user4048: I calculated that for a film of 2 hours with 30 fps, over a 1920*1080 (16:9) display with 32 bps (True Colour) had to weight : 2×3600×30×1920×1080×4 Bytes which is 1,7915904×10¹² so 1,8 TB...
Is there something I didn't get right? Or is it that uncompressed, I could not have more than 1 film stored on my computer to watch on my very standard 1920*1200 Display?
Are the films that are shot and projected in 1080p really that size? 
And my question is : if so how are Blu-Rays compressed into a measly 25-100 GB disk? I thought DVDs and such were lossless encodings and even if not how can we lose 95%+ of the information (100GB/2TB) and be ok watching it?

user2836: For distribution to theaters, typically the files are several hundred gigabytes to a terrabyte per film (depending on the quality level that they choose to distribute).  Even for cinema, some compression is generally used though the quality is still higher than bluray.  Even if they choose lossless, compression can still probably cut it down by a significant margin in most cases, the file sizes for lossless 24bit HD is still very high though and it gets even worse when moving to 4K.
For distribution on DVD and Bluray, very heavy compression is used.  Over-simplified a little, they use a technique that involves storing "keyframes" of the video and then storing information only about what changed from the last key-frame that was seen.  Since video, by nature, has a lot of similarities between one frame and the next, this kind of compression scheme works exceptionally well with minimal quality loss despite the massive amounts of raw data being thrown away.
It does have some costs, such as making it harder to work with the file and making it so that generations of loss are a factor for editing, but for average consumer viewing, it is still plenty high quality to be very near the look and feel of the uncompressed original.

user4050: Your guess is correct. That would be fully uncompressed, true to its settings.
You're right, there's only so much data that disks can handle, so there's compression that does need to take place. We can't always notice it. That's why there's different formats (MOV, MP4, WMV). They all have their individual compression types. Compression, really, is just removing data that is deemed "unnecessary" and remove. 
To put it in a perspective, here's an example:
You have a bag of air. It has Hydrogen and Oxygen in it. Let's say you want to keep the Oxygen, but get rid of the Hydrogen, because the Hydrogen is extra space (like extra lines in the binary code of files). When you compress it, and take out Hydrogen, you have a smaller bag, but you also still have Oxygen. The more you want that bag to be smaller, the more Oxygen you'll have to take out. Therefore, there will still be Oxygen there, it's just not as good.

user1530: DVD and Blu-Ray content makes massive use of lossy encoding.
An easy way to see for yourself is to find a dark scene in a film, and then pause it. If the only issue was a limited number of black levels, you would expect to see a stepped gradient curve from the darkest region to the lightest region. Of course, if true 24-bit color was being used, the "stepping" would be too subtle for your eye to see. But the video isn't even that good! You will most likely see large solid rectangles in the black part of the screen. This is the result of the compression algorithm "guessing" that a square is a good enough approximation, and everything within the square is close enough to the same color to just use one color.
This works well most of the time, but tends to fall apart in films that make extensive use of dramatic dark scenes.
Added from comments: Encoders "cheat" on everything they possibly can. If you can think of something new to "cheat" on that most people won't notice, you can make a lot of money licensing your codec in the video production world.

4048: <p>I calculated that for a film of 2 hours with 30 fps, over a 1920*1080 (16:9) display with 32 bps (True Colour) had to weight : <code>2×3600×30×1920×1080×4 Bytes</code> which is <code>1,7915904×10¹²</code> so <code>1,8 TB</code>...</p>

<p>Is there something I didn't get right? Or is it that uncompressed, I could not have more than 1 film stored on my computer to watch on my very standard <code>1920*1200</code> Display?
Are the films that are shot and projected in 1080p really that size? </p>

<p>And my question is : if so how are Blu-Rays compressed into a measly 25-100 GB disk? I thought DVDs and such were lossless encodings and even if not how can we lose 95%+ of the information (100GB/2TB) and be ok watching it?</p>
 2836: <p>For distribution to theaters, typically the files are several hundred gigabytes to a terrabyte per film (depending on the quality level that they choose to distribute).  Even for cinema, some compression is generally used though the quality is still higher than bluray.  Even if they choose lossless, compression can still probably cut it down by a significant margin in most cases, the file sizes for lossless 24bit HD is still very high though and it gets even worse when moving to 4K.</p>

<p>For distribution on DVD and Bluray, very heavy compression is used.  Over-simplified a little, they use a technique that involves storing "keyframes" of the video and then storing information only about what changed from the last key-frame that was seen.  Since video, by nature, has a lot of similarities between one frame and the next, this kind of compression scheme works exceptionally well with minimal quality loss despite the massive amounts of raw data being thrown away.</p>

<p>It does have some costs, such as making it harder to work with the file and making it so that generations of loss are a factor for editing, but for average consumer viewing, it is still plenty high quality to be very near the look and feel of the uncompressed original.</p>
 4050: <p>Your guess is correct. That would be fully uncompressed, true to its settings.</p>

<p>You're right, there's only so much data that disks can handle, so there's compression that does need to take place. We can't always notice it. That's why there's different formats (MOV, MP4, WMV). They all have their individual compression types. Compression, really, is just removing data that is deemed "unnecessary" and remove. </p>

<p>To put it in a perspective, here's an example:</p>

<p>You have a bag of air. It has Hydrogen and Oxygen in it. Let's say you want to keep the Oxygen, but get rid of the Hydrogen, because the Hydrogen is extra space (like extra lines in the binary code of files). When you compress it, and take out Hydrogen, you have a smaller bag, but you also still have Oxygen. The more you want that bag to be smaller, the more Oxygen you'll have to take out. Therefore, there will still be Oxygen there, it's just not as good.</p>
 1530: <p>DVD and Blu-Ray content makes <em>massive</em> use of lossy encoding.</p>

<p>An easy way to see for yourself is to find a dark scene in a film, and then pause it. If the only issue was a limited number of black levels, you would expect to see a stepped gradient curve from the darkest region to the lightest region. Of course, if true 24-bit color was being used, the "stepping" would be too subtle for your eye to see. But the video isn't even that good! You will most likely see large solid rectangles in the black part of the screen. This is the result of the compression algorithm "guessing" that a square is a good enough approximation, and everything within the square is close enough to the same color to just use one color.</p>

<p>This works well most of the time, but tends to fall apart in films that make extensive use of dramatic dark scenes.</p>

<p>Added from comments: Encoders "cheat" on <em>everything</em> they possibly can. If you can think of something new to "cheat" on that most people won't notice, you can make a lot of money licensing your codec in the video production world.</p>
