Editing an Image
Layers, masks, curves and export — the craft behind a photo that still holds up at full size.
- Level
- Nothing assumed
- Lessons
- 78
- Reading time
- 702 min
- Price
- Free, no sign-up to read
Raster editing as a working craft. What resolution can and cannot be recovered, why every edit belongs on its own layer, how masks separate people who can edit from people who cannot, selections down to hair, Levels and Curves instead of Brightness/Contrast, the difference between healing and cloning and generative fill, sharpening done last, and export settings that survive a slow connection. Photoshop is named; Photopea, GIMP and Krita are the paths that cost nothing.
Opens after the Design, Before Any Software exam
Sign in, finish that course, and pass its exam. You can read this syllabus meanwhile.
Go to Design, Before Any SoftwareModule 1
What an image is made of
Before any tool, the file. A photograph is a grid of numbers with a colour meaning attached, and almost every mystery in editing — the soft enlargement, the banded sky, the grey fringe round a logo, the colours that will not print — is a property of that grid rather than a bug in the software. This block takes the file apart: channels, bit depth, gamma, alpha, the raw mosaic, the inside of a JPEG, gamut, your monitor, and what the lens did before any of it reached you.
By the end you can
Open an unfamiliar image file and state what it can and cannot survive — its bit depth, colour space, alpha, compression history and lens geometry — and predict from those facts which edits will band, fringe, clip or fail to print, before making any of them
- 1The photo you were sent is too small, and nothing fixes thatEnlarging can only average pixels you already have; AI upscaling invents new ones, which is a different thing from recovering them.
- 2Three numbers per pixel, and why green counts mostA colour pixel is three independent brightness values whose contributions to perceived lightness are roughly 21/71/7 per cent, so the green channel is your cleanest and averaging the three is never a correct greyscale.
- 3Why the sky turns into stripesBanding is empty space in the histogram — a stretch redistributes the levels you have rather than creating new ones — so do tonal work in 16-bit, dither what remains, and remember that converting a JPEG upward adds no levels at all.
- 4Mid-grey is not half the lightStored pixel values are spaced to match your eye, not the light, so any operation that averages or adds pixels — resizing, blurring, transparency, compositing — is arithmetically wrong unless it is done in linear light.
- 5Transparency is a number, and it carries a colour with itAlpha is a single coverage number per pixel that leaves the pixel's colour untouched, so transparency problems split into three separate causes — the wrong premultiply convention, contaminated edge colour, and formats that only store on-or-off — each with its own fix.
- 6Two thirds of the colour in your photo was calculatedA raw file is linear sensor data before demosaic, white balance, tone curve and 8-bit conversion, which is why white balance costs nothing and a stop of highlights can be rebuilt from the channels that did not clip.
- 7Eight by eight, and what the encoder threw awayJPEG loses data at exactly one step — rounding the 8×8 transform coefficients — so blocking, ringing and colour fringes each map to a specific stage of the encoder and tell you whether to change the quality, the subsampling or the format.
- 8Some colours have no numberA colour space is a set of primaries defining a triangle of reachable colours, so converting to a smaller one either clips saturated areas into flat patches or compresses everything slightly, and the rendering intent is where you choose which.
- 9The monitor is a measuring instrument nobody calibratedCalibration does not make your screen match anyone else's — it makes your own judgements repeatable and eliminates the display as a suspect — and the cheapest version of it is checking every delivery on three different devices.
- 10Straight lines that are not straight, and colour that is not thereGeometry corrections resample the whole frame, so distortion, vignetting and lateral chromatic aberration go first, before cropping or retouching — and longitudinal fringing, flare and diffraction are losses no profile can undo.
Module 2
The non-destructive workshop
A layered file is a recipe, not a picture. This block builds the workshop you will do everything else in: what a blend mode computes, why the order of the stack changes the answer, what a group actually does to rendering, smart objects and linked files, the Channels panel as storage and arithmetic, versions and backups that survive a client changing their mind twice, and what to do when the file gets too heavy for the machine you own.
