How do I write a C++ program to read the JPG format picture, square the dots per pixel and output the image?

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You probably don’t actually want to do what you are asking how to do. As Jim Clack pointed out in his answer, a JPEG file doesn’t store a raster image, it stores a compressed image as a set of transforms. When you convert this image into a bitmap in memory, that’s a process called rendering. Say...
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You probably don’t actually want to do what you are asking how to do. As Jim Clack pointed out in his answer, a JPEG file doesn’t store a raster image, it stores a compressed image as a set of transforms. When you convert this image into a bitmap in memory, that’s a process called rendering. Say I had a JPEG image, and the image was, in theory, “naturally” 400x400 pixels in size. I could render it into a bitmap in memory as a 400x400 image, and that would get me a 400x400 raster image, as a bitmap. And then I take that image, and do as you suggest, to magnify it into an 800x800 image, turning each pixel into a square composed of 4 pixels, at two pixels on a side. You’re going to get a pretty crappy image, and unless your intent is to make the image look as if it were being displayed on an old-time 8-bit computer, like an Atari 2600, or a Commodore 64, or an old game console, such as the original Nintendo/Famicon system, or an old Mattel Intellivision, etc. — you’re going to end up with a pretty terrible image. If, instead, you created an 800x800 canvas (an empty bitmap), and rendered the JPEG into it at that resolution directly — then the math is going to work out in such a way as to make the image look a whole heck of a lot better. The reason for this is that the 2x2 pixel regions won’t all be the same color; instead, what gets rendered into an individual pixel is going to depend on the adjacent information — in other words, it’s going to decompress into different pixel colors, for each of the pixels in the 2x2 region. read less
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The logic of this program includes the size of an image. Assuming the size of an image as 250x250. function img_read( ) { image(i,j)=0; for(i=0;i<=249;i++) { for(j=0;j<=249;j++) { image(i,j) = image(i,j)+25; } } printf("The read image with enhanced contrast is:"ima...
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