Added gif file writter library

Setup for a new amazing feature! ;)

1 files changed, 931 insertions, 0 deletions

diff --git a/src/external/gif.h b/src/external/gif.h
new file mode 100644
index 00000000..1c0aeb18
--- /dev/null
+++ b/src/external/gif.h
@@ -0,0 +1,931 @@
+/**********************************************************************************************
+*
+*   gif.h by Charlie Tangora [ctangora -at- gmail -dot- com]
+*   adapted to C99 and reformatted by Ramon Santamaria (@raysan5)
+*
+*   This file offers a simple, very limited way to create animated GIFs directly in code.
+*
+*   Those looking for particular cleverness are likely to be disappointed; it's pretty 
+*   much a straight-ahead implementation of the GIF format with optional Floyd-Steinberg 
+*   dithering. (It does at least use delta encoding - only the changed portions of each 
+*   frame are saved.) 
+*
+*   So resulting files are often quite large. The hope is that it will be handy nonetheless
+*   as a quick and easily-integrated way for programs to spit out animations.
+*
+*   Only RGBA8 is currently supported as an input format. (The alpha is ignored.)
+*
+*   CONFIGURATION:
+*
+*   #define GIF_IMPLEMENTATION
+*       Generates the implementation of the library into the included file.
+*       If not defined, the library is in header only mode and can be included in other headers
+*       or source files without problems. But only ONE file should hold the implementation.
+*
+*   USAGE:
+*       1) Create a GifWriter struct. Pass it to GifBegin() to initialize and write the header.
+*       2) Pass subsequent frames to GifWriteFrame().
+*       3) Finally, call GifEnd() to close the file handle and free memory.
+*
+*
+*   LICENSE: public domain (www.unlicense.org)
+*
+*   This is free and unencumbered software released into the public domain.
+*   Anyone is free to copy, modify, publish, use, compile, sell, or distribute this
+*   software, either in source code form or as a compiled binary, for any purpose,
+*   commercial or non-commercial, and by any means.
+*   
+*   In jurisdictions that recognize copyright laws, the author or authors of this
+*   software dedicate any and all copyright interest in the software to the public
+*   domain. We make this dedication for the benefit of the public at large and to
+*   the detriment of our heirs and successors. We intend this dedication to be an
+*   overt act of relinquishment in perpetuity of all present and future rights to
+*   this software under copyright law.
+*   
+*   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+*   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+*   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+*   AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+*   ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+*   WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+*
+**********************************************************************************************/
+
+#ifndef GIF_H
+#define GIF_H
+
+#include <stdio.h>          // Required for: FILE
+#include <stdint.h>         // Required for for integer typedefs
+
+//#define GIF_STATIC
+#ifdef GIF_STATIC
+    #define GIFDEF static              // Functions just visible to module including this file
+#else
+    #ifdef __cplusplus
+        #define GIFDEF extern "C"      // Functions visible from other files (no name mangling of functions in C++)
+    #else
+        #define GIFDEF extern          // Functions visible from other files
+    #endif
+#endif
+
+//----------------------------------------------------------------------------------
+// Defines and Macros
+//----------------------------------------------------------------------------------
+//#define MAX_RESOURCES_SUPPORTED   256
+
+//----------------------------------------------------------------------------------
+// Types and Structures Definition
+//----------------------------------------------------------------------------------
+typedef struct GifWriter {
+    FILE *f;
+    uint8_t *oldImage;
+    bool firstFrame;
+} GifWriter;
+
+//----------------------------------------------------------------------------------
+// Global variables
+//----------------------------------------------------------------------------------
+//...
+
+//----------------------------------------------------------------------------------
+// Module Functions Declaration
+//----------------------------------------------------------------------------------
+
+// NOTE: By default use bitDepth = 8, dither = false
+GIFDEF bool GifBegin(GifWriter *writer, const char *filename, uint32_t width, uint32_t height, uint32_t delay, int32_t bitDepth, bool dither);
+GIFDEF bool GifWriteFrame(GifWriter *writer, const uint8_t *image, uint32_t width, uint32_t height, uint32_t delay, int bitDepth, bool dither);
+GIFDEF bool GifEnd(GifWriter *writer);
+
+#endif // GIF_H
+
+
+/***********************************************************************************
+*
+ *  GIF IMPLEMENTATION
+*
+************************************************************************************/
+
+#if defined(GIF_IMPLEMENTATION)
+
+#include <stdio.h>          // Required for: FILE, fopen(), fclose()
+#include <string.h>         // Required for: memcpy()
+#include <stdint.h>         // Required for for integer typedefs
+
+// Define these macros to hook into a custom memory allocator.
