diff options
Diffstat (limited to 'doc/Images.txt')
| -rw-r--r-- | doc/Images.txt | 114 |
1 files changed, 114 insertions, 0 deletions
diff --git a/doc/Images.txt b/doc/Images.txt new file mode 100644 index 00000000..1f70b0ca --- /dev/null +++ b/doc/Images.txt @@ -0,0 +1,114 @@ + February 2001, --Jcid + + ------ + IMAGES + ------ + +* When a image tag is found within a HTML page, Html_tag_open_img +handles it by: + + - Parsing & getting attribute values. + - Creating a new image structure (DilloImage) and its + associated widget (DwImage). + i.e. If 'Image' is the var for the structure, then + 'Image->dw' is the widget. + - Requesting the image to be feeded by the cache. + - Sending some info to the browser interface. + +* The cache can either request the image data from the net, or +feed it directly from the dicache (decompressed image cache). + +* Both processes are somewhat different because the first one +requires to decode the image data into RGB format, and the second +one has the whole data already decoded. + +* Regardless of the RGB-data feeding method, the decoded data is +passed to the widget (DwImage) and drawn by GdkRGB in a streamed +way. + Note that INDEXED images are also decoded into RGB format. + + +--------------------- +Fetching from the net +--------------------- + +* a_Cache_open_url initiates the resource request, and when +finally the answer arrives, the HTTP header is examined for MIME +type and either the GIF or PNG or JPEG decoder is set to handle +the incoming data stream. + Decoding functions: a_Gif_image, a_Jpeg_image and a_Png_image. + +* The decoding function calls the following dicache methods as +the data is processed (listed in order): + + a_Dicache_set_parms + a_Dicache_set_cmap (only for indexed-GIF images) + a_Dicache_write + a_Dicache_close + +* The dicache methods call the necessary functions to connect +with the widget code. This is done by calling image.c functions: + + a_Image_set_parms + a_Image_set_cmap + a_Image_write + a_Image_close + +* The functions in image.c make the required a_Dw_image_... +calls. + + +------------------------- +Fetching from the dicache +------------------------- + +* a_Cache_open_url tests the cache for the image, and directly +enqueues a cache client for it, without asking the network for +the data. When the client queue is processed (a bit later), the +decoder is selected based on the cache entry Type. + +* When the decoder is called, it tests the dicache for the image; +if the image is found, then it sets a_Dicache_callback as the +handling function and gets out of the way (no image decoding is +needed). + +* Later on, the DwImage buffer is set to reference the +dicache-entry's buffer and the rest of the functions calls is +driven by a_Dicache_callback. + + +----------- +Misc. notes +----------- + +* Repeated images generate new cache clients, but only one of +them (the first) is handled with a_Dicache_* functions, the rest +is done with a_Dicache_callback.. + +* The cache-client callback is set when the Content-type of the +image is got. It can be: a_Png_image, a_Gif_image or a_Jpeg_image +Those are called 'decoders' because their main function is to +translate the original data into RGB format. + +* Later on, the decoder can substitute itself, if it finds the +image has been already decoded, with a_Dicache_callback function. +This avoids decoding it twice. + +* The dicache-entry and the Image structure hold bit arrays that +represent which rows had been decoded. + +* The image processing can be found in the following sources: + + - image.[ch] + - dicache.[ch] + - gif.[ch], png.[ch], jpeg.[ch] + - dw_image.[ch] + +* Bear in mind that there are three data structures for image +code: + + - DilloImage (image.h) + - DwImage (dw_image.h) + - DICacheEntry (dicache.h) + + |