixel format, you can obtain pixel data in the format of your choice. Once you have that pixel data, it's a simple matter of indexing into it to obtain the individual pixel values.

That's it for today! Friday Q&A is driven by reader ideas as always, so if you have any suggestions for topics you'd like to see covered in a future installment, please send them in.

Did you enjoy this article? I'm selling whole books full of them! Volumes II and III are now out! They're available as ePub, PDF, print, and on iBooks and Kindle. Click here for more information.


A nice introduction! A frequent use for this technology is to load textures into OpenGL. The advantage is that any format can be used as a texture, including PSDs and even PDF files. That means no conversion step when testing out new art.

(Personally, though, I prefer to use CGImage and CGContext for this. It takes a bit more work, but one has more control especially over where the Alpha value ends up)
The nice thing is that once you understand the basics of this stuff, using CGBitmapContext is pretty easy.
Marginally related question: on iOS, if I have some pixels rendered into a CGBitmapContext, if I want to render a rotated version of that image, do I have to grab an image and draw it into a new context with a transform applied? Or is there a quicker way?

presumably you're using the image you've gotten from your CGBitmapContext in a UIView or on a CALayer? you can just apply an affine transform to the view/layer and it'll be rotated for you.
Or if you're rendering manually somewhere, you can just set a transform before you render.
bytesPerRow and friends are used not just for memory alignment but also for copy-free subimages. It's a significant feature and design consideration that's worth going into.
Nice introduction!

Your NSBitmapImageRep initializer is missing the bitsPerSample argument though
Johan Sørensen: Thanks, that was silly of me. Fixed now.
Very good description.

I am trying to write a cross platform (windows + OSX) image class and have difficulties in the MAC part.
I am using NSImage as the basic element and the bmp representation to draw pixels in it.
While this works ok when I load an image from a file into NSimage and then there is a ready bmp representation to modify its pixels before I draw the nsimage on an nsview.

My problems begin when I want to create a blank nsimage and modify its pixels.
I am failing at all different sequences of attaching a bmp rep ont the image.

I will examine your proposals.

Comments RSS feed for this page

Add your thoughts, post a comment:

Spam and off-topic posts will be deleted without notice. Culprits may be publicly humiliated at my sole discretion.

The Answer to the Ultimate Question of Life, the Universe, and Everything?
Formatting: <i> <b> <blockquote> <code>.
NOTE: Due to an increase in spam, URLs are forbidden! Please provide search terms or fragment your URLs so they don't look like URLs.
Code syntax highlighting thanks to Pygments.
Hosted at DigitalOcean.