[IOS] Change the display time for each frame of GIF animation in Pythonista3.

Introduction

Previously, when writing Articles addicted to GIF animation, [Details of GIF format](http://www.tohoho-web.com/wwwgif. I found a great article called htm # ImageBlock). Each image in the GIF file has a delay time set in 2 bytes, and it seems that it can be set in 0.01 second units.

The goal is [Graphic Control Extension]

It is natural that there is image data in the GIF file, but it seems that there is an 8-byte block called ** Graphic Control Extension ** in front of the image data. Graphic Control Extension --Extension Introducer (1B) = 0x21 <= mark 1 --Graphic Control Label (1B) = 0xf9 <= mark 2 --Block Size (1B) = 0x04 <= mark 3

• R(3b),DM(3b),UIF(1b),TCF(1b) --Delay Time (2B) <= This is the delay time
• Transparent Color Index(1B)
• Block Terminator(1B) = 0x00

Check with a binary editor

I checked the GIF animation with the delay time set to 1 second with a binary editor.

There is a delay time 0x6400 following the marks 1-30x21f904 properly. Since it is set in ** 1 second **, it is ** 100 ** when divided by ** 0.01 **, so 100 = 0x0064, and when stored in little endian, it is definitely 0x6400. You can change the display time for each frame ** by changing this value.

let's try it

ChangeDelay.py

#Open the GIF file as binary data.
FileName = 'test.gif'
with open(FileName,'rb') as f:
	Bin = f.read()

#Find the Graphic Control Extention and record the position and delay value in the List.
GCE = b'\x21\xf9\x04'
DelayList = []
point = 0
while point < len(Bin):
	start = Bin.find(GCE,point)
	if start<0:break
	delay = int.from_bytes(Bin[start+4:start+6],byteorder="little")
	DelayList.append([start+4,delay])
	point = start+1

#Change the first one to 1 second and the latter half to 5 times the length.
DelayList[0][1]=100
for i in range(1,len(DelayList)//2):
	DelayList[-i][1]*=5

#Rewrite binary data.
for point,delay in DelayList:
	Bin = Bin[:point] + delay.to_bytes(2,byteorder="little") + Bin[point+2:]
	

#Save binary data as GIF.
with open('temp.gif','wb') as f:
	f.write(Bin)

Before All 20 images are set to 0.05 seconds.

After The first one was set to 1 second, and the latter half was rewritten 5 times.