By the end you can
Build and hand over a layered document another editor can take apart — correctly ordered, grouped, masked, named and versioned — and diagnose from the stack alone why an adjustment is affecting the wrong layers or why the file has become too slow to work in
- 11Every edit should still be undoable next weekStack instructions on top of the pixels instead of changing them, and save the layered file before you save anything else.
- 12The eraser is the tool that costs you the second revisionBlack conceals, white reveals — mask instead of erasing, and every revision becomes a brushstroke rather than a restart.
- 13Twenty-seven blend modes, six ideasEvery blend mode is a short per-channel formula, so white vanishes in Multiply, black vanishes in Screen, mid-grey vanishes in Soft Light, and setting a tonal adjustment to Luminosity separates contrast from the saturation shift that normally comes with it.
- 14The same three edits in a different order give a different pictureImage operations are not commutative, so a layer stack is an ordered recipe — repairs under tone, tone under colour, sharpening last — and a stamped flatten is a snapshot that silently goes stale the moment anything beneath it changes.
- 15Why an adjustment stopped working when you put it in a folderA group defaults to Pass Through, where adjustments inside still reach the layers below, and giving the group any other blend mode or an opacity below 100 isolates it into its own buffer so those adjustments become local.
- 16One file, forty mockups, one changeA smart object stores the original and re-renders it through your transform and filters each time, which is what makes scaling reversible, filters re-editable and contents swappable — at the cost of file size, render lag, and a portability gap that GIMP does not close.
- 17The panel where selections go to liveAlpha channels are permanent storage for selections, colour channels are ready-made masks waiting to be found, and multiplying two masks intersects them while screening them unions them.
- 18The file you will need is the one from TuesdayHistory dies when the document closes and a synced folder replicates your mistakes, so the record that actually rescues a job is named versions on disk plus three copies on two kinds of media with one of them elsewhere.
- 19Eleven shortcuts and a scratch diskSpeed comes from never leaving the keyboard for the dozen commands you use hourly and from configuring scratch space and memory so the application stops stalling, not from working faster at the ones you use rarely.
- 20Working on a laptop that cannot hold the jobA slow document has four separable causes — pixel count, layer count, smart-filter re-rendering and scratch thrashing — and the fixes run from hiding groups and stamping checkpoints up to splitting the job into two documents, with browser-based tools hitting a hard memory ceiling rather than degrading gradually.
Module 3
Selections, cut-outs and edges
Getting a subject out of its background is the most requested skill in this trade and the one where the difference between adequate and professional is visible at a glance. This block goes past the automatic button: bezier paths for manufactured edges, painting a selection by hand, colour and luminosity as selection sources, what the segmentation models are actually doing, the equation that makes hair mathematically hard, and how to cut out things that are not opaque at all.
By the end you can
Choose and execute the right extraction method for a given edge — path, painted mask, colour range, luminosity, model or blend-mode composite — decontaminate the result so it carries no trace of its old background, and run a batch of cut-outs to a consistent standard with a sampling check that catches the failures
- 21Selections are easy until you reach hairEdge pixels are mixtures of subject and background, so a good cut-out is about partial transparency and colour decontamination, not about finding a line.
- 22Two handles and a curve: making the pen tool obeyA bezier path is anchors plus direction handles, so accuracy comes from placing few anchors at direction changes and shaping the curve with handles — and the finished cut-out lives or dies on a sub-pixel feather that matches the photograph's own edge softness.
- 23Stop fighting the selection tools and paint itQuick Mask turns a selection into a paintable greyscale image, so the fast route to a difficult cut-out is an automatic tool for the bulk followed by a low-opacity brush for the part the tool got wrong.
- 24Tolerance is a wall, fuzziness is a slopeThe Magic Wand's tolerance produces an in-or-out selection while Colour Range's fuzziness produces partial values, which is why one is for flat graphics and the other for anything that shades — and neither can separate a subject from a background that shares its values.
- 25Masks with no edge anywhereA luminosity mask makes each pixel's participation equal to its own brightness, so an adjustment fades continuously and has no boundary to halo along — and intersecting the mask with itself narrows it to the highlights, the shadows or a midtone band.
