Lesson video

Hello, my name is Mr. Hogan.

I am so excited to be learning with you today.

We are going to have such a great time learning all about data representation of images and sounds.

I will be supporting you with our learning during these lessons.

I'm so pleased that you have decided to complete your learning with me today.

We are going to do brilliantly.

Welcome to today's lesson from the unit Data representation: images and sound.

This lesson is called File size of digital images.

I'm really looking forward to this lesson and by the end of it, you will be able to describe how image file sizes are calculated and how changes to image quality and color accuracy adjust this.

Let's move on.

I'm really looking forward to it.

So there are three keywords in today's lesson.

So resolution: this is dimensions of an image in pixels or the number of pixels per inch, sometimes known as PPI, which affects image quality.

Color depth: this is the number of bits used to represent a color, which is also known as bit depth.

And then our final keyword is file size.

So this is the amount of data a file contains.

Remember, you can come back to this slide at any time if you forget what these keywords mean by just go and be rewinding the video.

So the lesson splits into three parts.

The first part is calculating the file size of an image.

Then we move on to the effects of using different image resolutions.

And then finally, the effects of using different color depths in an image.

So let's start with calculating the file size of an image.

A digital image is composed of individual elements arranged in a rectangular grid.

The elements of a digital image are called pixels.

The number of pixels in a digital image is the image resolution.

To work out the resolution in pixels, we take the width in pixels and multiply it by the height in pixels.

So where the width is 720 pixels and the height is 720 pixels, like in this image, we calculate the resolution by multiplying their width and height together to give the number of pixels, in this case 518,400 pixels in this picture's resolution.

Let's have a quick check.

See if you can calculate the resolution of this image.

Is it A: 112 pixels, B: 22 pixels or C: 100 pixels? You may pause the video.

Maybe you want to do some counting of pixels.

So you can pause the video or you can rewind it to see how you work out the resolution and then come back to this check.

Let's have a look at the answer.

It is A: 112 pixels.

That's because the width is 8 pixels, the height is 14 pixels.

So to work out the resolution, we have 8 X 14, which equals 112 pixels.

To calculate the number of bits in an image that uses 2 bits per pixel, we do the following: So there are 10 by 10 pixels, so therefore we multiply them together, which gives 100 pixels or picture elements, and 2 bits are used for the color of each pixel.

You can now see in the image.

Total number of binary digits for the image is worked out by taking the 100 pixels, multiplying it by 2 bits per pixel.

So this gives us 200 bits.

So that's how to calculate the number of bits in an image that uses 2 bits per pixel For every pixel, a sequence of binary digits is used to represent its color.

The number of binary digits used to represent each pixel color is the color depth.

So the resolution of this image is worked out by multiplying the width and height together.

So that was 518,400 pixels.

And the color depth is 1 bit because it's two possible colors, there's only black or white, represented by 1 and 0.

It may look a bit gray, but actually if you zoom in, you'll be able to see that it's just formed of black and white pixels.

If we add more colors to the image, so the like the gray represented by 1 1 and the blue represented by 1 0, the color depth is 2 bits, four possible colors.

If we add eight colors represented by 3 bits, the color depth is now 3 bits.

This image actually has 24 bits for its color depth, and that's a typical value of a image's color depth, something like 24 or sometimes 32 binary digits per pixel.

Let's have a check.

What does the color depth of an image describe? Is it A: the height of an image in pixels, B: the number of different colors it can show or C: the number of different colors such as red, green, and blue that are combined in each pixel? You can pause the video at any time or rewind it to look at previous slides.

Let's have a look at the answer.

It is B, the number of different colors it can show.

Moving on.

To calculate the file size of an image, we do the following: So we take the width in pixels, multiply it by the height in pixels, and then multiply it by the color depth.

So let's have a look at this image.

So we've got 720 pixels, height and width, and we multiply it by 24 bits 'cause it's got 24 bit color depth like we looked at before.

So that gives us 12,441,600 bits.