+// GIF_TEMP_MALLOC and GIF_TEMP_FREE will only be called in stack fashion - frees in the reverse order of mallocs
+// and any temp memory allocated by a function will be freed before it exits.
+#if !defined(GIF_TEMP_MALLOC)
+    #include <stdlib.h>
+    
+    #define GIF_TEMP_MALLOC malloc
+    #define GIF_TEMP_FREE free
+#endif
+
+// Check if custom malloc/free functions defined, if not, using standard ones
+// GIF_MALLOC and GIF_FREE are used only by GifBegin and GifEnd respectively, 
+// to allocate a buffer the size of the image, which is used to find changed pixels for delta-encoding.
+#if !defined(GIF_MALLOC)
+    #include <stdlib.h>     // Required for: malloc(), free()
+
+    #define GIF_MALLOC(size)  malloc(size)
+    #define GIF_FREE(ptr)     free(ptr)
+#endif
+
+//----------------------------------------------------------------------------------
+// Defines and Macros
+//----------------------------------------------------------------------------------
+#define GIFMIN(a, b) (((a)<(b))?(a):(b))
+#define GIFMAX(a, b) (((a)>(b))?(a):(b))
+#define GIFABS(x) ((x)<0?-(x):(x))
+
+//----------------------------------------------------------------------------------
+// Types and Structures Definition
+//----------------------------------------------------------------------------------
+
+//Gif palette structure
+typedef struct GifPalette {
+    int bitDepth;
+
+    uint8_t r[256];
+    uint8_t g[256];
+    uint8_t b[256];
+    
+    // k-d tree over RGB space, organized in heap fashion
+    // i.e. left child of node i is node i*2, right child is node i*2 + 1
+    // nodes 256-511 are implicitly the leaves, containing a color
+    uint8_t treeSplitElt[255];
+    uint8_t treeSplit[255];
+} GifPalette;
+
+
+// Simple structure to write out the LZW-compressed 
+// portion of the imageone bit at a time
+typedef struct GifBitStatus {
+    uint8_t bitIndex;  // how many bits in the partial byte written so far
+    uint8_t byte;      // current partial byte
+    
+    uint32_t chunkIndex;
+    uint8_t chunk[256];   // bytes are written in here until we have 256 of them, then written to the file
+} GifBitStatus;
+
+// The LZW dictionary is a 256-ary tree constructed 
+// as the file is encoded, this is one node
+typedef struct GifLzwNode {
+    uint16_t m_next[256];
+} GifLzwNode;
+
+//----------------------------------------------------------------------------------
+// Global Variables Definition
+//----------------------------------------------------------------------------------
+const int kGifTransIndex = 0;
+
+//----------------------------------------------------------------------------------
+// Module specific Functions Declaration
+//----------------------------------------------------------------------------------
+static void GifGetClosestPaletteColor(GifPalette *pPal, int r, int g, int b, int *bestInd, int *bestDiff, int treeRoot);
+static void GifSwapPixels(uint8_t *image, int pixA, int pixB);
+static int GifPartition(uint8_t *image, const int left, const int right, const int elt, int pivotIndex);
+static void GifPartitionByMedian(uint8_t *image, int left, int right, int com, int neededCenter);
+static void GifSplitPalette(uint8_t *image, int numPixels, int firstElt, int lastElt, int splitElt, int splitDist, int treeNode, bool buildForDither, GifPalette *pal);
+static int GifPickChangedPixels(const uint8_t *lastFrame, uint8_t *frame, int numPixels);
+static void GifMakePalette(const uint8_t *lastFrame, const uint8_t *nextFrame, uint32_t width, uint32_t height, int bitDepth, bool buildForDither, GifPalette *pPal);
+static void GifDitherImage(const uint8_t *lastFrame, const uint8_t *nextFrame, uint8_t *outFrame, uint32_t width, uint32_t height, GifPalette *pPal);
+static void GifThresholdImage(const uint8_t *lastFrame, const uint8_t *nextFrame, uint8_t *outFrame, uint32_t width, uint32_t height, GifPalette *pPal);
+static void GifWriteBit(GifBitStatus *stat, uint32_t bit);
+static void GifWriteChunk(FILE *f, GifBitStatus *stat);
+static void GifWriteCode(FILE *f, GifBitStatus *stat, uint32_t code, uint32_t length);
+static void GifWritePalette(const GifPalette *pPal, FILE *f);
+static void GifWriteLzwImage(FILE *f, uint8_t *image, uint32_t left, uint32_t top, uint32_t width, uint32_t height, uint32_t delay, GifPalette *pPal);
+
+//----------------------------------------------------------------------------------
+// Module Functions Definition
+//----------------------------------------------------------------------------------
+
+// Creates a gif file.
+// The input GIFWriter is assumed to be uninitialized.
