By Rich Marcus, Digital Projection Coordinator
Confused by the rules for submitting your digital projection entry? So was I when I took over as DPC last month, especially when entries started arriving that apparently didn’t conform with those rules. The rules state that an allowable image is “JPEG, 72 dpi [dots per inch], approximately 500KB, with dimensions not to exceed either 1024 pixels in width or 768 pixels in height.” The following discussion will attempt to clarify what this means in practice. I’ll assume that everyone knows what a JPEG is and will focus on the other requirements. Ill also assume that everyone knows what a pixel is.
Image files come out of your camera with a certain size in terms of pixel dimensions. The pixel dimensions and can be set in your camera’s menu system. For example, my 10 megapixel Pentax K10D allows a choice of 3 image sizes: 3872 pixels wide by 2592 pixels high (for a total of 10 megapixels), 3008 x 2000 (6 megapixels), and 1824 x 1216 (2 megapixels). You can see these dimensions in your editor program after you open your image by going to the Image – Resize function. Note that higher pixel dimensions result in a larger file size.
Many computer monitors and, especially, the display screens on most laptops can’t display this many pixels. Usually maximum display resolution is 1024 pixels wide x 768 pixels high, and this is true of the laptops we use at competitions. Since the competition projector merely projects the pixels on the display of the laptop it’s connected to, the projector is limited to showing images with the computer screen’s pixel dimensions. If your image exceeds these dimensions, the computer will throw out some of the pixels (a process called “resampling” or “downsampling”) to reach the desired number, and that is what is projected. The bigger the image you send, the more pixels get thrown out, possibly affecting the quality of the projected image. While it may be hard to see any quality change in an image projected on the beaded glass screen on the stage, judges do have the option of looking at the laptop display where such a change might be more noticeable.
Note that for an image in portrait mode, height is greater than width but is still limited to 768 pixels. For images which exceed this limit, height will be subject to downsampling during projection and width will be downsampled proportionally to maintain aspect ratio (nobody in the picture gets thin or fat). Note also that a square image no greater than 768 x 768 pixels (and we had one last month) is perfectly fine.
If your image’s pixel dimensions are SMALLER than the display’s, perhaps the result of cropping, your image will be displayed smaller on the screen, In a recent competition, one image came in at 307 x 224 and hence projected at less than 1/9 of the size of other images. If this is the case, you should try resizing the image UP for maximum impact. However, too much “upsampling” (the addition of new pixels invented by the computer) can result in a change in image quality.
What about that vague 500 KB file size rule? This is a recommendation, not a requirement, but I think you’ll find that a 1024 x 768 JPEG image will be right around there, perhaps as high as 1 MB using the highest quality setting. If it’s much greater than that, check your pixel dimensions again. The rule exists because in the old days, big files presented a problem for transmission and storage, but that is less of a problem today, at least in my (limited) experience.
Finally, what about the 72 dpi resolution requirement? Resolution is something that is tagged onto a file, much like EXIF information, so changing it in your editor will not affect file size. Many cameras use 72 dpi as the default output resolution because that is the typical resolution of a computer monitor. Actually, this parameter is only relevant in the context of print output (perhaps the subject of another article). However, since 72 dpi it is stated as a requirement for projection images, go ahead and change resolution to 72 dpi when you’re in the Resize dialog box. It won’t hurt anything.
RECOMMENDATIONS: Based on these factors, what should you do? Before submitting your image, go to the Resize (sometimes called “Image resize” or “Image size”) function in your editor and adjust the pixel dimensions accordingly: width close to but no greater than 1024 pixels, height close to no greater than 768 pixels. (Don’t confuse image dimensions in pixels with print dimensions in inches or other units.) You will have to check the resample box in order to change these figures. This puts the resampling process under your control, rather than subject to the whims of our laptop. Check your editor’s Help function or other information source for advice on the various options. While in the Resize dialog box, adjust resolution to 72 dpi. Not related to size, but it is suggested that you use the sRGB color space for your image, as that is what the projector is set for. Lastly, when saving your image file, please name the file descriptively and according to the convention stated in the rules (DIV-Descriptive Title can use spaces and Capitals-LastName-FirstName. Those are hyphens, not underscores.). Also, check to be sure that the file size is not grossly over 1 MB.
Thanks to Judy M. and to Tom and Barbara S. for their insights and assistance in preparing this article.