Freescale Semiconductor
Application Note
Document Number: AN4339
Rev. 0, 8/2011
Using the Run Length Encoding
Features on the MPC5645S
by:
Ioseph Martinez
Guadalajara
Mexico
Contents
1 Introduction
Run length encoding (RLE) is a method that allows data
compression for information in which symbols are repeated
constantly. The method is based on the fact that the repeated
symbol can be substituted by a number indicating how many
times the symbol is repeated and the symbol itself. Data which
is compressed using RLE method falls into two categories:
• Repeated symbols: This subset of data consists of the
symbols that can be compressed by replacing them with
a number indicating how many times the symbol is
repeated and the repeated symbol.
• Non-repeated symbols: This subset of data consists of
the symbols that cannot be compressed because they are
not repeated and cannot be predicted. This is the noncompressed data.
For example, consider the following text string:
YYYYYYNNNNNYYYYYYYYYNNNNNNYNYN. The
string consists of 30 elements with two different symbols. By
applying RLE we can get the following reduced string:
6Y5N9Y6NYNYN, consisting only in 12 elements achieving
a compression of 2.5 times.
There are several ways to encode raw data using RLE. In the
case of the MPC5645S, the data is encoded pixel by pixel,
where pixel = symbol, from the starting address of the bitmap
raw data as a single dimension array. Figure 1 shows how this
is done.
© 2011 Freescale Semiconductor, Inc.
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Introduction..............................................................1
2
RLE on the MPC5645S...........................................2
2.1
DCU3 and DCULite embedded RLE......................2
2.2
Standalone RLE decoder..........................................3
2.2.1
Decoding non-graphical RLE data...........................4
3
Use case examples...................................................5
4
Example code...........................................................6
RLE on the MPC5645S
Figure 1. RLE encoding direction
2 RLE on the MPC5645S
The MPC5645S has runtime RLE decompression optimized for pixel data. The module can decode the following different
pixel formats for source data: 8 bpp, 16 bpp, 24 bpp, and 32 bpp. This allows the selection of atomicity of source data for
more efficient compression.
The RLE capabilities of the MPC5645S can be divided into two sections:
• DCU3 and DCULite embedded RLE: The DCU3 and the DCULite can automatically read RLE compressed data from
images and display them seamlessly. Both DCU3 and DCULite have a dedicated decompression module and can be
used simultaneously with the standalone RLE module.
• Standalone RLE decoder: This module allows the decompression of RLE data independently from the DCU3 or the
DCULite. It can be used for any type of RLE data, including non-graphics data.
There are different ways to compress data using the RLE method. The MPC5645S RLE compressed data consists of frames
of two elements: a command byte and the data.
• The command is an 8-bit value: CMD[7:0].
• The most significantive bit, CMD[7], indicates if the following data is compressed or not.
• The remaining bits, CMD[6:0], indicate the number of pixels that will be decoded. This number is offset by 1 so the
number of pixels, N = CMD[6:0] + 1.
• The data atomicity will depend on the selected pixel format, 8, 16, 24, or 32 bpp.
• The data size when it is compressed, CMD[7] = 1, will be always one pixel. This means that CMD[6:0] + 1 pixels need
to be unrolled.
• The data size when it is not compressed, CMD[7] = 0 will be always CMD[6:0] + 1.
Figure 2. RLE compressed data example
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RLE on the MPC5645S
2.1 DCU3 and DCULite embedded RLE
The DCU3 and the DCULite have the ability decode and display RLE compressed images on the fly. Using an RLE image
has the following limitations:
• Can be used only on either layer 0 or layer 1 on a single layer
• Supports 8 bpp, 16 bpp –except YUV, 24 bpp, and 32 bpp
• Tile mode is not permitted when RLE mode is active
From the application point of view, there is no difference between using a non-RLE and an RLE image except that by using
an RLE image a lot of memory space can be saved. It can be very useful if a video or an animation needs to be played and all
of the images can be compressed, or the image dimensions are big and it has a good compression ratio.
