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>> [MUSIC PLAYING]

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>> ZAMYLA CHAN: Let's blow
things up with resize.

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In resize, the user will pass in, via
the command line, a bitmap image that

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they want you to scale by a number
that they also pass in,

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in the command line.

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How do we do this?

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>> Well, first things first, let's
break this down into steps.

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You're going to want to open the infile
that they give you, as well as

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create and open the outfile that
you're going to put the

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resized image in.

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Then, because you're resizing, and
because it's a bitmap, it a header, so

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you're going to also update the header
information for the outfile,

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and write that in.

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Then, you're going to read into
the scanline of the infile,

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pixel-by-pixel, resizing horizontally
and writing those pixels into the

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outfile, as specified
by the user's scale.

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>> You're going to remember to
add padding as necessary.

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More on that later.

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And then, also resize vertically.

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OK.

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So this is going to be a little bit more
complicated than Who Done It, but

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what's similar is that copy.c will,
again, prove very useful.

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Remember that copy.c opens a file,
updates the header information for the

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outfile, then reads into the scanline,
pixel-by-pixel, writing every pixel

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into the output file's scanline.

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>> So again, your first step might probably
be to cp, copy.c, resize.c

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into your PSET5 directory.

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Remember though, before you copy it,
to make sure that you understand

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copy.c very thoroughly.

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OK.

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>> So let's open a file.

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You know how to do that.

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I'm going to leave that to you.

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Next, update the header information
for the outfile.

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Because we have a new bitmap,
we have new header info.

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What's changing here?

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Well, the file size is going to change
because we're going to have more

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pixels than before.

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The image size is, thus, also
going to change, as is the

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width and the height.

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>> So which variables are those, exactly?

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Well, if you look into the header
information, you see there is

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biSizeImage, which represents the total
size of the image in bytes,

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including pixels and padding.

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biWidth is the width of the image
in pixels, minus the padding.

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biHeight is the height of
the image in pixels.

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And so those are contained in the
structs BITMAPFILEHEADER and

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BITMAPINFOHEADER.

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You can tell which one is which by
going to bmp.h and looking at the

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BITMAPINFOHEADER struct and seeing
which variables are listed there.

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>> So to update the outfiles header
information, you're going to want to

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change those values of the
height and the width.

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But chances are, you might need some
of the infile's header information

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later, so best to keep track of both.

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But be very clear with your variable
names so that you don't accidentally

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write the incorrect values in
the header for the outfile.

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>> So now let's get to reading into
the scanline pixel-by-pixel.

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Again, we're going to turn to our trusty
file I/O library, and look at

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the fread function.

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fread takes in a pointer to a struct
that will contain the bytes that

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you're reading in, the size of each
element that you're reading--

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again, sizeof is going to be useful
function here, the number of the

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elements of size, size, that you're
reading in, and then finally, the

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inpointer, the file that
you're reading from.

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So you're taking number elements
of size from inpointer, and

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putting them into data.

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>> Now it's time to resize horizontally.

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if n equals 2, then for each pixel in
the infile, we're going to write it

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twice in the outfile.

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How do we write files?

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Well, we have the fwrite function, so
we've taken the pointer to the struct

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that contains the bytes that you're
writing from, and then we pass in

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size, number, and the output, where
you're going to be writing that.

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And then to repeat a process, will be
able a simple iterative for loop.

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>> But we need to remember
to add padding in.

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The concept of padding is that, well,
each pixel is three bites, but the

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size of each scanline must
be a multiple of 4 bytes.

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So if the number of pixels isn't a
multiple of 4, we need to add some

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padding, which is just zeroes.

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Now, unlike Who Done It, and unlike
copy, then infile image and the

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outfile file have different have
different padding because they're

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different widths.

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>> OK.

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So perhaps a formula would
come in handy here.

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I'll leave it to you to find it out, but
tell you that, to write padding,

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well, it's just a simple fputc function,
passing in the character

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that you want to write, and then
the file pointer that you

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want to write to.

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So now that we've resized horizontally,
and then used padding,

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remember that you need to move your file
position indicator, because you

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can't fread into padding.

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So you want to make sure that your file
position indicator in the infile

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is at the correct point.

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Because we also want to
resize vertically.

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We can't just stretch it horizontally,
because every row needs

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to be copied n times.

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>> Now, there are several different
ways to do this.

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So one, we can use a rewrite method, in
that we remember all of the pixels

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of a given row in an array, and
then we write that array as

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many times as needed.

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Or there's the recopy method where,
after reading in one row in the infile

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and then writing that into the outfile,
adding the padding, we fseek

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back to the start of the original row,
and then repeat the horizontal

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resizing from there.

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Regardless of the method, though, you'll
want every pixel to be repeated

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n times, and every row to be
repeated n times as well.

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>> With that, you'll have bitmap
larger than life.

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My name is Zamyla, and this is CS50.

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