php-GD-Animated-webp

First attempt to create an animated webp using GD

Based on https://es.stackoverflow.com/q/159217/81450

Note: Missing the gif frames extraction things

PART 1 : encoding each frame

1. we use GD's imagewebp to encode the image frame and capture the data with ob_start
2. discard file header, keep framedata payload (starts with "VP8 " and goes until the eof)
3. save into an array(buffer): framedata + width + height + frame duration
4. repeat until no mo frames

PART 2 : build our own animated webp file according to specs https://developers.google.com/speed/webp/docs/riff_container

An animated image with EXIF metadata may look as follows:

RIFF/WEBP
+- VP8X (descriptions of features used)
+- ANIM (global animation parameters)
+- ANMF (frame1 parameters + data)
+- ANMF (frame2 parameters + data)
+- ANMF (frame3 parameters + data)
+- ANMF (frame4 parameters + data)
+- EXIF (metadata) <- optional we wont use it

1. header fileformat (RIFF+filesize+WEBP)

1.a) calculate filesize (uint32) total bytes in file -8 from header, or total bytes in content chunks + 4

2. chunk header VP8X

2.a) calculate chunksize (uint32) = 10

2.b) set bit flags alpha and animation to 1

2.c) store canvas width and height (uint24) (-1)

3. chunk header ANIM

3.a) calculate chunksize (uint32) = 6

3.b) background color: BGRA (0,0,0,0)

3.c) loop count (uint16) 0=infinityandbeyond

4. for each frame stored in array (buffer) PART 1.3

4.a) chunk header ANMF

4.b) calculate chunksize (uint32) = 16 + total bytes in frameData

4.c) frame origin X,Y (uint24) (/2?)

4.d) frame width and height (uint24) (-1)

4.e) duration in miliseconds (uint24)

4.f) reserved (6 bits) + alpha blending (1 bit) + discard frame (1 bit)

4.g) frameData from array (buffer) PARTE 1.3

5. save to disk, send to browser or close stream