Programmer 7500

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Text File | 1988-07-28 | 13.0 KB | 274 lines

Some notes about screen dumps Written by Michael Day 25 July 1988 First, display screens are less accurate than printers. The sizes can vary even when the screen is specified as the same size. Further, the adjustments of the screen can affect size and aspect ratio. A standard 12 inch monitor will provide a display area of around 20cm by 15cm (or 8" by 6"). This provides an aspect ratio of 4:3. In order for the printout to look the same as the screen, it is important that the aspect ratio on the printer be the same as the screen. Although the overall screen aspect ratio normally stays around 4:3, the pixel aspect ratio can change depending on the mode selected. As an example, consider a CGA color graphics screen which displays 320x200 pixels. This means that there are 40 pixels per inch horizontally, and 33.33 pixels per inch vertically. This gives the pixels an aspect ratio of 2.5:3. Meaning that if we were to magnify a single pixel large enough it would be 2.5 inches wide by 3 inches tall. When we switch the screen to the monochrome graphics mode we get 640x200 pixels on the screen. This means that the pixel aspect ratio is now 1.25:3. The pixel has shrunk in half in the horizontal direction, but remained the same in the vertical direction. The pixel aspect ratio is also affected when other modes or displays are used, such as 640x350 for EGA, 640x480 for VGA, or 720x348 for Hercules. Note: When we say that the pixel aspect ratio is 2.5:3, that is describing the area allocated on the screen to the pixel, and not the actual spot size of the electron beam. Case in point, on the CGA display, the pixel definition is crowded so close together in the horizontal direction that on most displays the dots overlap, but in the vertical direction the spot size is too small to completely fill in the area between the dots. On the VGA display this is corrected by double scanning (scanning two display lines for every CGA line). This effectively doubles the spot size in the vertical direction to fill in the gaps between the CGA lines. The spot size on a display is usually referred to as "pitch", and the allocated display area is usually referred to as a "pixel". The pitch size is a function of the monitor. The pixel size is a function of the display card. The printer: On the printer we have similar problems. The printer's aspect ratio is usually not the same as the display's aspect ratio, so some correction must be done if proper aspect ratio is to be maintained. Without proper aspect ratio, circles on the display will come out as ovals on the printer. The Epson printer is the most common printer used in conjunction with the PC. This is because it is the printer that IBM selected to provide with their computer. In it's "normal" graphics mode the Epson printer will print 60 dots per inch horizontally, and 72 dots per inch vertically. The resolution of 72 dots per inch vertically was chosen because character size is measured in "points" and a point is defined as being 1/72 inch. The position of the wires in the print head determine the vertical resolution, and since they are fixed at 1/72 inch apart, the vertical resolution is fixed at 1/72 inch. It is possible to simulate a higher resolution by moving the paper slightly and printing a second pass over the line that was just printed. The Epson provides for this with the ESC 3 command which causes the printer to advance the paper by the specified n/216 inch increment when a line feed is received. However, this means that three passes for each line must be used to obtain this resolution, which slows printing down by a factor of three. Thus it is not often used. Another alternative is to use a 24 pin printer which uses three rows of 8 pins to achieve the same effect on a single pass. In the horizontal direction the printer resolution can change depending on the mode selected. In normal graphics mode on the Epson this is set to 60 dots per inch. The reason behind this is quite simple, the normal resolution of a character printed in 10 pitch on the Epson is 6 dots horizontally (discounting the half dot fudging Epson uses to boost apparent resolution). The paper used is usually eight inches wide, and to fit eighty characters onto the line at 6 dots per character area you end up with 480 dots on the line, or 60 dots per inch. Six (five active pixels plus one separator) is the minimum number of pixels wide that can be used to fully represent all the ASCII characters. As was mentioned, the Epson has a half dot fudge which allows it to put twice as many dots on the same line. It does this by simply moving the head only half as far to print the next dot. By selecting double density mode you can get 120 dots per inch which gives you a resolution of 1/120 inch. This doesn't come free. In order to print twice as many dots on a line, the printer has to print twice as many dots. That means that it will be slower. The reason it is slower is because it takes a finite period of time to move the print wires in the head. Enough time must be allowed for the wires to retract in preparation for the next print. The 'fudge' in character mode printing is that the dots are specified in such a way that there are no two consecutive half dots in a row, thus giving enough time for the wire to retract without having to slow the printer down. But since in graphics mode the printer can't know ahead of time whether there will be any consecutive half dots, it must assume that there will be if you select the double density mode. Selecting double density mode does not mean that the printer will slow down to half print speed, there are other factors that come into play that control this, including head return time, and line feed time. In later model printers there is also an added super density mode which allows four dots to be printed in the space of one. And on some 24 pin printers there is even a "hex" density mode, allowing for 360 