先来了解一下Gif图片的构成
大概是这样的组成
GIF图片的编码结构中各部分所占字节数的具体情况如下：
-
文件头（File Header）：通常占据6个字节。其中，GIF标识符 “GIF” 占3个字节，版本信息（如 “89a” 或 “87a”）占3个字节。
-
全局颜色表（Global Color Table）：占据的字节数取决于全局颜色表的大小。如果没有全局颜色表，这部分会被省略。每个颜色条目通常占3个字节（RGB颜色信息），因此颜色表的总字节数为3乘以颜色数量。
-
图形控制扩展（Graphic Control Extension）：占据8个字节。这部分包括固定的标识符（1字节），块大小（1字节），其它控制信息（6字节）。
-
图像数据（Image Data）：这部分的大小会变化，取决于图像的尺寸和颜色数。通常，每个图像数据块的前面都有一个图形控制扩展块（8字节）。
-
应用扩展（Application Extension）：占据至少14个字节。这部分包括固定的标识符（1字节），块大小（1字节），应用程序标识符（8字节），应用程序认证码（3字节），以及应用程序特定的数据。
-
注释（Comment Extension）：占据的字节数会根据注释的长度而变化，通常包含注释信息。
-
图像结束（Image Terminator）：通常占据1个字节，标志着图像数据的结束。
-
文件结束（Trailer）：通常占据1个字节，表示GIF文件的结束。
附上一张gif图片的编码图，因为没有可以读取的编码方式所以就是乱码了，不过不影响可以看一下前六个字节GIF89a这是标识版本，文件头（File Header）
突然有事，回来补上，未完待续
先把源码贴上吧，有急用的可以先拿去
package com.example.chapter12.util;
import java.io.InputStream;
import java.util.Vector;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
//Handler for read & extract Bitmap from *.gif
public class GifImage {
// to store *.gif data, Bitmap & delay
public static class GifFrame {
// to access image & delay w/o interface
public Bitmap image;
public int delay;
public GifFrame(Bitmap im, int del) {
image = im;
delay = del;
}
}
// to define some error type
public static final int STATUS_OK = 0;
public static final int STATUS_FORMAT_ERROR = 1;
public static final int STATUS_OPEN_ERROR = 2;
private int status;
private InputStream in;
private int width; // full image width
private int height; // full image height
private boolean gctFlag; // global color table used
private int gctSize; // size of global color table
private int loopCount = 1; // iterations; 0 = repeat forever
private int[] gct; // global color table
private int[] lct; // local color table
private int[] act; // active color table
private int bgIndex; // background color index
private int bgColor; // background color
private int lastBgColor; // previous bg color
private int pixelAspect; // pixel aspect ratio
private boolean lctFlag; // local color table flag
private boolean interlace; // interlace flag
private int lctSize; // local color table size
private int ix, iy, iw, ih; // current image rectangle
private int lrx, lry, lrw, lrh;
private Bitmap image; // current frame
private Bitmap lastImage; // previous frame
private int frameindex = 0;
public int getFrameindex() {
return frameindex;
}
public void setFrameindex(int frameindex) {
this.frameindex = frameindex;
if (frameindex > frames.size() - 1) {
frameindex = 0;
}
}
private byte[] block = new byte[256]; // current data block
private int blockSize = 0; // block size
// last graphic control extension info
private int dispose = 0;
// 0=no action; 1=leave in place; 2=restore to bg; 3=restore to prev
private int lastDispose = 0;
private boolean transparency = false; // use transparent color
private int delay = 0; // delay in milliseconds
private int transIndex; // transparent color index
private static final int MaxStackSize = 4096;
// max decoder pixel stack size
// LZW decoder working arrays
private short[] prefix;
private byte[] suffix;
private byte[] pixelStack;
private byte[] pixels;
private Vector<GifFrame> frames; // frames read from current file
private int frameCount;
// to get its Width / Height
public int getWidth() {
return width;
}
public int getHeigh() {
return height;
}
/**
* Gets display duration for specified frame.