+// The delay value is the time between frames in hundredths of a second - note that not all viewers pay much attention to this value.
+GIFDEF bool GifBegin(GifWriter *writer, const char *filename, uint32_t width, uint32_t height, uint32_t delay, int32_t bitDepth, bool dither)
+{
+#if _MSC_VER >= 1400
+	writer->f = 0;
+    fopen_s(&writer->f, filename, "wb");
+#else
+    writer->f = fopen(filename, "wb");
+#endif
+    if (!writer->f) return false;
+    
+    writer->firstFrame = true;
+    
+    // allocate 
+    writer->oldImage = (uint8_t*)GIF_MALLOC(width*height*4);
+    
+    fputs("GIF89a", writer->f);
+    
+    // screen descriptor
+    fputc(width & 0xff, writer->f);
+    fputc((width >> 8) & 0xff, writer->f);
+    fputc(height & 0xff, writer->f);
+    fputc((height >> 8) & 0xff, writer->f);
+    
+    fputc(0xf0, writer->f);  // there is an unsorted global color table of 2 entries
+    fputc(0, writer->f);     // background color
+    fputc(0, writer->f);     // pixels are square (we need to specify this because it's 1989)
+    
+    // now the "global" palette (really just a dummy palette)
+    // color 0: black
+    fputc(0, writer->f);
+    fputc(0, writer->f); 
+    fputc(0, writer->f);
+    // color 1: also black
+    fputc(0, writer->f);
+    fputc(0, writer->f);
+    fputc(0, writer->f);
+    
+    if (delay != 0)
+    {
+        // animation header
+        fputc(0x21, writer->f); // extension
+        fputc(0xff, writer->f); // application specific
+        fputc(11, writer->f); // length 11
+        fputs("NETSCAPE2.0", writer->f); // yes, really
+        fputc(3, writer->f); // 3 bytes of NETSCAPE2.0 data
+        
+        fputc(1, writer->f); // JUST BECAUSE
+        fputc(0, writer->f); // loop infinitely (byte 0)
+        fputc(0, writer->f); // loop infinitely (byte 1)
+        
+        fputc(0, writer->f); // block terminator
+    }
+    
+    return true;
+}
+
+// Writes out a new frame to a GIF in progress.
+// The GIFWriter should have been created by GIFBegin.
+// AFAIK, it is legal to use different bit depths for different frames of an image -
+// this may be handy to save bits in animations that don't change much.
+GIFDEF bool GifWriteFrame(GifWriter *writer, const uint8_t *image, uint32_t width, uint32_t height, uint32_t delay, int bitDepth, bool dither)
+{
+    if (!writer->f) return false;
+    
+    const uint8_t *oldImage = writer->firstFrame? NULL : writer->oldImage;
+    writer->firstFrame = false;
+    
+    GifPalette pal;
+    GifMakePalette((dither? NULL : oldImage), image, width, height, bitDepth, dither, &pal);
+    
+    if (dither)
+        GifDitherImage(oldImage, image, writer->oldImage, width, height, &pal);
+    else
+        GifThresholdImage(oldImage, image, writer->oldImage, width, height, &pal);
+    
+    GifWriteLzwImage(writer->f, writer->oldImage, 0, 0, width, height, delay, &pal);
+    
+    return true;
+}
+
+// Writes the EOF code, closes the file handle, and frees temp memory used by a GIF.
+// Many if not most viewers will still display a GIF properly if the EOF code is missing,
+// but it's still a good idea to write it out.
+GIFDEF bool GifEnd(GifWriter *writer)
+{
+    if (!writer->f) return false;
+    
+    fputc(0x3b, writer->f); // end of file
+    fclose(writer->f);
+    GIF_FREE(writer->oldImage);
+    
+    writer->f = NULL;
+    writer->oldImage = NULL;
+    
+    return true;
+}
+
+//----------------------------------------------------------------------------------
+// Module specific Functions Definition
+//----------------------------------------------------------------------------------
+// walks the k-d tree to pick the palette entry for a desired color.
+// Takes as in/out parameters the current best color and its error -
+// only changes them if it finds a better color in its subtree.
+// this is the major hotspot in the code at the moment.