Configuring the DCU3 or the DCULite to display an RLE image is similar to configuring it to display any other kind of
image. You must only perform the following additional steps to the layer initialization:
• Set DDR_MODE in DCU_MODE
• Set the compressed size of the bitmap on the Compression Image Size Register COMP_IMSIZE
• Set the RLE_EN bit on the control descriptor 4 register CTRLDESCLn_4
2.2 Standalone RLE decoder
RLE compression is not only useful to compress images but other types of data such as sound and program code. The
standalone RLE decoder in the MPC5645S allows decoding any type of data whether it is an image or not.
To decode RLE data, configure the eDMA channels to move the data to and from the RLE decoder module. DMA transfers
will be triggered when required by input and output FIFOs.
Figure 3. RLE application block
The RLE data in application diagram shown in Figure 3 can be fetched from any of the available memories and the decoded
data can be put into SRAM, GRAM, or DRAM memory.
When images are decoded, the RLE Decoder also has the ability to extract a smaller image or portion of the original RLEcompressed image. The images are divided into a virtual grid of pixels where it is possible to select the start position and end
position. Figure 4 is an example of an image with the grid division.
NOTE
The grid start point is located at (1,1), the end point is located at (width, height). The start
and the end points of the selection rectangle are included in the partial image area.
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RLE on the MPC5645S
Figure 4. 20x20 pixels image example with grid
To configure the RLE module, use the steps below. A software example is provided at the end of this application note.
• Configure the read eDMA channel, Channel A in Figure 3
a. The channel reading the RLE data must point to the starting address where the RLE data is located and as
destination the memory mapped write FIFO.
b. 8, 16, or 32 bit transfer size accesses can be used. Configure Source and Destination size accordingly.
c. Destination offset will be zero because the write FIFO is memory mapped while Source offset depends on the
transfer size.
d. The minor loop count depends on the configured threshold of the write FIFO. The minor loop count must be
calculated such that it never overflows the write FIFO.
e. Set the major loop count to 1 and configure the DMAMUX to trigger transfers each time the write FIFO is below
the threshold.
• Configure the write eDMA channel, Channel B in Figure 3
a. The channel reading the uncompressed data must point to the read FIFO where the decoded data is placed and as
destination the memory where to output the raw data.
b. 8, 16, or 32 bit transfer size accesses can be used. Configure Source and Destination size accordingly.
c. Source offset will be zero because the read FIFO is memory mapped while Destination offset depends on the
transfer size.
d. The minor loop count depends on the configured threshold of the read FIFO. The minor loop count must be
calculated such that it never overruns the read FIFO.
e. Set the major loop count to 1 and configure the DMAMUX to trigger transfers each time the read FIFO is above
the threshold.
• Configure the RLE module
a. First, make sure the module is disabled, MDIS=1.
b. Set the compressed size of the RLE data in the register RLE_DEC_CISR.
c. Set the X and Y original image size in pixels in the register RLE_DEC_ DICR.
d. Configure the area of interest: set the start pixel coordinates, RLE_DEC_SPCR, and final pixel coordinates,
RLE_DEC_EPCR. The area of interest can be the same original image or a smaller part from the original image.
Check Figure 4 coordinates example.
e. Configure the pixel format, WIDTH, on the register, RLE_DEC_ICR.
f. Enable the module so the decoding process starts, MDIS=0.
2.2.1 Decoding non-graphical RLE data
It is possible to decode other types of RLE data, such as sound samples or program code, that is not defined as a 2D image.
This type of data doesn’t have X and Y attributes, therefore note the following recommendations:
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Use case examples
• Select a pixel format for your non-graphical data according the basic format of the data. For instance, if your data is 16bit sound samples, use 16-bpp.
• If the data decoded size is <65536 symbols, configure X to be the number of symbols and Y = 1 in the register
RLE_DEC_ DICR.
