Image Surfaces

Image Surfaces — Rendering to memory buffers

Synopsis




enum                cairo_format_t;
int                 cairo_format_stride_for_width       (cairo_format_t format,
                                                         int width);
cairo_surface_t*    cairo_image_surface_create          (cairo_format_t format,
                                                         int width,
                                                         int height);
cairo_surface_t*    cairo_image_surface_create_for_data (unsigned char *data,
                                                         cairo_format_t format,
                                                         int width,
                                                         int height,
                                                         int stride);
unsigned char*      cairo_image_surface_get_data        (cairo_surface_t *surface);
cairo_format_t      cairo_image_surface_get_format      (cairo_surface_t *surface);
int                 cairo_image_surface_get_width       (cairo_surface_t *surface);
int                 cairo_image_surface_get_height      (cairo_surface_t *surface);
int                 cairo_image_surface_get_stride      (cairo_surface_t *surface);

Description

Image surfaces provide the ability to render to memory buffers either allocated by cairo or by the calling code. The supported image formats are those defined in cairo_format_t.

Details

enum cairo_format_t

typedef enum _cairo_format {
    CAIRO_FORMAT_ARGB32,
    CAIRO_FORMAT_RGB24,
    CAIRO_FORMAT_A8,
    CAIRO_FORMAT_A1
    /* The value of 4 is reserved by a deprecated enum value.
     * The next format added must have an explicit value of 5.
    CAIRO_FORMAT_RGB16_565 = 4,
    */
} cairo_format_t;

cairo_format_t is used to identify the memory format of image data.

New entries may be added in future versions.

CAIRO_FORMAT_ARGB32

 each pixel is a 32-bit quantity, with alpha in the upper 8 bits, then red, then green, then blue. The 32-bit quantities are stored native-endian. Pre-multiplied alpha is used. (That is, 50% transparent red is 0x80800000, not 0x80ff0000.)

CAIRO_FORMAT_RGB24

 each pixel is a 32-bit quantity, with the upper 8 bits unused. Red, Green, and Blue are stored in the remaining 24 bits in that order.

CAIRO_FORMAT_A8

 each pixel is a 8-bit quantity holding an alpha value.

CAIRO_FORMAT_A1

 each pixel is a 1-bit quantity holding an alpha value. Pixels are packed together into 32-bit quantities. The ordering of the bits matches the endianess of the platform. On a big-endian machine, the first pixel is in the uppermost bit, on a little-endian machine the first pixel is in the least-significant bit.

cairo_format_stride_for_width ()

int                 cairo_format_stride_for_width       (cairo_format_t format,
                                                         int width);

This function provides a stride value that will respect all alignment requirements of the accelerated image-rendering code within cairo. Typical usage will be of the form: 

int stride;
unsigned char *data;
cairo_surface_t *surface;

stride = cairo_format_stride_for_width (format, width);
data = malloc (stride * height);
surface = cairo_image_surface_create_for_data (data, format,
					  width, height);

format :

 A cairo_format_t value

width :

 The desired width of an image surface to be created.

Returns :

 the appropriate stride to use given the desired format and width, or -1 if either the format is invalid or the width too large.

Since 1.6

cairo_image_surface_create ()

cairo_surface_t*    cairo_image_surface_create          (cairo_format_t format,
                                                         int width,
                                                         int height);

Creates an image surface of the specified format and dimensions. Initially the surface contents are all 0. (Specifically, within each pixel, each color or alpha channel belonging to format will be 0. The contents of bits within a pixel, but not belonging to the given format are undefined).

format :

 format of pixels in the surface to create

width :

 width of the surface, in pixels

height :

 height of the surface, in pixels

Returns :

 a pointer to the newly created surface. The caller owns the surface and should call cairo_surface_destroy() when done with it. This function always returns a valid pointer, but it will return a pointer to a "nil" surface if an error such as out of memory occurs. You can use cairo_surface_status() to check for this.

cairo_image_surface_create_for_data ()

cairo_surface_t*    cairo_image_surface_create_for_data (unsigned char *data,
                                                         cairo_format_t format,
                                                         int width,
                                                         int height,
                                                         int stride);

Creates an image surface for the provided pixel data. The output buffer must be kept around until the cairo_surface_t is destroyed or cairo_surface_finish() is called on the surface. The initial contents of buffer will be used as the initial image contents; you must explicitly clear the buffer, using, for example, cairo_rectangle() and cairo_fill() if you want it cleared.

Note that the stride may be larger than width*bytes_per_pixel to provide proper alignment for each pixel and row. This alignment is required to allow high-performance rendering within cairo. The correct way to obtain a legal stride value is to call cairo_format_stride_for_width() with the desired format and maximum image width value, and the use the resulting stride value to allocate the data and to create the image surface. See cairo_format_stride_for_width() for example code.

data :

 a pointer to a buffer supplied by the application in which to write contents. This pointer must be suitably aligned for any kind of variable, (for example, a pointer returned by malloc).

format :

 the format of pixels in the buffer

width :

 the width of the image to be stored in the buffer

height :

 the height of the image to be stored in the buffer

stride :

 the number of bytes between the start of rows in the buffer as allocated. This value should always be computed by cairo_format_stride_for_width() before allocating the data buffer.

Returns :

 a pointer to the newly created surface. The caller owns the surface and should call cairo_surface_destroy() when done with it. This function always returns a valid pointer, but it will return a pointer to a "nil" surface in the case of an error such as out of memory or an invalid stride value. In case of invalid stride value the error status of the returned surface will be CAIRO_STATUS_INVALID_STRIDE. You can use cairo_surface_status() to check for this. See cairo_surface_set_user_data() for a means of attaching a destroy-notification fallback to the surface if necessary.

cairo_image_surface_get_data ()

unsigned char*      cairo_image_surface_get_data        (cairo_surface_t *surface);

Get a pointer to the data of the image surface, for direct inspection or modification.

surface :

 a cairo_image_surface_t

Returns :

 a pointer to the image data of this surface or NULL if surface is not an image surface.

Since 1.2

cairo_image_surface_get_format ()

cairo_format_t      cairo_image_surface_get_format      (cairo_surface_t *surface);

Get the format of the surface.

surface :

 a cairo_image_surface_t

Returns :

 the format of the surface

Since 1.2

cairo_image_surface_get_width ()

int                 cairo_image_surface_get_width       (cairo_surface_t *surface);

Get the width of the image surface in pixels.

surface :

 a cairo_image_surface_t

Returns :

 the width of the surface in pixels.

cairo_image_surface_get_height ()

int                 cairo_image_surface_get_height      (cairo_surface_t *surface);

Get the height of the image surface in pixels.

surface :

 a cairo_image_surface_t

Returns :

 the height of the surface in pixels.

cairo_image_surface_get_stride ()

int                 cairo_image_surface_get_stride      (cairo_surface_t *surface);

Get the stride of the image surface in bytes

surface :

 a cairo_image_surface_t

Returns :

 the stride of the image surface in bytes (or 0 if surface is not an image surface). The stride is the distance in bytes from the beginning of one row of the image data to the beginning of the next row.

Since 1.2

See Also