gif

commit f30dbdf7867e888d08480a141af3b0572fd09084
parent f9bcb3eb69976b7eefa42da8438f659e502e2b37
Author: phoebos <ben@bvnf.space>
Date:   Wed,  1 Sep 2021 22:36:39 +0100

image block header handling

Diffstat:
M
gif.c
 | 
62
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----

1 file changed, 58 insertions(+), 4 deletions(-)
diff --git a/gif.c b/gif.c
@@ -39,6 +39,14 @@ struct Gif {
 };
 struct Image {
     struct Color_table *lct;
+    uint16_t left, top, width, height;
+    unsigned char flags; /* AB000CCC
+                            A: 0 -> use GCT, ignore C
+                               1 -> LCT follows, use C
+                            B: 0 -> image is in Sequential order
+                               1 -> image is in Interlaced order
+                            C+1: bits per pixel for this image
+                         */
     unsigned char *pixels;
 };
 
@@ -51,10 +59,56 @@ print_colortable(struct Color_table *c){
     }
 }
 
+/* gif_decode_image should be called with buf pointing to the separator
+ * character, ','. gct should be a pointer to the global color table.
+ */
 int
-gif_decode_image(struct Image *img, unsigned char *buf){
-    (void) img;
-    (void) buf;
+gif_decode_image(struct Image *img, struct Color_table *gct, unsigned char *buf){
+    if (*buf++ != ','){
+        fprintf(stderr, "not an image block\n");
+        return FAIL_GIF;
+    }
+    struct Color_table *ct = gct;
+
+    /* Get the top, left, width, and height from the beginning of buf.
+     * They are all 2 byte numbers with the LSB first. */
+    uint16_t *tmp;
+    tmp = (uint16_t *)buf;
+    img->left = *tmp;
+    buf += 2;
+    tmp = (uint16_t *)buf;
+    img->top = *tmp;
+    buf += 2;
+    tmp = (uint16_t *)buf;
+    img->width = *tmp;
+    buf += 2;
+    tmp = (uint16_t *)buf;
+    img->height = *tmp;
+    buf += 2;
+
+    fprintf(stderr, "image block starts at %d,%d with dimensions %d,%d\n",
+            img->left, img->top, img->width, img->height);
+
+    img->flags = *buf++;
+
+    if (img->flags && 0x80) {
+        /* local color table follows */
+        fprintf(stderr, "using LCT(%d), ", img->flags && 7);
+    } else {
+        /* use GCT */
+        fprintf(stderr, "using GCT, ");
+    }
+
+    if (img->flags && 0x40) {
+        /* image is interlaced */
+        fprintf(stderr, "interlaced\n");
+        /* TODO */
+        return -1;
+    } else {
+        /* image is sequential */
+        fprintf(stderr, "sequential\n");
+    }
+
     return 0;
 }
 
@@ -122,7 +176,7 @@ gif_decode(unsigned char *buf, size_t len){
                     img = malloc(sizeof(struct Image));
                     img->lct = malloc(sizeof(struct Color_table));
                     img->lct->len = g.gct->len;
-                    gif_decode_image(img, buf);
+                    gif_decode_image(img, g.gct, buf);
                 }
                 break;
             case ';':