+static void GifGetClosestPaletteColor(GifPalette *pPal, int r, int g, int b, int *bestInd, int *bestDiff, int treeRoot)
+{
+    // base case, reached the bottom of the tree
+    if (treeRoot > (1<<pPal->bitDepth)-1)
+    {
+        int ind = treeRoot-(1<<pPal->bitDepth);
+        if (ind == kGifTransIndex) return;
+        
+        // check whether this color is better than the current winner
+        int r_err = r - ((int32_t)pPal->r[ind]);
+        int g_err = g - ((int32_t)pPal->g[ind]);
+        int b_err = b - ((int32_t)pPal->b[ind]);
+        int diff = GIFABS(r_err)+GIFABS(g_err)+GIFABS(b_err);
+        
+        if (diff < *bestDiff)
+        {
+            *bestInd = ind;
+            *bestDiff = diff;
+        }
+        
+        return;
+    }
+    
+    // take the appropriate color (r, g, or b) for this node of the k-d tree
+    int comps[3]; comps[0] = r; comps[1] = g; comps[2] = b;
+    int splitComp = comps[pPal->treeSplitElt[treeRoot]];
+    
+    int splitPos = pPal->treeSplit[treeRoot];
+    if (splitPos > splitComp)
+    {
+        // check the left subtree
+        GifGetClosestPaletteColor(pPal, r, g, b, bestInd, bestDiff, treeRoot*2);
+        
+        if (*bestDiff > (splitPos - splitComp))
+        {
+            // cannot prove there's not a better value in the right subtree, check that too
+            GifGetClosestPaletteColor(pPal, r, g, b, bestInd, bestDiff, treeRoot*2 + 1);
+        }
+    }
+    else
+    {
+        GifGetClosestPaletteColor(pPal, r, g, b, bestInd, bestDiff, treeRoot*2 + 1);
+        
+        if (*bestDiff > splitComp - splitPos)
+        {
+            GifGetClosestPaletteColor(pPal, r, g, b, bestInd, bestDiff, treeRoot*2);
+        }
+    }
+}
+
+static void GifSwapPixels(uint8_t *image, int pixA, int pixB)
+{
+    uint8_t rA = image[pixA*4];
+    uint8_t gA = image[pixA*4 + 1];
+    uint8_t bA = image[pixA*4+2];
+    uint8_t aA = image[pixA*4+3];
+    
+    uint8_t rB = image[pixB*4];
+    uint8_t gB = image[pixB*4 + 1];
+    uint8_t bB = image[pixB*4+2];
+    uint8_t aB = image[pixA*4+3];
+    
+    image[pixA*4] = rB;
+    image[pixA*4 + 1] = gB;
+    image[pixA*4+2] = bB;
+    image[pixA*4+3] = aB;
+    
+    image[pixB*4] = rA;
+    image[pixB*4 + 1] = gA;
+    image[pixB*4+2] = bA;
+    image[pixB*4+3] = aA;
+}
+
+// just the partition operation from quicksort
+static int GifPartition(uint8_t *image, const int left, const int right, const int elt, int pivotIndex)
+{
+    const int pivotValue = image[(pivotIndex)*4+elt];
+    GifSwapPixels(image, pivotIndex, right-1);
+    int storeIndex = left;
+    bool split = 0;
+    for (int ii=left; ii<right-1; ++ii)
+    {
+        int arrayVal = image[ii*4+elt];
+        if (arrayVal < pivotValue)
+        {
+            GifSwapPixels(image, ii, storeIndex);
+            ++storeIndex;
+        }
+        else if (arrayVal == pivotValue)
+        {
+            if (split)
+            {
+                GifSwapPixels(image, ii, storeIndex);
+                ++storeIndex;
+            }
+            split = !split;
+        }
+    }
+    GifSwapPixels(image, storeIndex, right-1);
+    return storeIndex;
+}
+
+// Perform an incomplete sort, finding all elements above and below the desired median
+static void GifPartitionByMedian(uint8_t *image, int left, int right, int com, int neededCenter)
+{
+    if (left < right-1)
+    {
+        int pivotIndex = left + (right-left)/2;
+    
+        pivotIndex = GifPartition(image, left, right, com, pivotIndex);
+        
+        // Only "sort" the section of the array that contains the median
+        if (pivotIndex > neededCenter)
+            GifPartitionByMedian(image, left, pivotIndex, com, neededCenter);
+        
+        if (pivotIndex < neededCenter)
+            GifPartitionByMedian(image, pivotIndex + 1, right, com, neededCenter);
+    }
+}
+
+// Builds a palette by creating a balanced k-d tree of all pixels in the image
+static void GifSplitPalette(uint8_t *image, int numPixels, int firstElt, int lastElt, int splitElt, int splitDist, 
+                            int treeNode, bool buildForDither, GifPalette *pal)
+{
+    if (lastElt <= firstElt || numPixels == 0)
+        return;
+    
+    // base case, bottom of the tree
+    if (lastElt == firstElt + 1)
+    {
+        if (buildForDither)
+        {
+            // Dithering needs at least one color as dark as anything