- 26What the one-click button is actually doingA one-click cut-out is an object-recognition model predicting which pixels belong to a learned shape, which is why it fills enclosed gaps, loses thin structures and guesses at transparency — and why the mask must be inspected on its own before it is used.
- 27Three equations, seven unknowns: why hair is hardA boundary pixel is a mixture of foreground and background whose separation is mathematically underdetermined, so every extraction needs extra information, and the edge colour must be recomputed rather than contracted, feathered or contrasted away.
- 28The four subjects that are not opaqueNon-opaque subjects split into coverage, transmission and emission, and each needs a different method — matting and decontamination for hair, Multiply and Screen plates for glass, and a plain Screen with no mask at all for smoke and flame.
- 29Four hundred cut-outs, and the twelve that are wrongA batch of cut-outs is a pipeline plus a sampling check — a grey-background contact sheet of the masks finds the errors a white-background grid hides — and the predictable failure categories should be pulled out for manual work before the batch runs, not after.
Module 4
Tone and colour
Light and colour are where a photograph is actually made or lost, and where most of the reversible damage happens. This block covers reading the measurement rather than trusting the screen, white balance when there are two light sources, shaping form by hand with dodge and burn, what local contrast tools are really computing, saturation that does not clip, monochrome conversion that separates rather than flattens, grading with curves and lookup tables, skin tones without a false universal standard, and matching forty photographs so a catalogue reads as one shoot.
By the end you can
Take a set of unevenly lit photographs to a consistent, defensible tonal and colour result — diagnosing casts from per-channel evidence, correcting mixed lighting locally, shaping form without banding or halos, and stating what you measured rather than what looked right on your screen
- 30Brightness/Contrast is one knob for a problem with fiveSet the black and white points first, shape the middle with Curves, and remember that contrast in the midtones is borrowed from the shadows and highlights.
- 31There is no correct histogramThe histogram only reports value distribution, so its real jobs are catching per-channel clipping and missing tones — and the camera's version is computed from the JPEG preview, which is why the raw file usually holds highlights the warning says are gone.
- 32Two light sources, one global sliderWhite balance has a blue-yellow axis and a green-magenta axis, so a green office cast needs tint rather than temperature — and two light sources in one frame cannot be corrected globally at all, only by masking a second correction onto the minority light.
- 33Shaping light by hand, on a grey layerDodge and burn on a 50 per cent grey Soft Light layer at 5 to 10 per cent brush opacity is fully reversible local tonal control, and a steep-curve black-and-white viewing layer is how you see the blotches before your client does.
- 34Why Clarity makes halos, and what Dehaze is estimatingShadows/Highlights, Clarity, Texture and sharpening are one operation at four radii — comparing each pixel to a blurred version of its surroundings — so halos are inherent to the method, while Dehaze is a separate estimate of an atmospheric veil that fails wherever the scene genuinely has no dark pixels.
- 35Saturation is a slider that destroys detail quietlyRaising saturation pushes channels apart until the leading one clips and flattens a shaded surface into a single colour, which is why Vibrance weights the increase against existing saturation and why the three-channel histogram and a gamut warning are the two checks that catch the damage.
- 36Monochrome is a decision about which colours become which greysMonochrome conversion is a choice of how each hue maps to a grey, so the six-channel Black and White adjustment reproduces the old coloured lens filters — and a photograph whose subject and background differ in hue but not in lightness cannot be separated by any conversion.
- 37A look is a mapping, and a LUT is a table of itA grade is a mapping from input colour to output colour, so a LUT applies it identically every time and therefore gives a different result on an image whose exposure and contrast were not normalised first.
- 38There is no correct number for skinSkin has no universal correct value, so the method is to neutralise from a known reference, check hue consistency across the face rather than against a target, and recognise that automatic exposure and white balance were tuned on distributions that systematically under-serve darker skin.
- 39Forty photographs that have to look like one shootSet consistency is bought at capture with a grey card and manual settings, maintained by syncing one hero frame's settings and then adjusting only exposure, and verified on a mid-grey contact sheet with sampled numbers rather than by eye.