Sam is asking: "12,441,600 bits.

That's a huge number.

Is there a way to convert it into other units to make it easier to understand?" Yes, there are, Sam.

There are more common units of digital storage.

So we use a byte, and in a byte there are 8 bits.

So if we do 12,441,600 divided by 8, it becomes 1,555,200 bytes.

1,000 bytes in a kilobytes.

So we can do 1,555,200 bytes divided by 1,000 gives us 1,555 kilobytes.

And 1,000 kilobytes are in a megabyte.

So we can do 1555 kilobytes divided by 1,000.

So that gives us 1.

55 megabytes.

Let's have a practice.

One: Sam has made a logo for a game.

The logo is a bit map image of 3000 by 3000 pixels.

What is the logo's resolution? Two: Sam has also made an icon for the maze solving game she has created.

It is 32 pixels wide and 32 pixels high.

How many pixels does the image have in total? Remember you can pause the video at any time or rewind it.

Three: In Sam's game, there is a character that is 32 pixels tall by 16 pixels wide.

The games console can represent 64 colors, which is represented by 6 bits per pixel.

What is the file size of the character in bytes? Remember, pause the video at any time or rewind it.

So let's have a look at the answers.

We've got: one, Sam has made a logo for a game.

The logo is a bitmap image of 3000 by 3000 pixels.

What is the logo's resolution? So we take the height and width and multiply them together.

So 3000 pixels multiplied by 3000 pixels equals 9 million pixels.

Two: Sam has also made an icon for the maze solving game she has created.

It is 32 pixels wide and 32 pixels high.

How many pixels does the image have in total? So, similarly to the last question, we take the height and width, multiply them together.

So in this case we get 32 pixels multiplied by 32 pixels, and that gives us 1024 pixels.

In Sam's game, there is a character that is 32 pixels tall by 16 pixels wide.

The games console can represent 64 colors, which is represented by 6 bits per pixel.

What is the file size of the character in bytes? Okay, so we can work out the resolution.

So 32 by 16 pixels gives us 512 pixels and we can work out now the total bits because we know that 512 pixels multiplied by the color depth, which in this case is 6 bits.

So that gives us 3072 bits.

Now we convert it bits into bytes.

So 3072 bits divided by 8 gives us 384 bytes.

Well done if you've got this correct.

Really good.

Let's move on to the second part of the lesson.

So we're gonna look at the effects of using different image resolutions.

So we've calculated the file size of an image.

Now we're looking at the effects on using different resolutions.

When choosing a resolution size for an image, there is a balance between quality and efficiency.

Quality refers to the clarity and the detail of an image.

And high quality images are sharp and have really fine details in them.

Efficiency means doing something well without wasting resources like storage space, internet speed, or computing power.

For example, efficient images are changed and improved to make sure they look good without using too much space or slowing things down on your computer.

Why resolution matters.

Well, in gaming and graphic design, resolution affects how realistic the game looks.

In photography, higher resolution images makes the photographs more detailed.

And also in streaming or when using images on the web, choosing the right resolution balances this quality and efficiency.

Let's have a quick check.

What is the main benefit of using a high resolution for an image? Is it A: the image will use less storage space? Is it B: the image will appear clearer and more detailed? Or is it C: that image will load faster on a website? Remember, you can pause the video at any time.

Let's have a look at the answer.

It is B.

The image will appear clearer and more detailed.

Sam is asking, "For my school work, I don't look at the resolution of images that I use." Sofia is commenting: "I was creating a presentation and when I made the image larger, it became blurry.

So I had to find an image with a higher resolution." That's a good comment by Sofia.

I've come across images where I've enlarged them and they become blurry.

So I had to actually go and find images and look at their resolution.

This image is 2,560 pixels by 2027 pixels.

So in total this resolution is 5,189,120 pixels.

Let's have a look at this next one.

So this resolution has gone down slightly.

So it's 1,280 pixels by 1014 pixels, which gives us a resolution of 1,297,920 pixels.

