Wednesday, 19 February 2014

Parameter #1 - Resolution

Maximum Score 30/100

See current scoring methodology HERE

Image resolution is the detail an image holds.  It is one of the key parameters required for bird identification both in the field and from photos.

Resolution could be expressed in a number of ways.  In the field, resolution is determined by ones visual acuity, the distance from the bird and the magnification of the optics being used.

In print, resolution may be defined in terms of dots per inch or dots per centimetre on paper.

On the screen it is determined by the number of horizontal lines.  If we had no means to zoom into digital photographs on screen then the screen resolution would determine the maximum resolution for identification purposes.  

Luckily however every image viewing software package allows us to zoom into an image and inspect it's fine detail right down to its minute components, its pixels.  This zoom function is of huge advantage over photographic film.

So when it comes to digital images resolution is best expressed in terms of PIXEL RESOLUTION.


In the first draft of our Birding Image Quality Tool you will notice that I measured the actual number of pixels within the surface area of the bird rather than choosing the resolution of the whole image.  This makes sense.  When you take a picture of a bird in most cases the bird will only take up a small portion of the overall image.  So, for a true representation of pixel resolution it is important to zoom right in and extract the bird and its pixels out of the scene for closer examination.  There are useful tools available freely online that will count the actual number of pixels in a defined area of the image.  PAINT.NET is one such program.

Using simply use the Lasso Select function, trace around the subject and at the bottom of the screen the selected area will be displayed in total pixels.

Hold the Phone!

Surely birds come in all shapes and sizes and they photograph in all kinds of poses.  How could a simple measurement of pixel area be considered good enough in all scenarios.  Well lets put this theory to the ultimate test shall we.

The battle of Obesebird versus Skinny-legged Stickbird

In the left corner, weighing in at a colossal 16,050,000 pixels we have our heavy weight champion Obesebird, while in the right corner, at little more than 700,000 pixels we have featherweight Skinny-legged Stickbird.

In order for these two to meet in our ring we must slim down Obesebird to 700,000 pixels (a reduction of the image to approx. 21% of it's original size should do the trick) and put them in the ring together.  By the way don't ask me how aspect ratios work.  I arrived at 21% by trial and error (one to research another time).

Finally, to make things a bit more interesting I am going to reduce the final image to 60% (trial and error again).  That way both birds are now resolved at approx. 250,000 pixels each, which is at the upper end in terms of image quality in our first Rev., 1.0 of the Birding Image Quality Tool.

Here is the result.

Here is a portion of the image zoomed up.

While the smaller image has clearly lost some of its clarity and is pixilated, it still retains enough detail to make it useful for our ID purposes.  This gives me enough confidence to proceed with the logic that a single, simple measure and scale for pixel resolution may well be enough for our purposes.

Strike a Pose

The example above is at the extreme end of things.  I have been tinkering around with different bird images to come up with a standard set of poses for bird identification purposes.  More of that another time.  Below is an example of how pixel resolution in bird images varies around a mean. From my studies so far I am finding that in most cases where we are dealing with useful images for bird identification purposes it should be possible to find an optimal pixel resolution with + or - 25% margin around a mean of around 300,000 pixels.

The proof will be in the pudding so to speak.  The more we test the tool with real images of birds the more it can be refined.  Watch this space.

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