Module 5
Repair and retouching
Removing what should not be there, and rebuilding what has been lost. This block separates texture from tone so skin can be evened without being sanded, works through portraits and products with the restraint that keeps them honest, deals with reflections and glare, reconstructs large removed objects rather than smearing them, scans and restores damaged prints, and ends with a plain account of what no tool can bring back.
By the end you can
Repair a damaged or imperfect photograph to a professional standard using the right mechanism for each fault — separated frequencies, patch synthesis, manual reconstruction or warp — and state precisely which faults in a given file are unrecoverable and why
- 40Three tools that all remove a spot, and they are not interchangeableHealing blends into its surroundings, cloning copies exactly, content-aware borrows from elsewhere in the frame, and generative fill invents — and only the last one changes what the picture claims.
- 41Splitting texture from toneFrequency separation splits an image into a blurred tone layer and a neutral-grey detail layer that recombine exactly, so blemishes are healed on the detail layer where there is no tone to get wrong and colour is evened on the tone layer where there is no texture to destroy.
- 42Keep what is permanent, remove what is passingRemove what would be gone in a fortnight and keep what would not, do the retouch fully and then reduce the group's opacity to correct for the judgement drift that happens at high zoom, and treat eyes and teeth by removing colour rather than adding brightness.
- 43The product has to be the productThe governing test for product work is whether the buyer receives what they saw, which permits removing sample defects, rebuilding straight edges against a path, and mirroring a symmetrical half, but never altering proportions or the colour the product actually is.
- 44A reflection is a photograph of the light sourceA specular reflection is an image of the light source added on top of the surface, so it can be reduced by subtractive adjustments or replaced from a second frame shot with the reflection moved, but removing a dense one means inventing what the recorded pixels no longer contain.
- 45Rebuild the structure first, then the texturePatch-based fill copies texture from elsewhere in the same photograph and has no model of geometry, so large removals are done by rebuilding the structural lines by hand first, letting the fill handle only flat texture, and then unifying tone and grain across the repair.
- 46Moving pixels, and the straight line that gives it awayA warp is a displacement mesh that moves everything within its influence, so the reliable evidence of reshaping is a bent straight line in the background — which is exactly what the freeze mask, or warping the subject on its own layer, exists to prevent.
- 47Getting the photograph off the paperScan resolution is chosen from the intended output size with room for correction, moiré from a previously printed original is an interference pattern best removed by oversampling and downscaling, and infrared dust removal fails on silver black-and-white and Kodachrome because the image itself blocks infrared.
- 48Tears, mould and the cyan that faded firstRestoration runs large damage to small, the pink cast on old colour prints is the cyan dye layer having faded and can be neutralised but not recovered, and anything reconstructed rather than derived from surviving evidence belongs in a written note delivered with the file.
- 49Four losses that no tool undoesBlur, defocus, clipping and compression are absences of information rather than faults in it, so every tool that appears to reverse them is generating plausible content — and a five-minute triage at 100 per cent zoom tells you on day one what a file can become.
Module 6
Compositing: making two pictures one
Putting an element from one photograph into another is easy; making the result believable is a craft with a short list of rules. This block works through the six mismatches that give a composite away, the geometry that tells you where a camera stood, how to relight a cut-out and build its shadow and reflection, sky replacement, blending several exposures of one scene, stitching and stacking frames, and the grain and focus matching that removes the last tell.
By the end you can
Combine elements from different photographs into one image that survives inspection — matching perspective, scale, light direction and quality, edge softness, depth of field and grain — and merge multiple exposures, viewpoints or focus planes of one scene without halos, ghosting or parallax errors
- 50Six mismatches, and the eye finds them allComposites fail on six measurable mismatches — perspective, scale, light direction, light quality, edge softness and grain — and the fastest ways to find them are flipping the image, shrinking it and blurring it, because each reveals a different class of error.
- 51The horizon tells you where the camera stoodThe horizon sits at camera height and crosses every figure standing on the same ground plane at the same point on their body, which fixes an element's scale exactly — and an object's depth is read from where it meets the ground rather than from how large it is.
- 52Relighting a cut-out so it belongsRead direction, elevation, quality and the key-to-fill colour difference from the plate first, build the tight dark contact shadow before the directional cast shadow, and accept that shadows already baked into a hard-lit element cannot be relit from the opposite side.
