png

This is a node.js module, writen in C++, that uses libpng to produce a PNG image (in memory) from RGB or RGBA buffers.

It was written by Peteris Krumins ([e-mail]). His blog is at http://www.catonmat.net -- good coders code, great reuse.

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The module exports three objects: Png, FixedPngStack and DynamicPngStack.

The Png object is for creating PNG images from an RGB or RGBA buffer. The FixedPngStack is for joining a number of PNGs together (stacking them together) on a transparent blackground. The DynamicPngStack is for joining a number of PNGs together in the most space efficient way (so that the canvas border matches the leftmost upper corner of some PNG and the rightmost bottom corner of some PNG).

Png

The Png object takes 4 arguments in its constructor:

var png = new Png(buffer, width, height, buffer_type);

The first argument, buffer, is a node.js Buffer filled with RGB(A) values. The second argument is integer width of the image. The third argument is integer height of the image. The fourth argument is 'rgb', 'bgr', 'rgba or 'bgra'. Defaults to 'rgb'.

The constructed png object then has an encode method that when called, converts the RGB(A) values in buffer into a PNG image and returns as a binary string:

var png_image = png.encode();

You can now either send the png_image to the browser, or write to a file, or do something else with it. See examples/ directory for more examples.

FixedPngStack

The FixedPngStack object takes 3 arguments in its constructor:

var fixed_png = new FixedPngStack(width, height, buffer_type);

The first argument is integer width of the canvas image. The second argument is integer height of the canvas image. The third argument is 'rgb', 'bgr', 'rgba or 'bgra'. Defaults to 'rgb'.

Now you can use the push method of fixed_png object to push buffers to the canvas. The push method takes 5 arguments:

fixed_png.push(buffer, x, y, w, h);

It pushes an RGB(A) image in buffer of width w and height h to the canvas position (x, y). You can push as many buffers to canvas as you want. After that you should call encode method that will join all the pushed RGB(A) buffers together and return a single PNG.

All the regions that did not get covered will be transparent.

DynamicPngStack

The DynamicPngStack object doesn't take any dimension arguments because its width and height is dynamically computed. To create it, do:

var dynamic_png = new DynamicPngStack(buffer_type);

The buffer_type again is 'rgb', 'bgr', 'rgba' or 'bgra', depending on what type of buffers you're gonna push to dynamic_png.

It provides three methods - push, encode and dimensions. The push and encode methods are the same as in FixedPngStack. You push each of the RGB(A) buffers to the stack and after that you call encode.

The dimensions method is more interesting. It must be called only after encode as its values are calculated upon encoding the image. It returns an object with width, height, x and y properties. The width and height properties show the width and the height of the final image. The x and y propreties show the position of the leftmost upper PNG.

Here is an example that illustrates it. Suppose you wish to join two PNGs together. One with width 100x40 at position (5, 10) and the other with width 20x20 at position (2, 210). First you create the DynamicPngStack object:

var dynamic_png = new DynamicPngStack();

Next you push the RGB(A) buffers of the two PNGs to it:

dynamic_png.push(png1_buf, 5, 10, 100, 40);
dynamic_png.push(png2_buf, 2, 210, 20, 20);

Now you can call encode to produce the final PNG:

var png = dynamic_png.encode();

Now let's see what the dimensions are,

var dims = dynamic_png.dimensions();

The x position dims.x is 2 because the 2nd png is closer to the left. The y position dims.y is 10 because the 1st png is closer to the top. The width dims.width is 103 because the first png stretches from x=5 to x=105, but the 2nd png starts only at x=2, so the first two pixels are not necessary and the width is 105-2=103. The height dims.height is 220 because the 2nd png is located at 210 and its height is 20, so it stretches to position 230, but the first png starts at 10, so the upper 10 pixels are not necessary and height becomes 230-10= 220.

How to compile?

To get the node-png module compiled, you need to have libpng and node.js installed. Then just run:

node-waf configure build

to build node-png module. It will be called png.node. To use it, make sure it's in NODE_PATH.

See also http://github.com/pkrumins/node-jpeg module that produces JPEG images.

If you wish to stream PNGs over a websocket or xhr-multipart, you'll have to base64 encode it. Use my http://github.com/pkrumins/node-base64 module to do that.

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Have fun producing PNGs!

Sincerely, Peteris Krumins http://www.catonmat.net