Arduino eInk Image2LCD - Size of c-array

Question

This Image2LCD software ( https://www.buydisplay.com/default/image2lcd ) converts images to c-arrays. I want to write this basic operation myself, but I dont understand why the software outputs an array of length 5000 for an input image of size 200x200. For 400x400 the array size is 20000. It seems like its always 1/8 of the number of pixels.

The output array for the square 200x200 image begins and ends like this:

const unsigned char gImage_test[5000] = { /* 0X00,0X01,0XC8,0X00,0XC8,0X00, */
0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,
0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,
0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,
0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,
0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X60,0X00,0X00,0X00,0X00,
0X3C,0X60,0X00,0X0C,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,
0X00,0X00,0X00,0X00,0X70,0X00,0X00,0X00,0X00,0X7E,0X70,0X00,0X0E,0X00,0X00,0X00,
0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X78,0X00,0X00,
0X00,0X00,0X7F,0X78,0X00,0X0F,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,
0X00,0X00,0X00,0X00,0X00,0X00,0X7F,0XFC,0X3C,0X3E,0X3C,0X3F,0XF8,0X3C,0X7F,0X00,
0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X7F,

...

,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,
0X00,0X00,0X00,0X00,0X00,0X00,0X00,0X00,};

(Yes there is a lot of white in the image.)

Why don't you need one value for each pixel?

Answer 1

Shooting from the hip here, but if you're using monochrome, you only need one bit per pixel (Byte = 8 bits). These bits can be packed into bytes for storage efficiency. Say the first 8 pixels of your image are these:

0 1 0 0 0 0 0 1  

If we interpret these eight bits as one binary number, this is 1000001 , which is 65 in decimal - so just storing 65 in an 8-bit integer, taking up only one byte, will store all 8 monochrome pixels. The downside is that it's not as intuitive as having each pixel as a separate value in the array.

I may be wrong, but 1/8th points straight to this kind of compression.