- 53Constructing what the light would have doneA convincing shadow is dark and tight at the contact and progressively softer and lighter further away, coloured rather than black, and shaped by the surface it falls on — and the easiest way to get one right is to extract the real shadow from the element's own original frame.
- 54A new sky changes the light in the whole photographA sky is the scene's light source, so replacing it means matching direction, hardness, exposure and colour spill in the foreground as well — and a plate where the old sky reflects in windows, water or paintwork cannot take a new one without replacing every reflection too.
- 55Two exposures of one scene, and how not to make it look like HDRExposure fusion picks the best-exposed pixels and needs no tone mapping, while HDR merge plus tone mapping flattens global contrast and restores it locally, which is exactly why halos and the characteristic grey look appear — and a hand-masked blend of two exposures beats both.
- 56Many frames, one view, and the pivot point that mattersStitching solves for camera orientation from overlapping features, so it needs 30 to 40 per cent overlap and fails on parallax — rotating anywhere but the lens's entrance pupil makes near and far objects disagree between frames in a way no algorithm can reconcile.
- 57When depth of field is not enoughFocus stacking picks each pixel from the frame where local contrast is highest, so it needs frames whose sharp zones overlap and pixel-accurate alignment including the scale change from focus breathing — and it produces halos exactly where a sharp foreground edge meets a background sharp in a different frame.
- 58The last tell: texture and depthThe last tells are surface ones — depth of field, grain, lens artefacts, compression and black point — and they are fixed by applying a single unified treatment across the whole flattened composite rather than correcting the pasted element on its own.
Module 7
AI inside the image editor
Generative fill, upscaling, denoising and colourising are now buttons in ordinary editors, and every one of them produces content rather than recovering it. This block explains what the models are doing so their failures become predictable, shows how to run the free equivalents on a laptop with no graphics card, covers provenance and watermarking as they actually stand, and ends on the decision about when none of it should be used.
By the end you can
Use generative fill, outpainting, upscaling, denoising and colourising deliberately — predicting each one's characteristic failures from its mechanism, running free local equivalents where confidentiality or cost requires it — and state for a given image whether generated content is permissible and what has to be disclosed
- 59Noise in, picture out: inpainting explainedGenerative fill initialises a masked region with noise and denoises it towards something consistent with the surrounding pixels and your prompt, which is why filled areas are cleaner than photographs, softer than the original file, and wrong wherever text or regular geometry is involved.
- 60Getting a usable result in three attempts instead of thirtyLeave the prompt empty when removing and describe the light and material when adding, generate three variations and composite the good parts onto a masked layer rather than accepting a whole result, and fix grain, geometry and black point afterwards.
- 61Making a photograph bigger than it was takenOutpainting is inpainting with the mask outside the frame, so it works best in 20 to 30 per cent stages with generous overlap — and because viewers read extended content as part of the scene, it is the generative operation most in need of disclosure outside advertising and design.
- 62Which upscaler, for which materialUpscalers come in families with different tendencies to invent texture, they must be run after denoising and before sharpening, tiling seams are fixed with a larger overlap, and 2× is close to recovery while 8× is mostly the training data rather than your photograph.
- 63Reconstruction sold as noise reductionLearned denoisers predict a clean image rather than smoothing a noisy one, which is why they work on raw data whose noise statistics are known, why they deliver two to three stops, and why fine repeated texture and small faces come back plausible rather than correct.
- 64A guess about what colour the dress wasColourisation predicts chrominance from luminance using statistical associations learned from modern photographs, so it is reliable on sky, foliage and skin and a hedge towards muddy averages on anything manufactured — and none of it is recovered, because the greyscale file holds no colour information at all.
- 65On your own machine, with no graphics cardBackground removal, upscaling and denoising run usefully on a laptop with no graphics card, inpainting runs slowly but workably, and running locally buys confidentiality, zero per-image cost and independence from a service — while Colab's free GPU is a real option that is not local.
