ok call me crazy, but I have actually begun work on my own GIF decoder Arduino sketch.
If you really just see the GIF file format description as a "how-to" of how the bytes need to be decoded and analyzed, then it should be possible to just walk through all that, bit by bit (or rather: byte by byte 
), write code accordingly, and at the end hopefully display an image correctly.
Here's my code so far:
#include "logo.h"
void showGIF(uint16_t gifName, uint16_t width, uint16_t height, uint16_t xpos, uint16_t ypos) {
boolean goodFile = false;
char header[6];
byte headerElement;
// reading header
for (byte i = 0; i <= 5; i++) {
headerElement = pgm_read_byte(&MGlogo[i]);
header[i] = headerElement;
}
if (String(header).startsWith("GIF89a")) {
Serial.print("Header found: ");
Serial.println(header);
goodFile = true;
}
else Serial.println("Wrong file header found. Doing nothing.");
if (goodFile) {
//reading file width
int fileWidth = (pgm_read_byte(&MGlogo[6])) | (pgm_read_byte(&MGlogo[7]) << 8);
int fileHeight = (pgm_read_byte(&MGlogo[8])) | (pgm_read_byte(&MGlogo[9]) << 8);
Serial.println("-------------\nImage information:\n");
Serial.print("File dimensions: ");
Serial.print(fileWidth);
Serial.print(" x ");
Serial.print(fileHeight);
Serial.println(" px");
// Reading Packed Field byte and expanding it
byte packedField = pgm_read_byte(&MGlogo[10]);
Serial.print("Packed Field: ");
Serial.println(packedField, BIN);
byte globalColorTableFlag = bitRead(packedField, 7);
byte colorResolution = (bitRead(packedField, 6) <<2) | (bitRead(packedField, 5) << 1) | (bitRead(packedField, 4));
byte sortFlag = (bitRead(packedField, 4) >> 0);
byte globalColorTableSize = (bitRead(packedField, 0)) | (bitRead(packedField, 1) << 1) | (bitRead(packedField, 2) << 2);
globalColorTableSize = pow(2, globalColorTableSize) + 1;
Serial.print("Global color table: ");
if (globalColorTableFlag == 1) Serial.println("Y");
else Serial.println("N");
Serial.print("Global color resolution: ");
Serial.println(colorResolution);
Serial.print("Color sorting: ");
if (sortFlag == 1) Serial.println("decreasing");
else Serial.println("increasing");
Serial.print("Color table size: ");
Serial.print(globalColorTableSize);
Serial.println(" indexed colors");
}
}
void setup()
{
Serial.begin(115200);
showGIF(MGlogo, 30, 30, 0, 0);
}
void loop()
{
/* add main program code here */
}
I've attached all the files to this post.
A few hiccups so far; I can't do
pgm_read_byte(&gifName[i])
, it tells me "invalid types 'uint16_t {aka unsigned int}[byte {aka unsigned char}]' for array subscript". Not 100% sure what that's about.
Also, I'm still having trouble analyzing the "packed field" byte, which contains several bits of information. I get wrong values for color resolution (should be "8"), and the number of indexed colors (is 4 but should be 10).
According to sourceforge, the packed field byte looks like this:
My packed field byte I get from my code, when expanded, equals 10000100. So I am going to have to see why that is.
I may be wholely out of my depth here trying to write a GIF decoder function like this, but it kind of seems like a cool sub-project to take on while I will spend two weeks twiddling my thumbs waiting for my ESP32 module to arrive from China
mglogo-zip2.zip (2.38 KB)
