WGL_REF/TexSubImage2D

//:AUTHOR: John Mark Isaac Madison ( CODE_TOLD_FAST & CODE_TOLD_SLOW )
//:THIS_FILE_ON_PASTEBIN: https://pastebin.com/YeY2CHCX
//:YOUTUBE_VIDEO_ON_THIS: https://www.youtube.com/watch?v=x2dtAg5I6mk

//|¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯|//
//|                                                          |//
//| [ x_0 , y_0 ]                                            |//
//|        \                                y_0              |//
//|         +-------------+           +------|------+        |//
//|         |             |           |             |        |//
//|         |             |           |             |        |//
//|         |             |    x_0 ___|             |___ x_1 |//
//|         |             |           |             |        |//
//|         |             |           |             |        |//
//|         |             |           |             |        |//
//|         +-------------+           +------|------+        |//
//|                        \                y_1              |//
//|                   [ x_1 , y_1 ]                          |//
//|                                                          |//
//|__________________________________________________________|//


    |<---- pix_wid ---->|   ||   |<---- pix_wid ---->|
    +-------------------+   ||   +-------------------+
    |                   |   ||   |                   |
    |                   |   ||   |    | cpu_sub |    |
    |                   |   ||   |    +---------+    |
    |      cpu_img      |   ||   |    | 256-x-64|    |
    |                   |   ||   |    +---------+    |
    |                   |   ||   |                   |
    |                   |   ||   |                   |
    +-------------------+   ||   +-------------------+
             V              ||            V
      gl.texImage2D(...)    ||     gl.texSubImage2D(...)
             V              ||            V
    +-------------------+   ||   +-------------------+
    |                   |   ||   |                   |
    |                   |   ||   |    | gpu_sub |    |
    |                   |   ||   |    +---------+    |
    |      gpu_img      |   ||   |    | 256-x-64|    |
    |                   |   ||   |    +---------+    |
    |                   |   ||   |                   |
    |                   |   ||   |                   |
    +-------------------+   ||   +-------------------+

        G = YOUR_WEBGL_CONTEXT_OBJECT;
        G.activeTexture( G.TEXTURE0   +  tex_lot );
        G.bindTexture(   G.TEXTURE_2D ,  tex_han );
        const ALN = 4; //:ALN:"Alignment" GUESS?
        G.pixelStorei(G.UNPACK_ALIGNMENT, ALN);


        //:CALCULATE_STARTING_OFFSET_IN_BYTES:( src_oib)
        var p_x =( _.cpu_sub.x_0 ); //:TOP_LEF:X
        var p_y =( _.cpu_sub.y_0 ); //:TOP_LEF:Y
        var pix_dex = p_x + ( pix_wid * p_y);
        var src_oip = pix_dex * (  1  );//:PixelOffset
        var src_oib = pix_dex * pix_com;//:ComponentOffset

        G.pixelStorei( G.UNPACK_ROW_LENGTH , pix_wid );
        G.texSubImage2D(
        /*  01  */    G.TEXTURE_2D //:TARGET
        /*  02  */  , 0            //:LEVEL_OF_DETAIL (LOD)
        /*  03  */  , cpu_sub.x_0  //:TOP_LEFT
        /*  04  */  , cpu_sub.y_0  //:TOP_LEFT
        /*  --  */    ,
        /*  05  */    (   ( _.cpu_sub.x_1 )        //:WIDTH
        /*  05  */      - ( _.cpu_sub.x_0 ) + 1 )  //:WIDTH
        /*  --  */    ,
        /*  06  */    (   ( _.cpu_sub.y_1 )        //:HEIGHT
        /*  06  */      - ( _.cpu_sub.y_0 ) + 1 )  //:HEIGHT
        /*  07  */    ,   G.RGBA_INTEGER   //:SOURCE_FORMAT
        /*  08  */    ,   G.UNSIGNED_BYTE  //:SOURCE_TYPE
        /*  09  */    ,   pix_arr          //:ARRAY_BUFFER_VIEW
        /*  10  */    ,   src_oib          //:SOURCE_OFFSET
        );;