diff --git a/src/libImaging/AlphaComposite.c b/src/libImaging/AlphaComposite.c
index d1a1e7ad34a..fff9d8537e1 100644
--- a/src/libImaging/AlphaComposite.c
+++ b/src/libImaging/AlphaComposite.c
@@ -19,6 +19,10 @@ typedef struct {
     UINT8 a;
 } rgba8;
 
+/**
+ * Alpha-composite imSrc over imDst, returning a newly allocated result.
+ * Contract: imDst and imSrc are read-only and may alias each other.
+ */
 Imaging
 ImagingAlphaComposite(Imaging imDst, Imaging imSrc) {
     Imaging imOut;
diff --git a/src/libImaging/Bands.c b/src/libImaging/Bands.c
index d1fa16addff..4c0652a9978 100644
--- a/src/libImaging/Bands.c
+++ b/src/libImaging/Bands.c
@@ -17,6 +17,10 @@
 
 #include "Imaging.h"
 
+/**
+ * Extract a single band from imIn into a newly allocated single-band image.
+ * Contract: imIn is read-only and the returned image is a distinct allocation.
+ */
 Imaging
 ImagingGetBand(Imaging imIn, int band) {
     Imaging imOut;
@@ -68,6 +72,14 @@ ImagingGetBand(Imaging imIn, int band) {
     return imOut;
 }
 
+/**
+ * Split imIn into its component bands.
+ * The caller must provide an array of 4 Imaging pointers,
+ * which will be allocated and filled with the individual bands.
+ * The number of bands returned is the number of bands in imIn.
+ *
+ * Contract: imIn is read-only.
+ */
 int
 ImagingSplit(Imaging imIn, Imaging bands[4]) {
     int i, j, x, y;
@@ -173,6 +185,11 @@ ImagingSplit(Imaging imIn, Imaging bands[4]) {
     return imIn->bands;
 }
 
+/**
+ * Insert single-band imIn into `band` of imOut, in place.
+ *
+ * Contract: imIn and imOut MUST be distinct images and not alias.
+ */
 Imaging
 ImagingPutBand(Imaging imOut, Imaging imIn, int band) {
     int x, y;
@@ -219,6 +236,9 @@ ImagingPutBand(Imaging imOut, Imaging imIn, int band) {
     return imOut;
 }
 
+/**
+ * Fill a single band of imOut with a constant colour, in place.
+ */
 Imaging
 ImagingFillBand(Imaging imOut, int band, int color) {
     int x, y;
@@ -253,6 +273,11 @@ ImagingFillBand(Imaging imOut, int band, int color) {
     return imOut;
 }
 
+/**
+ * Merge the caller-supplied bands[] into a newly allocated multi-band image.
+ *
+ * Contract: the bands[] inputs are read-only, and the output is new.
+ */
 Imaging
 ImagingMerge(const ModeID mode, Imaging bands[4]) {
     int i, x, y;
diff --git a/src/libImaging/Blend.c b/src/libImaging/Blend.c
index 19662abf044..1589bf2bdc5 100644
--- a/src/libImaging/Blend.c
+++ b/src/libImaging/Blend.c
@@ -17,6 +17,12 @@
 
 #include "Imaging.h"
 
+/**
+ * Interpolate (or, for alpha outside [0, 1], extrapolate) between imIn1 and
+ * imIn2 by `alpha`, returning a newly allocated result.
+ *
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingBlend(Imaging imIn1, Imaging imIn2, float alpha) {
     Imaging imOut;
diff --git a/src/libImaging/Chops.c b/src/libImaging/Chops.c
index 557c2f52f10..bd18a06ace0 100644
--- a/src/libImaging/Chops.c
+++ b/src/libImaging/Chops.c
@@ -84,66 +84,124 @@ create(Imaging im1, Imaging im2, const ModeID mode) {
     return ImagingNewDirty(im1->mode, xsize, ysize);
 }
 
+/**
+ * Return a newly allocated image containing the lighter pixels of the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopLighter(Imaging imIn1, Imaging imIn2) {
     CHOP((in1[x] > in2[x]) ? in1[x] : in2[x]);
 }
 
+/**
+ * Return a newly allocated image containing the darker pixels of the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopDarker(Imaging imIn1, Imaging imIn2) {
     CHOP((in1[x] < in2[x]) ? in1[x] : in2[x]);
 }
 
