Handling sprites that the user actually sees
Real sprites are the graphical part of .grf files; they are what the user actually sees in-game.
A real sprite has the following format:
<sprite-number> <filename> <xpos> <ypos> <compression> <ysize> <xsize> <xrel> <yrel>
|<sprite-number>||dec||A sequential sprite number
|<filename>||string*||The name of the .pcx file in which the sprite will be found
|<xpos>||dec||The x position of the upper-left corner of the sprite in the file
|<ypos>||dec||The y position of the upper-left corner of the sprite in the file
|<compression>||B||The "compression"; see below
|<ysize>||dec||The y size of the sprite
|<xsize>||dec||The x size of the sprite
|<xrel>||dec||The x position of the upper-left corner of the sprite, relative to the "center" (usually negative)
|<yrel>||dec||The y position of the upper-left corner of the sprite, relative to the "center" (usually negative)|
* Unlike strings in pseudo-sprites, this string should not have a terminating "00" and should not be placed in quotes.
This specifies the name and path of the .pcx file. The path may be specified one of two ways:
- An absolute path. (eg C:\GRF\sprites\filename.pcx)
- A relative path; these are specified relative to the location of the .grf file. (eg sprites\filename.pcx)
For most purposes, relative paths are preferred, since .nfo files with relative paths are more portable than .nfo files with absolute paths.
<xpos, ypos, ysize, and xsize>
These are pretty easy to set, as long as you remember that the order is X, Y, Y, X, and not X, Y, X, Y.
Also, no sprite may contain zero pixels; ysize and xsize must both be at least 1.
There is a bug in grfcodec 0.9.7 and earlier that prevents them from properly encoding a sprite with a smaller ypos value than any earlier sprite. The best way to circumvent this bug is to upgrade to the newest version.
Unlike the other numbers, the compression is a hexadecimal bit-field. Currently, the following bits are supported:
|0||1||Color index 0 is transparent (should be always set; ignored if bit 3 is set)
|1||2||Store compressed sprite in memory
|3||8||Sprite is in chunked data format (aka "tile compression")
|6||40||Sprite should not be cropped. (obeyed by GRFCodec r1604 and later)|
The chunked data format is designed to compress tiles and diagonal views of vehicles; things with lots of transparency around the edges, and a contiguous block of non-transparent pixels in the middle. Using the chunked data format may provide advantages in drawing speed.
The most common values for the compression are 01 and 09. 03 is useful for large sprites that are infrequently used, as it reduces memory usage at the cost of slower drawing speed. It is generally a bad idea to use any other value, as that may cause problems. FF is an especially bad idea.
<xrel and yrel>
These two numbers will probably become the bane of your existence.
TTD stores a single point for every sprite. xrel and yrel specify how far right and down (respectively) to go from the TTD point to the upper-left hand corner of the sprite.
Halving xsize and ysize and negating the results produces a decent starting point for these values, and further adjustments may be done manually, with NFOEditor, or (in TTDPatch 2.0.1 alpha 69 and later) with the GRFAuthorHelperWindow
The ranges for the four numbers after the compression are as follows:
ysize: 1 .. 255
xsize: 1 .. 65535
xrel: -32768 .. 32767
yrel: -32768 .. 32767
Note that ysize*xsize may not exceed 65535.
It is a common practice, in action 1 blocks, to use the pseudosprite "-1 * 1 00" to take the place of a real sprite that will never be used. The most common use is for vehicle sets that will only be used in the purchase window. This reduces the size of the .grf file when compared to replacing them with one-pixel sprites (but not when compared to splitting the action 1 and making the second one define only one sprite), but does not save real-sprite slots. If TTD ever attempts to draw one of these sprites, it will, at best, crash.
Transparency and glass effects can be achieved by means of RecolorSprites
The best way to see real sprites is to decode one of TTD's standard .grf files. With very few exceptions, all sprites in those files are real sprites; the few that are not are RecolorSprites
Example real sprites can also be found at the bottom of Action1