• If the total data decoded size is >65535 symbols, the encoder must be configured so it generates RLE packets with a
maximum size of 65535. Each packet is decoded as an individual RLE data block.
• Set the RLE_DEC_SPCR register to the start point = (1,1).
• Set the RLE_DEC_EPCR register to the final point = (DICR.X, DICR.Y) to decode the complete data chunk.
3 Use case examples
It is possible to use statistical methods to analyze the entropy of certain images and decide whether the compression ratio will
be good or not, but typically it is done empirically by just looking at the images. Alternatively, performing an RLE
compression of an image is fairly fast in order to apply the trial and error method.
Because RLE is applied along the X axis, as it is how images are natively stored on memory, data having long runs of
repeated data on the X axis will be compressed efficiently while data having long runs only on the Y axis will not be
compressed.
Some images with good chances to have a good compression ratio are:
• Horizontal gradients
• Any type of shape having a solid background
• Images having single color areas
Images coming from the real world as pictures are usually not good choices for compression. The table below showcases
some examples of images with its compression ratios.
Table 1. RLE compression examples
Image
Description
Size
Comp. Size
Comp. Ratio
This image is a round shape with
a solid background and single
color areas around the center.
This type of graphics always
results in very good compression
ratios. 200x176@8bpp
35200 Bytes
11864 Bytes
3:1
This is a background image
having identifiable black color
areas between the green
diamonds and further black inside
them. This allows a 47% size
reduction. 480x272@8bpp
130560 Bytes
69015 Bytes
<2:1
Table continues on the next page...
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Example code
Table 1. RLE compression examples (continued)
Image
Description
Size
Comp. Size
Comp. Ratio
This example has a horizontal
600000 Bytes
color gradient with a big black
color area; all these
characteristics give this picture an
excellent RLE compression ratio.
500x400@24bpp
111031 Bytes
>5:1
Raster text is often a good
candidate for compression,
especially if it is big.
200x57@4bpp (Grayscale)
2781 Bytes
4:1
774304 Bytes
1:1
11400 Bytes
This is an example of what type of 786432 Bytes
images not to use for RLE
compression. The compression
ratio is almost unitary.
512x512@24bpp
4 Example code
The following example code shows how to configure the RLE decoder module and the eDMA to decode and transfer the data
to certain memory location:
/* Initialize eDMA channel TCD (0,1) to zeroes */
ptr = &EDMA.TCD[0].SADDR;
for(i = 0; i<16; i++){ *ptr++ = 0; }
/* configure eDMA channel 0 (for write FIFO) */
EDMA.TCD[0].SADDR= (uint32_t)rleBitmap;//Src Bitmap
EDMA.TCD[0].DADDR= 0x90000000 + 0x80;//mem mapped write FIFO
EDMA.TCD[0].SSIZE = 2;// src size 32bits
EDMA.TCD[0].DSIZE = 2;// dst size 32bits
EDMA.TCD[0].SOFF = 4;// src offset 4 bytes
EDMA.TCD[0].DOFF = 0;// dst offset 0 bytes
EDMA.TCD[0].BITER= 1;// major loop = 1
EDMA.TCD[0].CITER= 1;// major loop = 1