+            // in the image and at least one brightest color -
+            // otherwise it builds up error and produces strange artifacts
+            if (firstElt == 1)
+            {
+                // special case: the darkest color in the image
+                uint32_t r=255, g=255, b=255;
+                for (int ii=0; ii<numPixels; ++ii)
+                {
+                    r = GIFMIN(r, image[ii*4+0]);
+                    g = GIFMIN(g, image[ii*4 + 1]);
+                    b = GIFMIN(b, image[ii*4+2]);
+                }
+                
+                pal->r[firstElt] = r;
+                pal->g[firstElt] = g;
+                pal->b[firstElt] = b;
+                
+                return;
+            }
+            
+            if (firstElt == (1 << pal->bitDepth)-1)
+            {
+                // special case: the lightest color in the image
+                uint32_t r=0, g=0, b=0;
+                for (int ii=0; ii<numPixels; ++ii)
+                {
+                    r = GIFMAX(r, image[ii*4+0]);
+                    g = GIFMAX(g, image[ii*4 + 1]);
+                    b = GIFMAX(b, image[ii*4+2]);
+                }
+                
+                pal->r[firstElt] = r;
+                pal->g[firstElt] = g;
+                pal->b[firstElt] = b;
+                
+                return;
+            }
+        }
+        
+        // otherwise, take the average of all colors in this subcube
+        uint64_t r=0, g=0, b=0;
+        for (int ii=0; ii<numPixels; ++ii)
+        {
+            r += image[ii*4+0];
+            g += image[ii*4 + 1];
+            b += image[ii*4+2];
+        }
+        
+        r += numPixels / 2;  // round to nearest
+        g += numPixels / 2;
+        b += numPixels / 2;
+        
+        r /= numPixels;
+        g /= numPixels;
+        b /= numPixels;
+        
+        pal->r[firstElt] = (uint8_t)r;
+        pal->g[firstElt] = (uint8_t)g;
+        pal->b[firstElt] = (uint8_t)b;
+        
+        return;
+    }
+    
+    // Find the axis with the largest range
+    int minR = 255, maxR = 0;
+    int minG = 255, maxG = 0;
+    int minB = 255, maxB = 0;
+    for (int ii=0; ii<numPixels; ++ii)
+    {
+        int r = image[ii*4+0];
+        int g = image[ii*4 + 1];
+        int b = image[ii*4+2];
+        
+        if (r > maxR) maxR = r;
+        if (r < minR) minR = r;
+        
+        if (g > maxG) maxG = g;
+        if (g < minG) minG = g;
+        
+        if (b > maxB) maxB = b;
+        if (b < minB) minB = b;
+    }
+    
+    int rRange = maxR - minR;
+    int gRange = maxG - minG;
+    int bRange = maxB - minB;
+    
+    // and split along that axis. (incidentally, this means this isn't a "proper" k-d tree but I don't know what else to call it)
+    int splitCom = 1;
+    if (bRange > gRange) splitCom = 2;
+    if (rRange > bRange && rRange > gRange) splitCom = 0;
+    
+    int subPixelsA = numPixels  *(splitElt - firstElt) / (lastElt - firstElt);
+    int subPixelsB = numPixels-subPixelsA;
+    
+    GifPartitionByMedian(image, 0, numPixels, splitCom, subPixelsA);
+    
+    pal->treeSplitElt[treeNode] = splitCom;
+    pal->treeSplit[treeNode] = image[subPixelsA*4+splitCom];
+    
+    GifSplitPalette(image,              subPixelsA, firstElt, splitElt, splitElt-splitDist, splitDist/2, treeNode*2,   buildForDither, pal);
+    GifSplitPalette(image+subPixelsA*4, subPixelsB, splitElt, lastElt,  splitElt+splitDist, splitDist/2, treeNode*2 + 1, buildForDither, pal);
+}
+
+// Finds all pixels that have changed from the previous image and
+// moves them to the fromt of th buffer.
+// This allows us to build a palette optimized for the colors of the
+// changed pixels only.
+static int GifPickChangedPixels(const uint8_t *lastFrame, uint8_t *frame, int numPixels)
+{
+    int numChanged = 0;
+    uint8_t *writeIter = frame;
+    
+    for (int ii=0; ii<numPixels; ++ii)
+    {
+        if (lastFrame[0] != frame[0] ||
+           lastFrame[1] != frame[1] ||
+           lastFrame[2] != frame[2])
+        {
+            writeIter[0] = frame[0];
+            writeIter[1] = frame[1];
+            writeIter[2] = frame[2];
+            ++numChanged;
+            writeIter += 4;
+        }
+        lastFrame += 4;
+        frame += 4;
+    }
+    
+    return numChanged;
+}
+
+// Creates a palette by placing all the image pixels in a k-d tree and then averaging the blocks at the bottom.
