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156 wiersze
7.6 KiB
HTML
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<title>Image Data Format</title>
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<h2><a name="s3.2">3.2 Image Data Format</a></h2><a name="i0">
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<p>Arguably the most important aspect of an image acquisition system is how
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images are represented. The SANE approach is to define a simple yet powerful
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representation that is sufficient for vast majority of applications and
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devices. While the representation is simple, the interface has been defined
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carefully to allow extending it in the future without breaking backwards
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compatibility. Thus, it will be possible to accommodate future applications or
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devices that were not anticipated at the time this standard was created.
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<p>A SANE image is a rectangular area. The rectangular area is subdivided into a
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number of rows and columns. At the intersection of each row and column is a
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(preferable quadratic) pixel. A pixel consists of one or more sample values.
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Each sample value represents one channel (e.g., the red channel).
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<p>The SANE API transmits an image as a sequence of frames. Each frame covers
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the same rectangular area as the entire image, but may contain only a subset
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of the channels in the final image. For example, a red/green/blue image could
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either be transmitted as a single frame that contains the sample values for
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all three channels or it could be transmitted as a sequence of three frames:
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the first frame containing the red channel, the second the green channel, and
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the third the blue channel.
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<p>When transmitting an image frame by frame, the frontend needs to know what
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part of the image a frame represents (and how many frames it should expect).
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For that purpose, the SANE API tags every frame with a type and a format
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descriptor.
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<p><font color="darkgreen">
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There are two different types of frames: pixel oriented frames
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<tt>SANE_FRAME_RAW<a name="i1"></tt> and arbitrary data frames
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<tt>SANE_FRAME_MIME<a name="i2"></tt>. These types are discussed in detail in the
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following sections. The frame types used by version 1 of this standard
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(<tt>SANE_FRAME_GRAY<a name="i3"></tt>, <tt>SANE_FRAME_RGB<a name="i4"></tt>,
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<tt>SANE_FRAME_RED<a name="i5"></tt>, <tt>SANE_FRAME_GREEN<a name="i6"></tt>, and
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<tt>SANE_FRAME_BLUE<a name="i7"></tt>) are obsolete and superseded by
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<tt>SANE_FRAME_RAW<a name="i8"></tt>.
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<p> <h3><a name="s3.2.1">3.2.1 Pixel oriented frames</a></h3>
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<p> The type of pixel oriented frames is <tt>SANE_FRAME_RAW<a name="i9"></tt>. The
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frame contains one or more channels of data in a channel-interleaved format,
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that represents sample values from a property of the individual pixels that
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is subject to further description in the <tt>format_desc</tt> member of the
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<tt>SANE_Parameters</tt> structured type. See section <a href="doc012.html#s4.3.8">4.3.8</a>
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on page <a href="doc012.html#s4.3.8">4.3.8</a> for details about the format
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descriptions.
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</font>
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<p> Each sample value has a certain bit depth. The bit depth is fixed for the
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entire image and can be as small as one bit. Valid bit depths are 1, 8, or
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16 bits per sample. If a device's natural bit depth is something else, it is
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up to the driver to scale the sample values appropriately (e.g., a 4 bit
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sample could be scaled by a factor of four to represent a sample value of
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depth 8).
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<p><font color="darkgreen">
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The complete image may consist of several different channels. The number of channels
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is defined by member <tt>channels_per_image</tt> of <tt>SANE_Parameters</tt>.
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The image may be transmitted in an arbitary number of frames which can be
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determined by watching the <tt>SANE_PFLAG_LAST_FRAME</tt> flag in said type (or by
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counting the channels). Note: This frame type replaces all frame types of
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the SANE standard version 1.
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</font>
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<p>Conceptually, each pixel oriented frame is transmitted a byte at a time. Each
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byte may contain 8 sample values (for an image bit depth of 1), one full
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sample value (for an image bit depth of 8), or a partial sample value (for an
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image bit depth of 16 or bigger). In the latter case, the bytes of each
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sample value are transmitted in the machine's native byte order.
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<blockquote>
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<center>
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<b>Backend Implementation Note</b>
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</center>
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A network-based meta backend will have to ensure that the byte order
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in image data is adjusted appropriately if necessary. For example,
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when the meta backend attaches to the server proxy, the proxy may
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inform the backend of the server's byte order. The backend can then
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apply the adjustment if necessary. In essence, this implements a
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``receiver-makes-right'' approach.
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</blockquote>
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<p><p><a name="f2"></a>
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<center>
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<img width=390 height=196 src="img001.gif">
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<p><center>Figure 2: Transfer order of image data bytes</center>
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</center>
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<p>
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<p>The order in which the sample values in a frame are transmitted is illustrated
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in Figure <a href="doc008.html#f2">2</a>. As can be seen, the values are transmitted row by
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row and each row is transmitted from left-most to right-most column. The
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left-to-right, top-to-bottom transmission order applies when the image is
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viewed in its normal orientation (as it would be displayed on a screen, for
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example).
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<p>If a frame contains multiple channels, then the channels are transmitted in an
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interleaved fashion. Figure <a href="doc008.html#f3">3</a> illustrates this for the case
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where a frame contains a complete red/green/blue image with a bit-depth of 8.
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<p><p><a name="f3"></a>
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<center>
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<img width=624 height=111 src="img002.gif">
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<p><center>Figure 3: Bit and byte order of image data</center>
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</center>
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<p>
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<p>For a bit depth of 1, each byte contains 8 sample values of a <em>single</em>
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channel. In other words, a bit depth 1 frame is transmitted in a byte
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interleaved fashion. The first sample of each byte is represented by the most
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significant bit.
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<p><font color="darkgreen">
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For gray channels at a bit depth of 1 only two sample values are possible: 1
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represents minimum intensity (black) and 0 represents maximum intensity
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(white). For all other channel types and bit depths a sample value of 0
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represents minimum intensity and larger values represent increasing intensity.
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<p><h3><a name="s3.2.2">3.2.2 Arbitrary data frames</a></h3>
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<p>It also is possible to transmit arbitrary (not necessaryly pixel oriented)
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data. This allows transmission of compressed images like jpeg, tiff, etc.
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<p>The type of arbitrary data frames is <tt>SANE_FRAME_MIME<a name="i10"></tt>.
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The frame contains arbitrary data of the MIME (see RFC 1521/1522) type that is
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given in the <tt>format_desc</tt> member of the <tt>SANE_Parameters</tt>
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structured type (see See section <a href="doc012.html#s4.3.8">4.3.8</a> on
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page <a href="doc012.html#s4.3.8">4.3.8</a>). As such, it is assumed to be
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incomprehensible to the frontend, except for selected types the frontend is
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specifically capable of handling internally. The frontend is free to ignore
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those frames, or employ any appropriate means to otherwise handle this data
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(like saving them to disk or spawning an external viewer).
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</font>
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