title: "vbmDrawSpriteSliceE" description: TRSE built-in method (from IDE help)

`vbmDrawSpriteSliceE`

Systems: VIC20
Parameters: a, b, b

vbmDrawSpriteSliceE( [address1], [byte-start], [byte-end] );

[address1] - pre-shifted address table
[byte-start] - the row in the sprite to start drawing from, usually 0
[byte-end] - the row in the sprite to end drawing, usually the height of the sprite

Description

A flexible sprite routine to display a single 'slice' of a sprite on the bitmap using an EOR operation. The EOR operation will merge the sprite with the bitmap without destroying the bitmap, but it will look 'noisy' when pixels overlap. The way EOR works is:

If both the sprite and bitmap have no pixel - no pixel is drawn
If the sprite has a pixel but the bitmap does not - a pixel is drawn
If the bitmap has a pixel but the sprite does not - a pixel is drawn
If both the sprite and the bitmap has a pixel - a pixel is cleared

A side effect of this is that EOR can be used to both draw and remove the sprite without destroying what was already on the bitmap. When a sprite overlaps other pixels, the pxiel will clear which will look a bit noisy, but when drawn again, the sprite is removed and previous pixels restored. See example below.

The first parameter is an address table pointing to where each pre-shifted frame can be found in memory.

For example: $a000, $a080, $a1000, $a180

The address table will need to be defined if you import your sprites already pre-shifted. If you use the vbmSpriteShiftL / R commands, these will build the address table for you.

There are three ways to draw a sprite:

vbmDrawSpriteSlice - draws a sprite slice, merging it with the bitmap
vbmDrawSpriteSliceE - draws a sprite slice, merging it with the bitmap using the EOR operation
vbmClearSpriteSlice - clears a sprite slice from the bitmap, effectively 'cutting it out'

Example


    spr: incbin("spr24x40.bin", $4000); // this sprite is 24 x 40
    @define spra0 $4000
    @define spra1 $4028
    @define spra2 $4050

    ...    

    x,y :byte;

    // our sprite is 24 x 40.  So need 4 byte columns for pre-shifted data
    // will pre-shift in increments of 2 so 4 addresess per byte required
    spr0 : array[4] of integer; 
    spr1 : array[4] of integer; 
    spr2 : array[4] of integer; 
    spr3 : array[4] of integer; 

    ...

    // sprite is 24 x 40.  Need 40 bytes per shift ($28) x 4 shifts = $A0
    vbmSpriteShiftR( ^@spra0, ^$a000, 2, 40, spr0 ); 
    vbmSpriteShiftL( ^@spra0, ^$a0A0, 2, 40, spr1 ); 

    vbmSpriteShiftR( ^@spra1, ^$a140, 2, 40, spr2 ); 
    vbmSpriteStitch( ^$a140, ^$a0a0, 160 );  // 40 high * 4 = 160
    vbmSpriteShiftL( ^@spra1, ^$a140, 2, 40, spr2 ); 

    vbmSpriteShiftR( ^@spra2, ^$a1E0, 2, 40, spr3 ); 
    vbmSpriteStitch( ^$a1E0, ^$a140, 160 );
    vbmSpriteShiftL( ^@spra2, ^$a1E0, 2, 40, spr3 ); 

    y := 10;

    for x := 0 to 100 do
    begin

        // draw a large sprite
        vbmSetPosition2( x, y ); // position the sprite, 2 pixel increments
        vbmDrawSpriteSliceE( spr0, 0, 40 );
        vbmNextColumn();
        vbmDrawSpriteSliceE( spr1, 0, 40 );
        vbmNextColumn();
        vbmDrawSpriteSliceE( spr2, 0, 40 );
        vbmNextColumn();
        vbmDrawSpriteSliceE( spr3, 0, 40 );  

        waitforraster( 0 ); // wait for raster

        // drawing the large sprite again at the same position 'wipes' it out, restoring anything
        // prevuiously on the bitmap
        vbmSetPosition2( x, y ); // position the sprite, 2 pixel increments
        vbmDrawSpriteSliceE( spr0, 0, 40 );
        vbmNextColumn();
        vbmDrawSpriteSliceE( spr1, 0, 40 );
        vbmNextColumn();
        vbmDrawSpriteSliceE( spr2, 0, 40 );
        vbmNextColumn();
        vbmDrawSpriteSliceE( spr3, 0, 40 );

    end;

vbmDrawSpriteSliceE