+// This is known as the "modified median split" technique
+static void GifMakePalette(const uint8_t *lastFrame, const uint8_t *nextFrame, uint32_t width, uint32_t height, int bitDepth, bool buildForDither, GifPalette *pPal)
+{
+    pPal->bitDepth = bitDepth;
+    
+    // SplitPalette is destructive (it sorts the pixels by color) so
+    // we must create a copy of the image for it to destroy
+    int imageSize = width*height*4*sizeof(uint8_t);
+    uint8_t *destroyableImage = (uint8_t*)GIF_TEMP_MALLOC(imageSize);
+    memcpy(destroyableImage, nextFrame, imageSize);
+    
+    int numPixels = width*height;
+    if (lastFrame)
+        numPixels = GifPickChangedPixels(lastFrame, destroyableImage, numPixels);
+    
+    const int lastElt = 1 << bitDepth;
+    const int splitElt = lastElt/2;
+    const int splitDist = splitElt/2;
+    
+    GifSplitPalette(destroyableImage, numPixels, 1, lastElt, splitElt, splitDist, 1, buildForDither, pPal);
+    
+    GIF_TEMP_FREE(destroyableImage);
+    
+    // add the bottom node for the transparency index
+    pPal->treeSplit[1 << (bitDepth-1)] = 0;
+    pPal->treeSplitElt[1 << (bitDepth-1)] = 0;
+    
+    pPal->r[0] = pPal->g[0] = pPal->b[0] = 0;
+}
+
+// Implements Floyd-Steinberg dithering, writes palette value to alpha
+static void GifDitherImage(const uint8_t *lastFrame, const uint8_t *nextFrame, uint8_t *outFrame, uint32_t width, uint32_t height, GifPalette *pPal)
+{
+    int numPixels = width*height;
+    
+    // quantPixels initially holds color*256 for all pixels
+    // The extra 8 bits of precision allow for sub-single-color error values
+    // to be propagated
+    int32_t *quantPixels = (int32_t*)GIF_TEMP_MALLOC(sizeof(int32_t)*numPixels*4);
+    
+    for (int ii=0; ii<numPixels*4; ++ii)
+    {
+        uint8_t pix = nextFrame[ii];
+        int32_t pix16 = (int32_t)pix*256;
+        quantPixels[ii] = pix16;
+    }
+    
+    for (uint32_t yy=0; yy<height; ++yy)
+    {
+        for (uint32_t xx=0; xx<width; ++xx)
+        {
+            int32_t *nextPix = quantPixels + 4*(yy*width+xx);
+            const uint8_t *lastPix = lastFrame? lastFrame + 4*(yy*width+xx) : NULL;
+            
+            // Compute the colors we want (rounding to nearest)
+            int32_t rr = (nextPix[0] + 127) / 256;
+            int32_t gg = (nextPix[1] + 127) / 256;
+            int32_t bb = (nextPix[2] + 127) / 256;
+            
+            // if it happens that we want the color from last frame, then just write out
+            // a transparent pixel
+            if (lastFrame &&
+               lastPix[0] == rr &&
+               lastPix[1] == gg &&
+               lastPix[2] == bb)
+            {
+                nextPix[0] = rr;
+                nextPix[1] = gg;
+                nextPix[2] = bb;
+                nextPix[3] = kGifTransIndex;
+                continue;
+            }
+            
+            int32_t bestDiff = 1000000;
+            int32_t bestInd = kGifTransIndex;
+            
+            // Search the palete
+            GifGetClosestPaletteColor(pPal, rr, gg, bb, &bestInd, &bestDiff, 1);
+            
+            // Write the result to the temp buffer
+            int32_t r_err = nextPix[0] - (int32_t)(pPal->r[bestInd])*256;
+            int32_t g_err = nextPix[1] - (int32_t)(pPal->g[bestInd])*256;
+            int32_t b_err = nextPix[2] - (int32_t)(pPal->b[bestInd])*256;
+            
+            nextPix[0] = pPal->r[bestInd];
+            nextPix[1] = pPal->g[bestInd];
+            nextPix[2] = pPal->b[bestInd];
+            nextPix[3] = bestInd;
+            
+            // Propagate the error to the four adjacent locations
+            // that we haven't touched yet
+            int quantloc_7 = (yy*width+xx + 1);
+            int quantloc_3 = (yy*width+width+xx-1);
+            int quantloc_5 = (yy*width+width+xx);
+            int quantloc_1 = (yy*width+width+xx + 1);
+            
+            if (quantloc_7 < numPixels)
+            {
+                int32_t *pix7 = quantPixels+4*quantloc_7;
+                pix7[0] += GIFMAX(-pix7[0], r_err*7 / 16);
+                pix7[1] += GIFMAX(-pix7[1], g_err*7 / 16);
