cpct_setBlendMode

Establishes the blending mode that cpct_drawSpriteBlended will use from now on.

C Definition

void cpct_setBlendMode (CPCT_BlendMode mode) __z88dk_fastcall;

Input Parameters (1 bytes)

Assembly call (Input parameters on registers)

call cpct_setBlendMode_asm

Parameter Restrictions

• mode it could theoretically be any 8-bits value, but only some values will be really useful, whereas some others will have unexpected results, even making the program crash.  In C it is restricted by the CPCT_BlendMode enumeration to prevent errors.  The value of this parameter is a Z80 opcode that will be inserted into the blending function as blend operation.

Known limitations

• This function will produce no result if cpct_drawSpriteBlended is allocated in ROM space.  This function modifies cpct_drawSpriteBlended code, therefore it requires it to be in RAM space.

Details

Sets a new blending mode for the function cpct_drawSpriteBlended.  The new blending mode will be the one cpct_drawSpriteBlended will use after calling this function, until another blending mode is established, by calling this function again.

Internally, what this function does is modifying 1 byte of cpct_drawSpriteBlended function code.  This modified byte is the one that performs the operation between every pair of screen video memory and sprite bytes.  This operation is a single byte Z80 instruction, hence the modification on 1 byte.

Examples of use

cpct_drawSpriteBlended uses XOR mode as default.  If we wanted to draw a moving ball in a game using AND mode, we should do it this way:

// We want our blended sprites to use AND mode, so we set it first
// (once set, it will continue being AND mode unless manually modified)
cpct_setBlendMode(CPCT_BLEND_AND);

// This is the main loop of the game
while(1) {
   u8* pmem;   // Used to point to video memory when calling drawing functions

   // Update all game entities and other stuff here...
   // ...
   // ...

   // Draw the ball sprite, at its (x,y) location, blended with AND mode.
   // Ball sprite is 8x8 pixels, 4x8 bytes in mode 0.
   pmem = cpct_getScreenPtr(CPCT_VMEM_START, ball.x, ball.y);
   cpct_drawSpriteBlended(pmem, 8, 4, ball_sprite);

   // More drawing code and other things go here
   // ...
}

Destroyed Register values

A, HL

Required memory

6 bytes

Time Measures

 Case      | microSecs (us) | CPU Cycles
-------------------------------------------
 Any       |       9        |     36
-------------------------------------------