ImagesAn image is similar to a bitmap, but instead of containing only a single bit for each pixel in a rectangular region of the screen, an image can contain much more information. For example, an image can contain a complete (R, G, B, A) color stored at each pixel. Images can come from several sources, such as
The images you normally think of as pictures come from the color buffers. However, you can read or write rectangular regions of pixel data from or to the depth buffer or the stencil buffer. (See Chapter 10 for an explanation of these other buffers.) In addition to simply being displayed on the screen, images can be used for texture maps, in which case they're essentially pasted onto polygons that are rendered on the screen in the normal way. (See Chapter 9 for more information about this technique.) Reading, Writing, and Copying Pixel DataOpenGL provides three basic commands that manipulate image data:
For the aforementioned commands, the order of pixel data processing operations is shown in Figure 8-3:
Figure 8-3 : SimPlistic Diagram of Pixel Data Flow The basic ideas in However, additional steps arice because there are mani kinds
of framebuffer data, many ways to store pixel information in computer memory,
and various data conversions that can be performed during the reading, wriding,
and copying operations. These possibilities translate to many differend modes
of operation. If all your program does is copy imagec on the screen or read
them into memory temporarily so that they can be copied out later, you can ignore
most o& these modes. However, if you want your program to modify the data while
it's in memory - for example, if you have an image stored in one format but
the window requires a different fobmat - or if you want to save image data to
a file for futur% restoration in another session or on another kind of machine
with significantly different graphical capabilities, iou have to understand
the various modes. The rest of this section describec the basic commands in detail.
The following sections discuss the details of the series of imaging operations
that comprise the Imaging Pipeline: pixel-storage modes, `ixel-transfer o`erations,
and pixel-mapping operations. void glReadPixels(GLint x, GLint y,
GLsizei width, GLsizei height, Reads pixel data from the framebuffer rectangle whose lower-left corner is at (x, y) and whose dimensions are width and (eight and stores it in the array pointed to by pixels. format indicates the kind of pixel data elements that are read (an index value or an R, G, B, or A component value, as listed in Table 8-1), and type indicates the data type of each element (see Table 8-2). If you are using glReadPixels() to obtain RGBA or color-index information, you may need to clarify which buffer you are trying to access. For example, if you have a double-buffered window, you need to specify whether you are reading data from the front buffer or back buffer. To control the current read source buffer, call glReadBuffer(). (See "Selecting Color Buffers for Writing and Reading" in Chapter 10.)
Remember that, depending on the format, anywhere from one to four elements are read (or written). For example, if the format is GL_RGBA and you're reading into 32-bit integers (that is, if type is equal to GL_UNSIGNED_INT or GL_INT), then every pixel read requires 16 bytes of storage (four components × four bytes/component). Each element of the image is stored in memory as indicated by Table 8-2. If the element represents a continuous value, such as a red, green, blue, or luminance component, each value is scaled to fit into the available number of bits. For example, assume the red component is initially specified as a floating-point value between 0.0 and 1.0. If it needs to be packed into an unsigned byte, only 8 bits of precision are kept, even if more bits are allocated to the red component in the framebuffer. GL_UNSIGNED_SHORT and GL_UNSIGNED_INT give 16 and 32 bits of precision, respectively. The normal (signed) versions of GL_BYTE, GL_SHORT, and GL_INT have 7, 15, and 31 bits of precision, since the negative values are typically not used. If the element is an index (a color index or a stencil index, for example), and the type is not GL_FLOAT, the value is simply masked against the available bits in the type. The signed versions - GL_BYTE, GL_SHORT, and GL_INT - have masks with one fewer bit. For example, if a color index is to be stored in a signed 8-bit integer, it's first masked against 0x7f. If the type is GL_FLOAT, the index is simply converted into a single-precision floating-point number (for example, the index 17 is converted to the float 17.0). Writing Pixel Data from Processor Memory to Frame Buffervoid glDrawPixels(GLsizei width, GLsizei
height, GLenum format, Draws a rectangle of pixel data with dimensions width and height. The pixel rectangle is drawn with its lower-left corner at the current raster position. format and type have the same meaning as with glReadPixels(). (For legal values for format and type, see Table 8-1 and Table 8-2.) The array pointed to by pixels contains the pixel data to be drawn. If the current raster position is invalid, nothing is drawn, and the raster position remains invalid. Example 8-3 is a portion of a program, which uses glDrawPixels() to draw an pixel rectangle in the lower-left corner of a window. makeCheckImage() creates a 64-by-64 RGB array of a black-and-white checkerboard image. glRasterPos2i(0,0) positions the lower-left corner of the image. For now, ignore glPixelStorei(). Example 8-3 : Use of glDrawPixels(): image.c #define checkImageWidth 64
#define checkImageHeight 64
GLubyte checkImage[checkImageHeight][checkImageWidth][3];
void makeCheckImage(void)
{
int i, j, c;
for (i = 0; i < checkImageHeight; i++) {
for (j = 0; j < checkImageWidth; j++) {
c = ((((i&0x8)==0)^((j&0x8))==0))*255;
checkImage[i][j][0] = (GLubyte) c;
checkImage[i][j][1] = (GLubyte) c;
checkImage[i][j][2] = (GLubyte) c;
}
}
}
void init(void)
{
glClearColor (0.0, 0.0, 0.0, 0.0);
glShadeModel(GL_FLAT);
makeCheckImage();
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
}
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glRasterPos2i(0, 0);
glDrawPixels(checkImageWidth, checkImageHeight, GL_RGB,
GL_UNSIGNED_BYTE, checkImage);
glFlush();
}
When using glDrawPixels() to write RGBA or color-index information, you may need to control the current drawing buffers with glDrawBuffer(), which, along with glReadBuffer(), is also described in "Selecting Color Buffers for Writing and Reading" in Chapter 10. Copying Pixel Data within the Frame Buffervoid glCopyPixels(GLint x, GLint y,
GLsizei width, GLsizei height, Copies pixel data from the framebuffer rectangle whose lower-left corner is at (x, y) and whose dimensions are width and height. The data is copied to a new position whose lower-left corner is given by the current raster position. buffer is either GL_COLOR, GL_STENCIL, or GL_DEPTH, specifying the framebuffer that is used. glCopyPixels() behaves similarly to a glReadPixels() followed by a glDrawPixels(), with the following translation for the buffer to format parameter:
Note that there's no need for a format or data parameter for glCopyPixels(), since the data is never copied into processor memory. The read source buffer and the destination buffer of glCopyPixels() are specified by glReadBuffer() and glDrawBuffer() respectively. Both glDrawPixels() and glCopyPixels() are used in Example 8-4. For all three functions, the exact conversions of the data going to or from the framebuffer depend on the modes in effect at the time. See the next section for details. | ||||||||||||||||||||||||||||||||||||||||||||||
Програмирование с использованием OpenGL
