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ALLOCIMAGE(2)ALLOCIMAGE(2)

NAME

allocimage, allocimagemix, freeimage, nameimage, namedimage, setalpha, loadimage, cloadimage, unloadimage, readimage, writeimage, bytesperline, wordsperline – allocating, freeing, reading, writing images

SYNOPSIS

#include <u.h> #include <libc.h> #include <draw.h>

Image *allocimage(Display *d, Rectangle r,
ulong chan, int repl, ulong col)

Image *allocimagemix(Display *d, ulong one, ulong three)

int freeimage(Image *i)

int nameimage(Image *i, char *name, int in)

Image *namedimage(Display *d, char *name)

ulong setalpha(ulong color, uchar alpha)

int loadimage(Image *i, Rectangle r, uchar *data, int ndata)

int cloadimage(Image *i, Rectangle r, uchar *data, int ndata)

int unloadimage(Image *i, Rectangle r, uchar *data, int ndata)

Image *readimage(Display *d, int fd, int dolock)

int writeimage(int fd, Image *i, int dolock)

int bytesperline(Rectangle r, int d)

int wordsperline(Rectangle r, int d)

enum

DESCRIPTION

A new Image on Display d is allocated with allocimage; it will have the rectangle, pixel channel format, replication flag, and initial fill color given by its arguments. Convenient pixel channels like GREY1, GREY2, CMAP8, RGB16, RGB24, and RGBA32 are predefined. All the new image’s pixels will have initial value col. If col is DNofill, no initialization is done. Representative useful values of color are predefined: DBlack, DWhite, DRed, and so on. Colors are specified by 32-bit numbers comprising, from most to least significant byte, 8-bit values for red, green, blue, and alpha. The values correspond to illumination, so 0 is black and 255 is white. Similarly, for alpha 0 is transparent and 255 is opaque. The id field will have been set to the identifying number used by /dev/draw (see draw(3)), and the cache field will be zero. If repl is true, the clip rectangle is set to a very large region; if false, it is set to r. The depth field will be set to the number of bits per pixel specified by the channel descriptor (see image(6)). Allocimage returns 0 if the server has run out of image memory.

Allocimagemix is used to allocate background colors. On 8-bit color-mapped displays, it returns a 2×2 replicated image with one pixel colored the color one and the other three with three. (This simulates a wider range of tones than can be represented by a single pixel value on a color-mapped display.) On true color displays, it returns a 1×1 replicated image whose pixel is the result of mixing the two colors in a one to three ratio.

Freeimage frees the resources used by its argument image.

Nameimage publishes in the server the image i under the given name. If in is non-zero, the image is published; otherwise i must be already named name and it is withdrawn from publication. Namedimage returns a reference to the image published under the given name on Display d. These routines permit unrelated applications sharing a display to share an image; for example they provide the mechanism behind getwindow (see graphics(2)).

The RGB values in a color are premultiplied by the alpha value; for example, a 50% red is 0x7F00007F not 0xFF00007F. The function setalpha performs the alpha computation on a given color, ignoring its initial alpha value, multiplying the components by the supplied alpha. For example, to make a 50% red color value, one could execute setalpha(DRed, 0x7F).

The remaining functions deal with moving groups of pixel values between image and user space or external files. There is a fixed format for the exchange and storage of image data (see image(6)).

Unloadimage reads a rectangle of pixels from image i into data, whose length is specified by ndata. It is an error if ndata is too small to accommodate the pixels.

Loadimage replaces the specified rectangle in image i with the ndata bytes of data.

The pixels are presented one horizontal line at a time, starting with the top-left pixel of r. In the data processed by these routines, each scan line starts with a new byte in the array, leaving the last byte of the previous line partially empty, if necessary. Pixels are packed as tightly as possible within data, regardless of the rectangle being extracted. Bytes are filled from most to least significant bit order, as the x coordinate increases, aligned so x=0 would appear as the leftmost pixel of its byte. Thus, for depth 1, the pixel at x offset 165 within the rectangle will be in a data byte at bit-position 0x04 regardless of the overall rectangle: 165 mod 8 equals 5, and 0x80 >> 5 equals 0x04.

Cloadimage does the same as loadimage, but for ndata bytes of compressed image data (see image(6)). On each call to cloadimage, the data must be at the beginning of a compressed data block, in particular, it should start with the y coordinate and data length for the block.

Loadimage, cloadimage, and unloadimage return the number of bytes copied.

Readimage creates an image from data contained in an external file (see image(6) for the file format); fd is a file descriptor obtained by opening such a file for reading. The returned image is allocated using allocimage. The dolock flag specifies whether the Display should be synchronized for multithreaded access; single-threaded programs can leave it zero.

Writeimage writes image i onto file descriptor fd, which should be open for writing. The format is as described for readimage.

Readimage and writeimage do not close fd.

Bytesperline and wordsperline return the number of bytes or words occupied in memory by one scan line of rectangle r in an image with d bits per pixel.

EXAMPLE

To allocate a single-pixel replicated image that may be used to paint a region red,

	red = allocimage(display, Rect(0, 0, 1, 1), RGB24, 1, DRed);

SOURCE

/sys/src/libdraw

SEE ALSO

graphics(2), draw(2), draw(3), image(6)

DIAGNOSTICS

These functions return pointer 0 or integer –1 on failure, usually due to insufficient memory.

May set errstr.

BUGS

Depth must be a divisor or multiple of 8.