EDMA.TCD[0].NBYTESu.R = 28;// minor loop = 28
EDMA.TCD[0].BWC = 3;// BW control helps relief
EDMA.SERQ.R = 0;//enable requests
DMAMUX.CHCONFIG[0].B.ENBL= 1;//enable DMAMUX for channel 0
DMAMUX.CHCONFIG[0].B.SOURCE= 54;//write FIFO request
/* configure eDMA channel 1 (for read FIFO) */
EDMA.TCD[1].SADDR= 0x90000000 + 0xC0;//mem mapped Read FIFO
EDMA.TCD[1].DADDR = 0x60000000;//GRAM mem start
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Example code
EDMA.TCD[1].SSIZE = 2;// src size 32bits
EDMA.TCD[1].DSIZE = 2;// dst size 32bits
EDMA.TCD[1].SOFF = 0;// src offset 0 bytes
EDMA.TCD[1].DOFF = 4;// dst offset 4 bytes
EDMA.TCD[1].BITER = 1;// major loop = 1
EDMA.TCD[1].CITER = 1;// major loop = 1
EDMA.TCD[1].NBYTESu.R = 28;// minor loop = 28
EDMA.TCD[1].BWC = 3;// BW control helps relief
EDMA.SERQ.R = 1;//enable requests
DMAMUX.CHCONFIG[1].B.ENBL= 1;//enable DMAMUX for channel 1
DMAMUX.CHCONFIG[1].B.SOURCE = 53;//write FIFO request
/* Configure RLE module */
RLE.MCR.B.MDIS= 1;// disable module
RLE.CISR.B.SIZE= sizeof(bitmap);// compressed size
RLE.DICR.B.X = 240;// pixels/symbols width
RLE.DICR.B.Y = 180;
// pixels/symbols height
RLE.ICR.B.WIDTH= 0;// 8bpp size
// decompress full size (240x180)
RLE.SPCR.B.X= 1;// start x pixel (origin)
RLE.SPCR.B.Y= 1;// start y pixel (origin)
RLE.EPCR.B.X= 240;// final x pixel (width)
RLE.EPCR.B.Y= 180;// final y pixel (height)
RLE.MCR.R |= 4;// enable TX FIFO Flush
RLE.MCR.B.MDIS = 0;// enable module
The following example code shows how to configure Layer 0 to display a 24 bpp RLE graphic.
NOTE
The following code requires the system and DCU to be up and running. The bitmap data
used must be compressed using the RLE MPC5645S format.
/* Set DDR Mode ON on the DCU*/
DCU.DCU_MODE.B.DDR_MODE = 1; //DDR Mode On
/* Configure the Layer 0 control descriptors */
DCU.LAYER[0].CTRLDESCL1.B.HEIGHT = 272;//Height of layer
DCU.LAYER[0].CTRLDESCL1.B.WIDTH = 480;//Width of layer
DCU.LAYER[0].CTRLDESCL2.B.POSY = 0;//Place this distance from top
DCU.LAYER[0].CTRLDESCL2.B.POSX = 0;//Place this distance from left
// Address of image
DCU.LAYER[0].CTRLDESCL3.R = (uint32_t)rleBitmap;//Compressed bitmap address
DCU.LAYER[0].CTRLDESCL4.B.LUOFFS = 0;//In this case it's the first LUT
DCU.LAYER[0].CTRLDESCL4.B.BPP = 5;//Bits per pixel (= 24)
DCU.LAYER[0].CTRLDESCL4.B.TRANS = 0xFF;//Not transparent
DCU.LAYER[0].CTRLDESCL4.B.BB = 1;//Remove chroma
// Chroma setting
DCU.LAYER[0].CTRLDESCL5.B.CKMAX_R = 0x00;//Max red chroma
DCU.LAYER[0].CTRLDESCL6.B.CKMIN_R = 0x00;//Min red chroma
DCU.LAYER[0].CTRLDESCL5.B.CKMAX_G = 0x00;//Max green chroma
DCU.LAYER[0].CTRLDESCL6.B.CKMIN_G = 0x00;//Min green chroma
DCU.LAYER[0].CTRLDESCL5.B.CKMAX_B = 0x00;//Max blue chroma
DCU.LAYER[0].CTRLDESCL6.B.CKMIN_B = 0x00;//Min blue chroma
/* RLE Configuration registers */
DCU.LAYER[0].CTRLDESCL4.B.RLE_EN = 1;//Enable RLE on this layer
DCU.COMP_IMSIZE.B.SIZE = sizeof(rleBitmap);//Set the compressed image size
DCU.LAYER[0].CTRLDESCL4.B.EN = 1;//Enable layer
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Document Number: AN4339
Rev. 0, 8/2011
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