Okay, now let's have a look at this next image.

So this is slightly decreased in its resolution.

So this is 970 pixels by 768 pixels, which gives 744,960 pixels.

Okay, so the resolution has reduced.

This one's reduced even more.

So the resolution of this is 453,443 pixels.

This one's reduced even more.

I wonder if you can start telling that the quality's gone down.

So this one's 290,880 pixels.

This one's gone down.

So this is only 49,500 pixels.

This one's even less.

Now, this one's 7,900 pixels.

I wonder if you can tell the difference now between this one and the original one.

So let's put them side by side and hopefully you can see that the quality is different.

So although we sort of went through the slide and decreased the resolution or the quality of each slide, if you put our best resolution image with 5,189,120 pixels on the right hand side next to our image with our least resolution, which is 7,900 pixels on the left hand side, hopefully you can tell the difference.

So let's have a look at visual quality.

Higher resolution improves detail, especially on larger screens.

Low resolution images may look pixelated or lack detail when zoomed in or displayed on high definition monitors.

When looking at file size, images with higher resolutions have larger file sizes because they store more pixels.

Lower resolution take up less storage, but are lower quality.

Let's have a look at examples.

High resolution images are ideal for printing, professional photography and really large displays.

Low resolution images are suitable for web use, faster loading times and devices with smaller screens.

Let's have a check.

Sofia wants her website to load quickly.

Which image resolution should she choose to ensure fast loading times while maintaining good quality? Is it A: high resolution, which often leads to slower loading times, B: low resolution which may load quickly, but could compromise image quality, or C: optimized resolution, balancing quality and loading speed? Remember you can pause to have a think about the answer or rewind the video.

The answer is C, yeah, optimized resolution, balancing that quality and loading speed.

Well done for getting this far.

It's time for a practice, but you're doing really, really well.

Let's see if we can answer these questions.

One: Sofia is sending a photo attached to an email to Sam.

Why might Sofia choose to send a low resolution version of the photo instead of the original high resolution version? Two: Sam is printing a large photo to hang on her wall.

Should Sam choose a high resolution or low resolution image? Explain your answer.

Remember you can pause the video at any time or rewind to look at previous slides.

Three: Sofia has been asked to design an image to promote a healthy lifestyle on a digital billboard to be displayed in a city center.

What factors would you consider when choosing the resolution of the images for the billboard? Explain your reasoning.

Remember you can pause the video at any time.

Let's have a look at the answer for number one.

So Sofia is sending a photo attached to an email to Sam.

Why might Sofia choose to send a low resolution version of the photo instead of the original high version? So sending a low resolution photo through an email is better because it'll have a smaller file size.

This means it will send faster and be less likely to cause problems with email size limits.

While the quality might not be as good as the original, it will still be fine for viewing on a phone or a computer screen.

Hopefully you've got something very similar to this answer.

Well done.

Two: Sam is printing a large photo to hang on her wall.

Should Sam choose a high resolution or low resolution image? Explain your answer.

So Sam should choose a high resolution image for printing a photo.

High resolution images have more pixels, which means the image will be printed with more detail and clarity.

A low resolution image might look blurry or pixelated when printed in a large size.

Again, hopefully you've got the main points of these answers.

Three: What factors should you consider when choosing the resolution of the images for the billboard? I would choose a high resolution.

Billboards are very large, so the images need to have a lot of pixels to look clear and sharp from a distance.

Billboards are viewed from a distance, so image quality is prioritized over file size.

However, I would also need to consider the capabilities of the display screen and the storage capacity of the billboard's computer system.

There might be a limit to how high the resolution can be.

Great.

Hopefully you've got answers similar to this.

But if not, maybe you want to rewind the video and look over the slides again.

We're going to move on to the third part of the lesson.

So we've calculated the file size of an image, we've looked at the effects on the different resolutions, and now we are going to have a look at the effects of using different color depths.

Hopefully you're enjoying this lesson as much as I am.

I'm really looking forward to this last part.