+                pix7[2] += GIFMAX(-pix7[2], b_err*7 / 16);
+            }
+            
+            if (quantloc_3 < numPixels)
+            {
+                int32_t *pix3 = quantPixels+4*quantloc_3;
+                pix3[0] += GIFMAX(-pix3[0], r_err*3 / 16);
+                pix3[1] += GIFMAX(-pix3[1], g_err*3 / 16);
+                pix3[2] += GIFMAX(-pix3[2], b_err*3 / 16);
+            }
+            
+            if (quantloc_5 < numPixels)
+            {
+                int32_t *pix5 = quantPixels+4*quantloc_5;
+                pix5[0] += GIFMAX(-pix5[0], r_err*5 / 16);
+                pix5[1] += GIFMAX(-pix5[1], g_err*5 / 16);
+                pix5[2] += GIFMAX(-pix5[2], b_err*5 / 16);
+            }
+            
+            if (quantloc_1 < numPixels)
+            {
+                int32_t *pix1 = quantPixels+4*quantloc_1;
+                pix1[0] += GIFMAX(-pix1[0], r_err / 16);
+                pix1[1] += GIFMAX(-pix1[1], g_err / 16);
+                pix1[2] += GIFMAX(-pix1[2], b_err / 16);
+            }
+        }
+    }
+    
+    // Copy the palettized result to the output buffer
+    for (int ii=0; ii<numPixels*4; ++ii)
+    {
+        outFrame[ii] = quantPixels[ii];
+    }
+    
+    GIF_TEMP_FREE(quantPixels);
+}
+
+// Picks palette colors for the image using simple thresholding, no dithering
+static void GifThresholdImage(const uint8_t *lastFrame, const uint8_t *nextFrame, uint8_t *outFrame, uint32_t width, uint32_t height, GifPalette *pPal)
+{
+    uint32_t numPixels = width*height;
+    for (uint32_t ii=0; ii<numPixels; ++ii)
+    {
+        // if a previous color is available, and it matches the current color,
+        // set the pixel to transparent
+        if (lastFrame &&
+           lastFrame[0] == nextFrame[0] &&
+           lastFrame[1] == nextFrame[1] &&
+           lastFrame[2] == nextFrame[2])
+        {
+            outFrame[0] = lastFrame[0];
+            outFrame[1] = lastFrame[1];
+            outFrame[2] = lastFrame[2];
+            outFrame[3] = kGifTransIndex;
+        }
+        else
+        {
+            // palettize the pixel
+            int32_t bestDiff = 1000000;
+            int32_t bestInd = 1;
+            GifGetClosestPaletteColor(pPal, nextFrame[0], nextFrame[1], nextFrame[2], &bestInd, &bestDiff, 1);
+            
+            // Write the resulting color to the output buffer
+            outFrame[0] = pPal->r[bestInd];
+            outFrame[1] = pPal->g[bestInd];
+            outFrame[2] = pPal->b[bestInd];
+            outFrame[3] = bestInd;
+        }
+        
+        if (lastFrame) lastFrame += 4;
+        outFrame += 4;
+        nextFrame += 4;
+    }
+}
+
+
+// insert a single bit
+static void GifWriteBit(GifBitStatus *stat, uint32_t bit)
+{
+    bit = bit & 1;
+    bit = bit << stat->bitIndex;
+    stat->byte |= bit;
+    
+    ++stat->bitIndex;
+    if (stat->bitIndex > 7)
+    {
+        // move the newly-finished byte to the chunk buffer 
+        stat->chunk[stat->chunkIndex++] = stat->byte;
+        // and start a new byte
+        stat->bitIndex = 0;
+        stat->byte = 0;
+    }
+}
+
+// write all bytes so far to the file
+static void GifWriteChunk(FILE *f, GifBitStatus *stat)
+{
+    fputc(stat->chunkIndex, f);
+    fwrite(stat->chunk, 1, stat->chunkIndex, f);
+    
+    stat->bitIndex = 0;
+    stat->byte = 0;
+    stat->chunkIndex = 0;
+}
+
+static void GifWriteCode(FILE *f, GifBitStatus *stat, uint32_t code, uint32_t length)
+{
+    for (uint32_t ii=0; ii<length; ++ii)
+    {
+        GifWriteBit(stat, code);
+        code = code >> 1;
+        
+        if (stat->chunkIndex == 255)
+        {
+            GifWriteChunk(f, stat);
+        }
+    }
+}
+
+// write a 256-color (8-bit) image palette to the file
+static void GifWritePalette(const GifPalette *pPal, FILE *f)
+{
+    fputc(0, f);  // first color: transparency
+    fputc(0, f);
+    fputc(0, f);
+    
+    for (int ii=1; ii<(1 << pPal->bitDepth); ++ii)
+    {
+        uint32_t r = pPal->r[ii];
+        uint32_t g = pPal->g[ii];
+        uint32_t b = pPal->b[ii];
+        
+        fputc(r, f);
+        fputc(g, f);
