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UXPng

UXPng turns the bytes of a PNG file into a UXImage, and an image into a PNG file’s bytes. It is written in xc and has nothing behind it: its own inflate, the chunk grammar and the five scanline filters. It is the same code on every backend, and a backend with no image decoder of its own (GEM) can still load a PNG. Its sibling for JPEG is UXJpeg.

#use <UXKit>            // or #import "UXPng.xc"

Overview

Section titled “Overview”
UXImage* atlas = UXPng.decode(bytes, length);
if (atlas == (UXImage*)0)
    {
    // not a PNG, damaged, or a kind it refuses
    }

Pixels come out as 0xAARRGGBB with straight (not premultiplied) alpha.

Supported: non-interlaced images at 8 and 16 bits per sample (and the sub-byte grey and palette depths), colour types grey, RGB, palette, grey with alpha and RGBA, with tRNS transparency. A 16-bit sample is taken to 8 bits by its high byte, which is what browsers do (Chrome’s decode of a 16-bit RGBA sheet matches it byte for byte). ImageMagick rounds instead, (v + 128) / 257, so its 8-bit export of the same file differs by 1 wherever a sample is not a multiple of 257.

Refused, with a null return rather than a wrong picture: Adam7 interlaced images, a bad CRC, and a truncated or empty file.

Encoding writes 8-bit RGB when every pixel is opaque and 8-bit RGBA otherwise, non-interlaced, so decode of the result gives back the same words. Each row takes the filter whose output is smallest, and the rows are deflated with LZ77 matches over a 32K window and deflate’s fixed Huffman codes. A picture of a window, with its large flat areas, compresses to well under 1% of its pixels.

UXImage* shot = window.snapshot((UXRect*)0);
UXFileIO.write((u8*)"shot.png", UXPng.encode(shot));

Topics

Section titled “Topics”

Decoding · decode Encoding · encode

decode

Section titled “decode”
static UXImage* decode(u8* bytes, i32 n)

The image in the n bytes at bytes, or null when they are not a PNG this decoder supports. The bytes are only read, and the image is a new allocation.

encode

Section titled “encode”
static Data* encode(UXImage* img)

The image as a PNG file’s bytes, or null for an empty image. The image is only read.

See also

Section titled “See also”