+/**
+ * Return a newly allocated image containing the absolute per-pixel difference of the
+ * two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopDifference(Imaging imIn1, Imaging imIn2) {
     CHOP(abs((int)in1[x] - (int)in2[x]));
 }
 
+/**
+ * Return a newly allocated image containing the per-pixel product (scaled to 0-255) of
+ * the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopMultiply(Imaging imIn1, Imaging imIn2) {
     CHOP((int)in1[x] * (int)in2[x] / 255);
 }
 
+/**
+ * Return a newly allocated image containing the screen blend of the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopScreen(Imaging imIn1, Imaging imIn2) {
     CHOP(255 - ((int)(255 - in1[x]) * (int)(255 - in2[x])) / 255);
 }
 
+/**
+ * Return a newly allocated image containing the per-pixel sum, divided by `scale` and
+ * shifted by `offset`, clipped to 0-255.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopAdd(Imaging imIn1, Imaging imIn2, float scale, int offset) {
     CHOP(((int)in1[x] + (int)in2[x]) / scale + offset);
 }
 
+/**
+ * Return a newly allocated image containing the per-pixel difference, divided by
+ * `scale` and shifted by `offset`, clipped to 0-255.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopSubtract(Imaging imIn1, Imaging imIn2, float scale, int offset) {
     CHOP(((int)in1[x] - (int)in2[x]) / scale + offset);
 }
 
+/**
+ * Return a newly allocated "1" image that is the logical AND of the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopAnd(Imaging imIn1, Imaging imIn2) {
     CHOP2((in1[x] && in2[x]) ? 255 : 0, IMAGING_MODE_1);
 }
 
+/**
+ * Return a newly allocated "1" image that is the logical OR of the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopOr(Imaging imIn1, Imaging imIn2) {
     CHOP2((in1[x] || in2[x]) ? 255 : 0, IMAGING_MODE_1);
 }
 
+/**
+ * Return a newly allocated "1" image that is the logical XOR of the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopXor(Imaging imIn1, Imaging imIn2) {
     CHOP2(((in1[x] != 0) ^ (in2[x] != 0)) ? 255 : 0, IMAGING_MODE_1);
 }
 
+/**
+ * Return a newly allocated image containing the per-pixel sum of the two images,
+ * wrapping on overflow (no clipping).
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopAddModulo(Imaging imIn1, Imaging imIn2) {
     CHOP2(in1[x] + in2[x], IMAGING_MODE_UNKNOWN);
 }
 
+/**
+ * Return a newly allocated image containing the per-pixel difference of the two images,
+ * wrapping on underflow (no clipping).
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopSubtractModulo(Imaging imIn1, Imaging imIn2) {
     CHOP2(in1[x] - in2[x], IMAGING_MODE_UNKNOWN);
 }
 
+/**
+ * Return a newly allocated image containing the soft-light blend of the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopSoftLight(Imaging imIn1, Imaging imIn2) {
     CHOP2(
@@ -153,6 +211,10 @@ ImagingChopSoftLight(Imaging imIn1, Imaging imIn2) {
     );
 }
 
+/**
+ * Return a newly allocated image containing the hard-light blend of the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingChopHardLight(Imaging imIn1, Imaging imIn2) {
     CHOP2(
@@ -162,6 +224,10 @@ ImagingChopHardLight(Imaging imIn1, Imaging imIn2) {
     );
 }
 
+/**
+ * Return a newly allocated image containing the overlay blend of the two images.
+ * Contract: imIn1 and imIn2 are read-only and may alias each other.
+ */
 Imaging
 ImagingOverlay(Imaging imIn1, Imaging imIn2) {
     CHOP2(
diff --git a/src/libImaging/ColorLUT.c b/src/libImaging/ColorLUT.c
index 5559de689b2..55fc808f04a 100644
--- a/src/libImaging/ColorLUT.c
+++ b/src/libImaging/ColorLUT.c
@@ -55,6 +55,12 @@ table_index3D(int index1D, int index2D, int index3D, int size1D, int size1D_2D)
     Each element is signed 16-bit int where 0 is lowest output value
     and 255 << PRECISION_BITS (16320) is highest value.
 */
+/**
+ * Apply a 3D colour lookup table to imIn, writing the result to imOut.
+ *
+ * Contract: unlike the other point-style loops in libImaging,
+ * imIn and imOut MAY be the same image to support running in place.
+ */
 Imaging
 ImagingColorLUT3D_linear(
     Imaging imOut,
diff --git a/src/libImaging/Fill.c b/src/libImaging/Fill.c
index 25344cf3a7e..ee61ec8d8ec 100644
--- a/src/libImaging/Fill.c
+++ b/src/libImaging/Fill.c
@@ -19,6 +19,9 @@
 