*
* @param n int index of frame
* @return delay in milliseconds
*/
public int getDelay(int n) {
delay = -1;
if ((n >= 0) && (n < frameCount)) {
delay = ((GifFrame) frames.elementAt(n)).delay;
}
return delay;
}
public int getFrameCount() {
return frameCount;
}
public Bitmap getImage() {
return getFrame(0);
}
public int getLoopCount() {
return loopCount;
}
private void setPixels() {
int[] dest = new int[width * height];
// fill in starting image contents based on last image's dispose code
if (lastDispose > 0) {
if (lastDispose == 3) {
// use image before last
int n = frameCount - 2;
if (n > 0) {
lastImage = getFrame(n - 1);
} else {
lastImage = null;
}
}
if (lastImage != null) {
lastImage.getPixels(dest, 0, width, 0, 0, width, height);
// copy pixels
if (lastDispose == 2) {
// fill last image rect area with background color
int c = 0;
if (!transparency) {
c = lastBgColor;
}
for (int i = 0; i < lrh; i++) {
int n1 = (lry + i) * width + lrx;
int n2 = n1 + lrw;
for (int k = n1; k < n2; k++) {
dest[k] = c;
}
}
}
}
}
// copy each source line to the appropriate place in the destination
int pass = 1;
int inc = 8;
int iline = 0;
for (int i = 0; i < ih; i++) {
int line = i;
if (interlace) {
if (iline >= ih) {
pass++;
switch (pass) {
case 2:
iline = 4;
break;
case 3:
iline = 2;
inc = 4;
break;
case 4:
iline = 1;
inc = 2;
}
}
line = iline;
iline += inc;
}
line += iy;
if (line < height) {
int k = line * width;
int dx = k + ix; // start of line in dest
int dlim = dx + iw; // end of dest line
if ((k + width) < dlim) {
dlim = k + width; // past dest edge
}
int sx = i * iw; // start of line in source
while (dx < dlim) {
// map color and insert in destination
int index = ((int) pixels[sx++]) & 0xff;
int c = act[index];
if (c != 0) {
dest[dx] = c;
}
dx++;
}
}
}
image = Bitmap.createBitmap(dest, width, height, Config.RGB_565);
}
private Bitmap getFrame(int n) {
Bitmap im = null;
if ((n >= 0) && (n < frameCount)) {
im = ((GifFrame) frames.elementAt(n)).image;
}
return im;
}
public GifFrame[] getFrames() {
if (null != frames)
return frames.toArray(new GifFrame[0]);
return null;
}
public Bitmap nextBitmap() {
frameindex++;
if (frameindex > frames.size() - 1) {
frameindex = 0;
}
return ((GifFrame) frames.elementAt(frameindex)).image;
}
public int nextDelay() {
return ((GifFrame) frames.elementAt(frameindex)).delay;
}
// to read & parse all *.gif stream
public int read(InputStream is) {
init();
if (is != null) {
in = is;
readHeader();
if (!err()) {
readContents();
if (frameCount < 0) {
status = STATUS_FORMAT_ERROR;
}
}
} else {
status = STATUS_OPEN_ERROR;
}
try {
is.close();
} catch (Exception e) {
e.printStackTrace();
}
return status;
}
private void decodeImageData() {
int NullCode = -1;
int npix = iw * ih;
int available, clear, code_mask, code_size, end_of_information, in_code, old_code, bits, code, count, i, datum, data_size, first, top, bi, pi;
if ((pixels == null) || (pixels.length < npix)) {
pixels = new byte[npix]; // allocate new pixel array
}
if (prefix == null) {
prefix = new short[MaxStackSize];
}
if (suffix == null) {
suffix = new byte[MaxStackSize];
}
if (pixelStack == null) {
pixelStack = new byte[MaxStackSize + 1];
}
// Initialize GIF data stream decoder.