dots per inch resolution. The printer handles this by adding some intelligence in the printing. If the previous dot was printed then it will not print the next one even if told to. That way there is enough time for the wire to be retracted in preparation for the following dot. This allows the printer to print 240 dots per inch resolution or more while running at 120 dots per inch speed. Since most programs try to be compatible to the original Epson MX-80 printer, the super density mode is seldom used. Printer aspect ratio: As was previously mentioned, the Epson printer does not have the same aspect ratio as the display, though there are tricks that can be used to restore the over all image aspect ratio. This is performed by printing additional dots at certain times to change the apparent pixel aspect ratio of the printer. With the exception of the CGA display, the translation is usually not perfect. There are two ways in which the display can be printed. normal 'upright' mode, in which the display's horizontal axis appears on the printer's horizontal axis. And 'landscape' mode where the display is printed out sideways. A normal upright printout will provide an image that is the same size and orientation as the display, but has the disadvantage of requiring a lot of fiddling with the dots to produce the printout. The landscape (or sideways) printout does not require as much fiddling, but causes the picture to grow slightly in size (8.9" x 6.7" instead of 8" x 6"). A real big advantage is with the CGA display mode. When printing a CGA display sideways all that needs to be done is to double the display's horizontal scan lines on the printer for 640x200 mode, or double it in both directions in 320x200 mode. The printout will come out with the proper aspect ratio. The CGA display could be printed upright without any bit twiddling at all, but the aspect ratio will be way off, and the picture would be smaller. By bit twiddling it is possible to restore the picture to the same size as the display (8"x6"), but there will be some distortion at the pixel level of the printout. With Hercules, EGA, and VGA type displays it is normally not possible to perform a screen dump without some distortion. If it is important that the picture fit the 8"x6" display format then the upright display method should be used, but you should realize that there will be significant distortion at the pixel level to keep the aspect ratio the same. If minimal distortion at the pixel level is desired, then the landscape method should be used. The primary disadvantage is that the picture size will grow slightly (8.9"x6.7") in order to achieve the reduced distortion. Also note that the Hercules display will grow even further, expanding to 10"x7.5" to achieve the reduced pixel distortion. While it is possible to achieve the same display size in the landscape mode, there is no reason to do so since it would end up being even more distorted at the pixel level than the upright mode. The amount of pixel level distortion is a trade off of speed verses distortion. It is possible to achieve minimal distortion by selecting the super resolution mode and using six print passes per line. However that will cause the print time to be increased by a factor of six. This is seldom acceptable given that the print time in normal mode is too slow as it is. One final note on speed. The print time for a given mode is based entirely on how much data there is to print. The larger the picture, the longer it will take to print it. Scaling distortion: When scaling occurs, there will be trade-offs as a result. And since in the normal course of events when doing a screen dump scaling will occur, you will normally be encountering these trade-offs. The trade-offs cause distortion of the picture in one form or another. The two main forms of distortion are aspect ratio, and pixel distortion. When scaling down there is also an additional degradation caused by screen pixel data being lost. Aspect ratio distortion is where the overall picture ratio on the printer does not match the ratio provided on the screen. Sometimes this is intentional, but usually it is not desired. For engineering graphs, aspect ratio errors greater than 2% are considered unacceptable. For business charts aspect ratio errors greater than 5% begin to become unacceptable. In all cases aspect ratio errors greater than 10% usually are considered unacceptable as the picture will be noticeably distorted. There are times when it might be desirable to exceed the aspect ratio error factor. This might be done if the pixel level distortion is considered unacceptable. Pixel level distortion is most noticeable in the printer's Y (vertical) direction. This is because each distortion increment is 1/72 inch. This is readily noticeable. Sometimes for the sake of clarity it is desirable to forgo the overall aspect ratio in order to obtain a minimal pixel level distortion. There is no easy way to determine what is best, it is an individual thing. Pixel level distortion is caused by the inability to evenly divide the screen pixels into the same number or multiple of pixels on the printer. The usual end result is that occasionally there will be additional pixels added in the picture causing a slight pixel level distortion to maintain the overall picture aspect ratio. By slightly changing the aspect ratio of the printout you can sometimes eliminate the distortion, or at least cause the distortion to occur in a pattern which is more acceptable. When scaling up (expanding) a screen picture the distortion can be noticeable, but no data is lost, it is just slightly distorted due to the added pixels. When scaling down decisions must be made by the program on what data to retain and what to discard. This is usually handled on a priority basis. The colors are each assigned a priority. When two or more screen pixels must be placed in the position of one printer pixel then the screen pixel with the highest priority color will take precedence, and the rest will be discarded.