 #include "math.h"
 
+/**
+ * Fill an entire image with a constant colour, in place.
+ */
 Imaging
 ImagingFill(Imaging im, const void *colour) {
     ImagingSectionCookie cookie;
diff --git a/src/libImaging/Filter.c b/src/libImaging/Filter.c
index b5f971d40c2..1a8843526e1 100644
--- a/src/libImaging/Filter.c
+++ b/src/libImaging/Filter.c
@@ -120,6 +120,11 @@ kernel_i16(int size, UINT8 *in0, int x, const float *kernel, int bigendian) {
     return result;
 }
 
+/**
+ * Convolve `im` with a 3x3 kernel, writing the result to imOut.
+ *
+ * Contract: imOut and im *MUST* be distinct images.
+ */
 static void
 ImagingFilter3x3(Imaging imOut, Imaging im, const float *kernel, float offset) {
 #define KERNEL1x3(in0, x, kernel, d)                                     \
@@ -271,6 +276,11 @@ ImagingFilter3x3(Imaging imOut, Imaging im, const float *kernel, float offset) {
     memcpy(imOut->image[y], im->image[y], im->linesize);
 }
 
+/**
+ * Convolve `im` with a 5x5 kernel, writing the result to imOut.
+ *
+ * Contract: imOut and im *MUST* be distinct images.
+ */
 static void
 ImagingFilter5x5(Imaging imOut, Imaging im, const float *kernel, float offset) {
 #define KERNEL1x5(in0, x, kernel, d)                                             \
diff --git a/src/libImaging/Histo.c b/src/libImaging/Histo.c
index 7af60003511..c51677901c6 100644
--- a/src/libImaging/Histo.c
+++ b/src/libImaging/Histo.c
@@ -56,6 +56,11 @@ ImagingHistogramNew(Imaging im) {
     return h;
 }
 
+/**
+ * Accumulate a histogram over `im`, optionally restricted to imMask.
+ *
+ * Contract: Both im and imMask are read-only.
+ */
 ImagingHistogram
 ImagingGetHistogram(Imaging im, Imaging imMask, void *minmax) {
     ImagingSectionCookie cookie;
diff --git a/src/libImaging/Matrix.c b/src/libImaging/Matrix.c
index 02676c9e31a..cd317ba264e 100644
--- a/src/libImaging/Matrix.c
+++ b/src/libImaging/Matrix.c
@@ -17,6 +17,12 @@
 
 #define CLIPF(v) ((v <= 0.0) ? 0 : (v >= 255.0F) ? 255 : (UINT8)v)
 
+/**
+ * Convert `im` to `mode` by applying the colour matrix `m`,
+ * returning a newly allocated result.
+ *
+ * Contract: im is read-only.
+ */
 Imaging
 ImagingConvertMatrix(Imaging im, const ModeID mode, const float m[12]) {
     Imaging imOut;
diff --git a/src/libImaging/Negative.c b/src/libImaging/Negative.c
index 114e8608e87..ea04563851d 100644
--- a/src/libImaging/Negative.c
+++ b/src/libImaging/Negative.c
@@ -18,6 +18,11 @@
 
 #include "Imaging.h"
 
+/**
+ * Invert every byte of `im`, returning a newly allocated result.
+ *
+ * Contract: im is read-only.
+ */
 Imaging
 ImagingNegative(Imaging im) {
     Imaging imOut;
diff --git a/src/libImaging/Quant.c b/src/libImaging/Quant.c
index 99c2dbac3af..5712f6bd39c 100644
--- a/src/libImaging/Quant.c
+++ b/src/libImaging/Quant.c
@@ -1663,6 +1663,12 @@ quantize2(
     return 0;
 }
 
+/**
+ * Quantize `im` down to at most `colors` palette entries,
+ * returning a newly allocated result.
+ *
+ * Contract: im is read-only.
+ */
 Imaging
 ImagingQuantize(Imaging im, int colors, int mode, int kmeans) {
     int i, j;