+        fputc(b, f);
+    }
+}
+
+// write the image header, LZW-compress and write out the image
+static void GifWriteLzwImage(FILE *f, uint8_t *image, uint32_t left, uint32_t top, uint32_t width, uint32_t height, uint32_t delay, GifPalette *pPal)
+{
+    // graphics control extension
+    fputc(0x21, f);
+    fputc(0xf9, f);
+    fputc(0x04, f);
+    fputc(0x05, f); // leave prev frame in place, this frame has transparency
+    fputc(delay & 0xff, f);
+    fputc((delay >> 8) & 0xff, f);
+    fputc(kGifTransIndex, f); // transparent color index
+    fputc(0, f);
+    
+    fputc(0x2c, f); // image descriptor block
+    
+    fputc(left & 0xff, f);           // corner of image in canvas space
+    fputc((left >> 8) & 0xff, f);
+    fputc(top & 0xff, f);
+    fputc((top >> 8) & 0xff, f);
+    
+    fputc(width & 0xff, f);          // width and height of image
+    fputc((width >> 8) & 0xff, f);
+    fputc(height & 0xff, f);
+    fputc((height >> 8) & 0xff, f);
+    
+    //fputc(0, f); // no local color table, no transparency
+    //fputc(0x80, f); // no local color table, but transparency
+    
+    fputc(0x80 + pPal->bitDepth-1, f); // local color table present, 2 ^ bitDepth entries
+    GifWritePalette(pPal, f);
+    
+    const int minCodeSize = pPal->bitDepth;
+    const uint32_t clearCode = 1 << pPal->bitDepth;
+    
+    fputc(minCodeSize, f); // min code size 8 bits
+    
+    GifLzwNode *codetree = (GifLzwNode *)GIF_TEMP_MALLOC(sizeof(GifLzwNode)*4096);
+    
+    memset(codetree, 0, sizeof(GifLzwNode)*4096);
+    int32_t curCode = -1;
+    uint32_t codeSize = minCodeSize + 1;
+    uint32_t maxCode = clearCode + 1;
+    
+    GifBitStatus stat;
+    stat.byte = 0;
+    stat.bitIndex = 0;
+    stat.chunkIndex = 0;
+    
+    GifWriteCode(f, &stat, clearCode, codeSize);  // start with a fresh LZW dictionary
+    
+    for (uint32_t yy=0; yy<height; ++yy)
+    {
+        for (uint32_t xx=0; xx<width; ++xx)
+        {
+            uint8_t nextValue = image[(yy*width+xx)*4+3];
+            
+            // "loser mode" - no compression, every single code is followed immediately by a clear
+            //WriteCode(f, stat, nextValue, codeSize);
+            //WriteCode(f, stat, 256, codeSize);
+            
+            if (curCode < 0)
+            {
+                // first value in a new run
+                curCode = nextValue;
+            }
+            else if (codetree[curCode].m_next[nextValue])
+            {
+                // current run already in the dictionary
+                curCode = codetree[curCode].m_next[nextValue];
+            }
+            else
+            {
+                // finish the current run, write a code
+                GifWriteCode(f, &stat, curCode, codeSize);
+                
+                // insert the new run into the dictionary
+                codetree[curCode].m_next[nextValue] = ++maxCode;
+                
+                if (maxCode >= (1ul << codeSize))
+                {
+                    // dictionary entry count has broken a size barrier,
+                    // we need more bits for codes
+                    codeSize++;
+                }
+                if (maxCode == 4095)
+                {
+                    // the dictionary is full, clear it out and begin anew
+                    GifWriteCode(f, &stat, clearCode, codeSize); // clear tree
+                    
+                    memset(codetree, 0, sizeof(GifLzwNode)*4096);
+                    curCode = -1;
+                    codeSize = minCodeSize + 1;
+                    maxCode = clearCode + 1;
+                }
+                
+                curCode = nextValue;
+            }
+        }
+    }
+    
+    // compression footer
+    GifWriteCode(f, &stat, curCode, codeSize);
+    GifWriteCode(f, &stat, clearCode, codeSize);
+    GifWriteCode(f, &stat, clearCode + 1, minCodeSize + 1);
+    
+    // write out the last partial chunk
+    while (stat.bitIndex) GifWriteBit(&stat, 0);
+    if (stat.chunkIndex) GifWriteChunk(f, &stat);
+    
+    fputc(0, f); // image block terminator
+    
+    GIF_TEMP_FREE(codetree);
+}
+
+#endif // GIF_IMPLEMENTATION