data_size = read();
clear = 1 << data_size;
end_of_information = clear + 1;
available = clear + 2;
old_code = NullCode;
code_size = data_size + 1;
code_mask = (1 << code_size) - 1;
for (code = 0; code < clear; code++) {
prefix[code] = 0;
suffix[code] = (byte) code;
}
// Decode GIF pixel stream.
datum = bits = count = first = top = pi = bi = 0;
for (i = 0; i < npix; ) {
if (top == 0) {
if (bits < code_size) {
// Load bytes until there are enough bits for a code.
if (count == 0) {
// Read a new data block.
count = readBlock();
if (count <= 0) {
break;
}
bi = 0;
}
datum += (((int) block[bi]) & 0xff) << bits;
bits += 8;
bi++;
count--;
continue;
}
// Get the next code.
code = datum & code_mask;
datum >>= code_size;
bits -= code_size;
// Interpret the code
if ((code > available) || (code == end_of_information)) {
break;
}
if (code == clear) {
// Reset decoder.
code_size = data_size + 1;
code_mask = (1 << code_size) - 1;
available = clear + 2;
old_code = NullCode;
continue;
}
if (old_code == NullCode) {
pixelStack[top++] = suffix[code];
old_code = code;
first = code;
continue;
}
in_code = code;
if (code == available) {
pixelStack[top++] = (byte) first;
code = old_code;
}
while (code > clear) {
pixelStack[top++] = suffix[code];
code = prefix[code];
}
first = ((int) suffix[code]) & 0xff;
// Add a new string to the string table,
if (available >= MaxStackSize) {
break;
}
pixelStack[top++] = (byte) first;
prefix[available] = (short) old_code;
suffix[available] = (byte) first;
available++;
if (((available & code_mask) == 0)
&& (available < MaxStackSize)) {
code_size++;
code_mask += available;
}
old_code = in_code;
}
// Pop a pixel off the pixel stack.
top--;
pixels[pi++] = pixelStack[top];
i++;
}
for (i = pi; i < npix; i++) {
pixels[i] = 0; // clear missing pixels
}
}
private boolean err() {
return status != STATUS_OK;
}
// to initia variable
private void init() {
status = STATUS_OK;
frameCount = 0;
frames = new Vector<GifFrame>();
gct = null;
lct = null;
}
private int read() {
int curByte = 0;
try {
curByte = in.read();
} catch (Exception e) {
status = STATUS_FORMAT_ERROR;
}
return curByte;
}
private int readBlock() {
blockSize = read();
int n = 0;
if (blockSize > 0) {
try {
int count = 0;
while (n < blockSize) {
count = in.read(block, n, blockSize - n);
if (count == -1) {
break;
}
n += count;
}
} catch (Exception e) {
e.printStackTrace();
}
if (n < blockSize) {
status = STATUS_FORMAT_ERROR;
}
}
return n;
}
// Global Color Table
private int[] readColorTable(int ncolors) {
int nbytes = 3 * ncolors;
int[] tab = null;
byte[] c = new byte[nbytes];
int n = 0;
try {
n = in.read(c);
} catch (Exception e) {
e.printStackTrace();
}
if (n < nbytes) {
status = STATUS_FORMAT_ERROR;
} else {
tab = new int[256]; // max size to avoid bounds checks
int i = 0;
int j = 0;
while (i < ncolors) {
int r = ((int) c[j++]) & 0xff;
int g = ((int) c[j++]) & 0xff;
int b = ((int) c[j++]) & 0xff;
tab[i++] = 0xff000000 | (r << 16) | (g << 8) | b;
}
}
return tab;
}
// Image Descriptor
private void readContents() {
// read GIF file content blocks
boolean done = false;
while (!(done || err())) {
int code = read();
switch (code) {
case 0x2C: // image separator
readImage();
break;
case 0x21: // extension
code = read();
switch (code) {
case 0xf9: // graphics control extension
readGraphicControlExt();
break;
case 0xff: // application extension
readBlock();
String app = "";
for (int i = 0; i < 11; i++) {
app += (char) block[i];
}
