图像像素坐标

Question

I'm using PHP and ImageMagick to read colors from an image's edge.

$x = 1;
$y = 1;
$pixel = $image->getImagePixelColor($x, $y);
$colors = $pixel->getColor(); // rgba

But I don't quite get how the coordinates work. So I created this 4px png.

1,1 gives rgba(0,30,255,0) instead of the transparent pixel. Also, what if I select a large range of pixels?

Answer 1

The easiest way to understand this is to remember that when a pixel is transparent, its colour becomes irrelevant, yet it is retained.

So, if we make an opaque 512x512 image of random noise, like this:

convert -size 512x512 xc:gray +noise random a.png

and check its size, you can see it is 1.5MB:

-rw-r--r--@ 1 mark  staff   1.5M 26 Oct 09:14 a.png

If I now make an otherwise identical, transparent version:

convert -size 512x512 xc:gray +noise random -alpha transparent b.png

You can see the file has got bigger (at 1.7MB), because it is now storing transparency AND the original random data. I won't show the image because there is nothing to see:

-rw-r--r--@    1 mark  staff   1.7M 26 Oct 09:16 b.png

I can now make the transparency layer fully opaque and the random pixels become visible again - so they were still there all along - even when the image was transparent:

convert b.png -alpha opaque c.png

If, however, I make the pixels transparent again, AND also set them all to the colour of the image background (since their colour is irrelevant), like this:

 convert -size 512x512 xc:gray +noise random -alpha transparent -alpha background d.png 

-rw-r--r--@ 1 mark  staff   1.8K 26 Oct 09:22 d.png

And now suddenly the 1.5MB file is nearly 1,000 times smaller at 1.8K because it compresses much better as all the (invisible/transparent) colours are one single, solid background colour.

In answer to your question about coordinates. We can make a little image like this:

convert -size 1x1             \
       xc:red xc:cyan +append \
    \( xc:yellow xc:magenta +append \) -append a.png

And dump it like this:

convert a.png txt:

# ImageMagick pixel enumeration: 2,2,65535,srgb
0,0: (65535,0,0)  #FF0000  red
1,0: (0,65535,65535)  #00FFFF  cyan
0,1: (65535,65535,0)  #FFFF00  yellow
1,1: (65535,0,65535)  #FF00FF  magenta

And see that pixel coordinates in ImageMagick start at top-left corner.