Color depth refers to the number of bits used to represent the color of a single pixel in an image or display.

Higher color depth allows for more colors, leading to smoother gradients and more realistic images.

This image is split in two.

Each half has a different color depth.

So on the left hand side we've got a 24 bit color depth.

And then on the right hand side, I've changed the image to have 8 bit color depth, which only uses 256 colors compared to the 16.

7 million colors on the left.

And hopefully you can see that difference of quality.

Color depth is typically set to either 8 bits, 256 colors, that's normally used in old systems; 16 bit, which is 65,536 color.

So this is high color used in early multimedia; or 24 bits, which gives us 16.

7 million colors.

This is like known as the true color and its standard now for most modern day images and displays.

In real life, we can perceive a huge amount of color variation.

Therefore, the more colors you use in a graphic or an image, the closer the picture will look to reality.

So high color depths result in richer and more accurate color.

However, more bits per pixel mean larger file sizes because each pixel requires more data to store its color information.

So here we have a picture of a hot air balloon that I've divided into four sections.

The section on the left hand side will require more data than the section on the right hand side because it uses more colors.

You can see how much the file size decreases when the color depth is decreased in these images.

So 24 bits of color on the left hand side gives us those 16 million colors, which uses 2.

3 megabytes.

The next one is 8 bits.

So it gives us 256 colors and needs 23.

4 kilobytes to be stored.

The third one along is an 8 bit color.

It has 256 different types of grays, which is stored or needs to have 23.

4 kilobytes of storage.

So that's the same, the one that we just seen, but there's just different types of grays.

And this last one just uses two colors, white and black.

So therefore you need 2.

9 kilobytes to store this image.

The reason you may choose to reduce the color depth is to reduce the file size.

The more bits used to represent colors in an image, the more storage space is needed to save that image.

Sending smaller file size images via mobile phone messages allows them to be received more quickly and uses less of the sender's data allowance.

So for example, 8 bits could be a simple graphics, icons or retro style games.

16 bits of color depth may be used for basic multimedia or older games, or 24 or 32 bits color depth will be used for photography or video editing and high quality displays.

Let's have a practice.

So Sam is creating an image to promote her school's charity event.

She has two versions of the same image.

Image A has 414,948 different colors.

And image B has 256 different colors.

One: Which image would have the smallest file size and why? If you want to have a think about it, you can pause the video at any time or rewind it.

Two: If Sam wants to use the image on the school's website, which image would be more suitable and why? Three: Which image would be best for printing a large high-quality banner poster to be displayed on the school's gates? And see if you can say why.

Remember, you can rewind the video at any time or you can pause it.

Let's have a look at the answer.

So we have these two different images here.

One: Which image will be the smallest file size and why? So it'll be image B that will have the smallest file size 'cause it's means it's got a lower color depth, which means the image takes up less storage space.

Well done if you got the answer correct.

Two: If someone wants to use the image on the school's website, which image would be more suitable and why? Image B would be more suitable for the website.

Smaller file size are important for websites so they load up quickly.

An image with a high color depth would make the webpage load slowly.

And three: Which image would be best for printing a large, high-quality banner poster to be displayed on the school's gates and why? So it would be image A because it's got a high color depth.

It would be best for the poster.

Images with a high color depth have more detail and will look clearer when printed in a large size.

So well done If you got those answers correct, or if you haven't, maybe you want to rewind the video to remind yourselves of the answers.

Well done.

You've come to the end of the lesson.

You've done so well.

I've really enjoyed learning with you today.

I hope you've enjoyed it too.

So over the lesson, you've learned that resolution refers to the number of pixels in an image.

Higher resolution means more pixels and a more detailed image.

You've also learned that color depth refers to the number of bits used to represent the color of each pixel.

Higher color depth means a wider range of colors.

And also you've learned there's a balance between image quality and file size.

Higher quality images, more pixels, more colors, usually have larger file sizes.

So you've learned a huge amount.

So, well done.

I'm really pleased.

Thank you.