if (app.equals("NETSCAPE2.0")) {
readNetscapeExt();
} else {
skip(); // don't care
}
break;
default: // uninteresting extension
skip();
}
break;
case 0x3b: // terminator
done = true;
break;
case 0x00: // bad byte, but keep going and see what happens
break;
default:
status = STATUS_FORMAT_ERROR;
}
}
}
private void readGraphicControlExt() {
read(); // block size
int packed = read(); // packed fields
dispose = (packed & 0x1c) >> 2; // disposal method
if (dispose == 0) {
dispose = 1; // elect to keep old image if discretionary
}
transparency = (packed & 1) != 0;
delay = readShort() * 10; // delay in milliseconds
transIndex = read(); // transparent color index
read(); // block terminator
}
// to get Stream - Head
private void readHeader() {
String id = "";
for (int i = 0; i < 6; i++) {
id += (char) read();
}
if (!id.toUpperCase().startsWith("GIF")) {
status = STATUS_FORMAT_ERROR;
return;
}
readLSD();
if (gctFlag && !err()) {
gct = readColorTable(gctSize);
bgColor = gct[bgIndex];
}
}
private void readImage() {
// offset of X
ix = readShort(); // (sub)image position & size
// offset of Y
iy = readShort();
// width of bitmap
iw = readShort();
// height of bitmap
ih = readShort();
// Local Color Table Flag
int packed = read();
lctFlag = (packed & 0x80) != 0; // 1 - local color table flag
// Interlace Flag, to array with interwoven if ENABLE, with order
// otherwise
interlace = (packed & 0x40) != 0; // 2 - interlace flag
// 3 - sort flag
// 4-5 - reserved
lctSize = 2 << (packed & 7); // 6-8 - local color table size
if (lctFlag) {
lct = readColorTable(lctSize); // read table
act = lct; // make local table active
} else {
act = gct; // make global table active
if (bgIndex == transIndex) {
bgColor = 0;
}
}
int save = 0;
if (transparency) {
save = act[transIndex];
act[transIndex] = 0; // set transparent color if specified
}
if (act == null) {
status = STATUS_FORMAT_ERROR; // no color table defined
}
if (err()) {
return;
}
decodeImageData(); // decode pixel data
skip();
if (err()) {
return;
}
frameCount++;
// create new image to receive frame data
image = Bitmap.createBitmap(width, height, Config.RGB_565);
// createImage(width, height);
setPixels(); // transfer pixel data to image
frames.addElement(new GifFrame(image, delay)); // add image to frame
// list
if (transparency) {
act[transIndex] = save;
}
resetFrame();
}
// Logical Screen Descriptor
private void readLSD() {
// logical screen size
width = readShort();
height = readShort();
// packed fields
int packed = read();
gctFlag = (packed & 0x80) != 0; // 1 : global color table flag
// 2-4 : color resolution
// 5 : gct sort flag
gctSize = 2 << (packed & 7); // 6-8 : gct size
bgIndex = read(); // background color index
pixelAspect = read(); // pixel aspect ratio
}
private void readNetscapeExt() {
do {
readBlock();
if (block[0] == 1) {
// loop count sub-block
int b1 = ((int) block[1]) & 0xff;
int b2 = ((int) block[2]) & 0xff;
loopCount = (b2 << 8) | b1;
}
} while ((blockSize > 0) && !err());
}
// read 8 bit data
private int readShort() {
// read 16-bit value, LSB first
return read() | (read() << 8);
}
private void resetFrame() {
lastDispose = dispose;
lrx = ix;
lry = iy;
lrw = iw;
lrh = ih;
lastImage = image;
lastBgColor = bgColor;
dispose = 0;
transparency = false;
delay = 0;
lct = null;
}
/**
* Skips variable length blocks up to and including next zero length block.
*/
private void skip() {
do {
readBlock();
} while ((blockSize > 0) && !err());
}
}
