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Terrain Tutorial Index
A TGA Library
Height Maps
Lighting
Artificial Terrain Generation
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Terrain Tutorial
A TGA library
This library is introduced here in order to make this tutorial self-contained. The library is pretty simple, handling only uncompressed images, RGBA or greyscale. No support for color palettes is provided. If you're interested in a full-blown TGA library I would suggest a look at Paul Groves' TGA loader. Check out his home page.
The functions in this library include loading and saving TGA images, colour reduction from RGB to greyscale, and a screen grabber.
TGA file structure
A TGA file has a header that consists of 12 fields. These are:
Some possible values for the image type are:
One note of interest is that a TGA stores the pixels in BGR mode, i.e. the red and blue components are swapped, relative to RGB. This implies that we'll have to swap them when we load or save the image.
Now for some library details. The following status codes where defined:
typedef struct {
int status;
unsigned char type, pixelDepth;
short int width, height;
unsigned char *imageData;
}tgaInfo;
The following functions are available in the library:
tgaInfo* tgaLoad(char *filename);
Parameters:
- filename - the name of the image file
This functions returns a structure with all the image info and pixels. Pixels are stored in imageData, which is an one-dimensional array with values from 0 to 255. In order to read this array correctly, we'll need to check the values of pixelDepth, width and height. The value of pixelDepth tells us which information is in the array. There are 3 possible values with the following meaning:
The next function allows us to save pixels as a TGA image
int tgaSave(char *filename, short int width, short int height, unsigned char pixelDepth, unsigned char *imageData);
Parameters:
- filename - The file name where we want to save the image (extension is required!).
- width - The width of the image
- height - The height of the image
- pixelDepth - The number of bits per pixel, 8 for greyscale, 24 for RGB, and 32 for RGBA
- imageData - The image pixels
This function returns a status code. TGA_OK means everything went smoothly.
The next function is provided so that we can save series of images. For instance, if the file name is "foo" then the first time this function is called it will save an image as "foo0.tga", the second time "foo1.tga", and so on.
int tgaSaveSeries(char *filename, short int width, short int height, unsigned char pixelDepth, unsigned char *imageData);
Parameters:
- filename - The file name where we want to save the image (no extensions in here please).
- width - The width of the image
- height - The height of the image
- pixelDepth - The number of bits per pixel, 8 for greyscale, 24 for RGB, and 32 for RGBA
- imageData - The image pixels
As always, it is a good idea to check the return value of the function to see if the operation was completed successfully. The next function allows you to take a screen shot and save it to a TGA image. With this function it is possible to grab the entire viewport, or just a part of it. The syntax is as follows:
int tgaGrabScreenSeries(char *filename, int x,int y, int width, int height);
Parameters:
- filename - the image file name
- x - The x component of the lower left corner of the image.
- y - The y component of the lower left corner of the image.
- width - The width of the image.
- height - The height of the image.
The following code snipet illutrates how to capture the whole viewport (assuming a viewport from 0,0 to w,h) , and only its top half.
tgaGrabSeries("bla", 0,0,w,h);
tgaGrabSeries("bla", 0,h/2,w,h/2);
The code for the above function is:
int tgaGrabScreenSeries(char *filename,
int xmin,int ymin,
int xmax, int ymax) {
int w, h;
unsigned char *imageData;
w = xmax - xmin;
h = ymax - ymin;
imageData = (unsigned char *)malloc(
sizeof(unsigned char)
* w * h * 4);
glReadPixels(xmin,ymin,xmax,ymax,
GL_RGBA,GL_UNSIGNED_BYTE, (GLvoid *)imageData);
return(tgaSaveSeries(filename,w,h,32,imageData));
}
The key is the OpenGL function glReadPixels. This function reads a rectangular area from the frame buffer and stores it in an array (this array must have previously allocated memory). This function has the following syntax:
void glReadPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLvoid *pixels);
Parameters:
- x - the x component of the lower left corner of the requested area
- y - the y component of the lower left corner of the requested area
- width - the width in pixels of the area
- height - the height in pixels of the area
- format - determines which data we're going to read. See bellow for some possible values.
- type - the data type of the data we're going to read.
- pixels - An array with the pixel information. The array data depends on the format and type.
The format specifies if we're getting RGBA data, or just the Green component. Some possible values are:
This next function converts a RGB image to a greyscale image. The formula used was posted on the openGL.org discussion groups some time ago.
greyscale = 0.30 * R + 0.59 * G + 0.11 * B
This function returns the modified image information. Besides imageData, the fields pixelDepth and type are also altered accordingly to reflect the new image type.
void tgaRGBtogreyscale(tgaInfo *info);
Parameters:
This last function releases the memory, so when using the image for a texture or a height map, the image can be destroyed afterwards.
void tgaDestroy(tgaInfo *info);
Parameters:
The next section presents the fully commented code for this library.
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