Creating an Image - Page 3
December 15, 2000
As mentioned above, the first thing we need to do is create a
blank image canvas for PHP to work on – a call to the
ImageCreate() function does just this. The only two
things we need to tell it are the width and height of the image
we wish to create. ImageCreate() will then return an
identifier that identifies that blank image in memory. All the
subsequent image functions we use will have to refer to the image
in memory by means of this identifier.
This identifier is similar to a file handle or database link
ID, as we've used in recent chapters, but refers to the location
of our image canvas in memory rather than an open file on the
disk or an active database connection.
$image = ImageCreate(200,150);
We've now defined $image as the identifier referring
to the new, blank canvas (200 pixels wide by 150 pixels high)
that ImageCreate() has just created for us. Now that
this 200 x 150 pixel image is in memory we can start drawing on
it. First though, we need to know what color we'll be drawing in.
Setting up Colors
Before we can tell PHP that we want to use a certain color on our
canvas, we have to create that color. We do this in a similar
fashion to that of creating the image canvas:
$gray = ImageColorAllocate($image,204,204,204);
$blue = ImageColorAllocate($image,0,0,255);
We've used the function ImageColorAllocate() to
define two colors for our image: a gray and a blue. Each color is
assigned a unique identifier and tied to an existing image
canvas. As you can see, this function requires 4 pieces of
information:
- The first is the identifier of the image canvas with which
this color will be used. This is the same identifier that we saw
returned to us earlier from the
ImageCreate()
function.
- The second, third and fourth things we need to tell
ImageColorAllocate() are the respective values of
red, green and blue components for that color, which must lie
between 0 and 255.
Computers make up color by mixing different quantities of Red,
Green and Blue. This is what is known as RGB mode color.
Each of red, green and blue can range from 0 to 255. Setting each
of red, green and blue to 0 will give us black (total absence of
color), a value of 255 for all three will give us white. If we
want to stick to web-safe colors, then we have to limit
our values of red, green and blue to multiples of 51. That gives
us 6 possible values for each of red, green and blue, for a total
of 216 colors.
$gray and $blue are now two new
identifiers to the colors gray and blue. We created the former by
specifying equal values of each color. Note that 204 was not a
number we pulled from the hat, but a multiple of 51, and we've
therefore defined a web-safe color. We defined $blue
by telling ImageColorAllocate()that the red
component was 0, the green component was 0 and that the blue
component was 255.
The Image Coordinate System
While we're dealing with theory here, we may as well take a look
at how the image coordinate system works. It may sound like a
mouthful but it's simply a way for us to precisely describe
points in the image.
If you're familiar with creating graphs, you'll probably be used
to the x and y values radiating outwards from the bottom left
hand corner. In PHP, all coordinates radiate outwards from the
top left-hand corner of the image, as the image below
shows:
Graph Coordinates PHP Coordinates
With this in mind, let's take a look at how our blank
$image is laid out:
The x-y coordinates for the top left corner of
$image are 0,0. This will be true for every image
that you create. The x-coordinates extend to the right for 200
pixels and y-coordinates extend down for 150 pixels, so the
bottom right-hand corner of the image has x-y coordinates of
199,149.
Laying a Foundation - Page 2
Beginning PHP4
Drawing On Our Image: Lines - Page 4
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