In reality, each R3G3B2 colour is translated internally by the ZX Spectrum Next into a full RGB3 colour by performing a binary OR of the first bit (MSB) of the blue component with 0 so for example colour 10111110 (8-bit) will become internally 101111101 as a full 9-bit colour.

Layer 1,0 (LoRes) and Layer 2 are very straightforward in how they store both colour as well as graphic data. Unlike the other modes, there's no separate area for colour and there is no interleaving in the order of storage or separate pointers to the area the data is stored. Each byte of memory represents one pixel on screen from the top left to the bottom right. The only two differences between them are the memory location where the screen contents are stored and their resolution. The former uses the standard DISP_FILE1 and DISP_FILE2 areas (each holding one half of the screen) and is usually stored in BANK 5, having a maximum resolution of 128 w x 96 h pixels (thus making it a total of 12 Kbytes in size) while the latter uses up to 5 banks. The default resolution of 256 w x 192 h pixels uses normally 3 (by default BANKS 9,10 and 11 but these are relocatable⁶), making its memory requirements 48 Kbytes while the higher resolutions of 320 w x 256 h and 640 w x 256 h use 5 16 Kbyte banks, requiring a total of 80 Kbytes.

Although the remaining, higher resolution, Layer 2 modes are not currently supported by NextBASIC (they will however in the near future) it is important to mention them here for two reasons:

First, because they both store their data (as we will see in the last section of the following chapter) in a sideways manner and secondly because especially for the highest resolution Layer 2 mode (640 w x 256 h) the colour is stored in a similar manner as Layer 3 below.

That being said, in all Layer 2 resolutions, colour is stored as part of the graphic data and therefore neither LoRes nor Layer 2 modes suffer from colour clash.

An additional side-effect of the linear nature of these modes, is that the concepts of FLASH and BRIGHT do not exist there. BRIGHT was just a way to squeeze more colours out of a very limited selection and FLASH can be reproduced by quickly inverting the contents of an area using a number of programming techniques available via NextBASIC.

Layer 3 colour storage

Layer 3 is special as it allows for complete usage of the full Spectrum Next screen area, therefore the entirety of the 320 w x 256 h pixels resolution is available (combining the standard graphic area with the width and height of the border) and uses either (like the Sprite system we will examine in Chapter 17) a palette offset + 4-bit index combination to store colour for each tile or a monochrome mode specifically suited to display text. The first method, achieves significant memory space savings without sacrificing colour capabilities (although at first it may look a bit restrictive): Each tile being 8 x 8 has 64 possible pixel locations; by using a 4-bit colour index number we can only have 2⁴ = 16 combinations/colours instead of 64 we theoretically could have. With a bit of prior arrangement of our image data however, we can achieve spectacular results and display very complex images (colour-wise) even with that restriction in place. The monochrome mode has obvious memory benefits we have explored with Layer 1,2 (HiRes) as well as increased speed.

Layer 2 priority colours

As we will see in length on the following section, since the ZX Spectrum Next display is layered, there is a way to rearrange the layer display priority, or rather the order in which these layers are stacked one on top of another. This provides unique flexibility however there are cases that you'd want to mesh the layers in a more complex way as for example in the case of a game where you would want the player's sprite to weave in-and-out the environment in order to get the impression of depth. Usually this is achieved by employing an algorithm that performs environmental masking; hiding in other words things that we don't

⁶ By relocatable, we mean that although the ZX Spectrum Next initially reserves BANKS 9 through 12 for Layer 2 graphic data, this can change either automatically or by the user. One should not assume the aforementioned banks of memory always hold Layer 2 graphic data. Check Chapters 22 and 23 for more information regarding the actual Layer 2 location.