- 66What can be proved about where an image came fromC2PA signs a chain of edits rather than certifying truth, invisible watermarks live in the pixels but only for vendors that apply them, and AI-detection services are too unreliable to accuse or defend anyone with — so the strongest provenance a working editor has is the kept original plus a written delivery note.
- 67The decision, and the parts nobody has settledAsk whether the picture is a record or a construction, because that determines whether generated content is permissible — and treat the copyright status of model training as genuinely unresolved, documenting what you used rather than assuming an answer.
Module 8
Out of the editor: size, sharpness and the delivered file
The edit is finished and nobody has seen it yet. Everything between the layered master and the file somebody opens is its own craft, and it is where careful work is most often thrown away: the resize that aliases a striped shirt into moiré, the sharpening tuned for a size the picture will never be seen at, the wide-gamut profile a platform strips on upload. This block takes the picture out — the arithmetic of how many pixels a destination needs, the resampling kernel doing the work, output sharpening, colour conversion, codecs newer than JPEG, motion, metadata, and what happens to your file after you press upload.
By the end you can
Take a finished edit to a delivered file for any named destination — computing the pixel dimensions from the destination rather than the source, choosing a resampling filter and a colour space by mechanism, sharpening last and at output size, selecting the codec and quality by measurement rather than habit, and predicting what a platform's own re-encoding pipeline will do to the result
- 68Nearest, bicubic, Lanczos: what the resize dialogue is computingA resize invents pixel values between the ones you have: point sampling aliases fine detail into moiré, Lanczos is sharp but rings at hard edges, and averaging gamma-encoded numbers rather than light makes downscaled bright detail on dark backgrounds come out roughly half as bright as it should be.
- 69The width to export, and why the browser will not fix it for youExport width is the slot's CSS width times the device pixel ratio, capped at 2x for photographs; handing the browser a larger file costs the full bytes, is downscaled without gamma correction and throws away the sharpening you tuned for the output size.
- 70Sharpening adds no detail, and it goes lastSharpening only exaggerates edges that already exist, so do it after resizing, at the output size, and after noise reduction rather than before.
- 71Why your image looks washed out on somebody else's phoneDeliver sRGB by converting rather than assigning, choose the format by what is in the picture, and treat file size as part of the job.
- 72Stop guessing the quality sliderA fixed quality setting gives different visual results on different pictures because it scales a quantisation table rather than targeting an outcome, so pick settings by flipping between versions at 100 per cent, automate that judgement with a perceptual metric, and argue in byte budgets rather than slider numbers.
- 73The formats that replaced JPEG, and the one that mightWebP and AVIF are smaller than JPEG because they use variable block sizes and intra prediction borrowed from video codecs, AVIF's characteristic failure is smoothing texture rather than blocking it, JPEG XL can losslessly shrink existing JPEGs but its browser support is genuinely unresolved, and quality numbers never transfer between encoders.
- 74GIF is a 1987 format and you are probably still exporting oneGIF costs ten to twenty times what the same motion costs as video because it stores 256 dithered colours per frame with no motion compensation, so deliver MP4 or WebM with muted autoplay, or animated WebP where an image tag is required, and keep GIF only for platforms that accept nothing else.
- 75The fields that say who made it, and which platforms keep themEXIF records the camera, IPTC records the picture's meaning and XMP is the container modern tools write, so fill Creator, Copyright, Credit and Description as an import preset — and treat embedded metadata as a record that most social re-encoding destroys rather than as any kind of protection.
- 76One picture, six files, and letting the browser chooseShip several widths and formats and let the browser choose with srcset, sizes and picture — sizes being the attribute that actually prevents oversized downloads — then lazy-load everything except the hero, which needs high fetch priority because it is what the load-time metric is measuring.
- 77Your file is re-encoded on upload, and you can plan for itPlatforms resize, re-compress and strip the profile from every upload, so convert to sRGB, match the width they actually serve, sharpen slightly less than for final delivery, and give their encoder a high-quality input rather than one you already compressed.
- 78One bottle, one window, twelve minutesThe order is fixed — shoot, develop, straighten, mask, retouch, tone, then resize, sharpen, convert to sRGB and export — and the layered file is saved before any of it.